migrate gestures to nullsafety (#62157)

dev/tools/dartdoc.dart

@@ -126,6 +126,7 @@ Future<void> main(List<String> arguments) async {
   final List<String> dartdocArgs = <String>[
     ...dartdocBaseArgs,
     '--allow-tools',
+    '--enable-experiment=non-nullable',
     if (args['json'] as bool) '--json',
     if (args['validate-links'] as bool) '--validate-links' else '--no-validate-links',
     '--link-to-source-excludes', '../../bin/cache',

packages/flutter/lib/gestures.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 /// The Flutter gesture recognizers.
 ///
 /// To use, import `package:flutter/gestures.dart`.

packages/flutter/lib/src/gestures/arena.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:async';
 
@@ -65,7 +64,7 @@ class _GestureArena {
   /// "eager winner". We look for an eager winner when closing the arena to new
   /// participants, and if there is one, we resolve the arena in its favor at
   /// that time.
-  GestureArenaMember eagerWinner;
+  GestureArenaMember? eagerWinner;
 
   void add(GestureArenaMember member) {
     assert(isOpen);
@@ -119,7 +118,7 @@ class GestureArenaManager {
   ///
   /// Called after the framework has finished dispatching the pointer down event.
   void close(int pointer) {
-    final _GestureArena state = _arenas[pointer];
+    final _GestureArena? state = _arenas[pointer];
     if (state == null)
       return; // This arena either never existed or has been resolved.
     state.isOpen = false;
@@ -141,7 +140,7 @@ class GestureArenaManager {
   ///  * [hold]
   ///  * [release]
   void sweep(int pointer) {
-    final _GestureArena state = _arenas[pointer];
+    final _GestureArena? state = _arenas[pointer];
     if (state == null)
       return; // This arena either never existed or has been resolved.
     assert(!state.isOpen);
@@ -175,7 +174,7 @@ class GestureArenaManager {
   ///  * [sweep]
   ///  * [release]
   void hold(int pointer) {
-    final _GestureArena state = _arenas[pointer];
+    final _GestureArena? state = _arenas[pointer];
     if (state == null)
       return; // This arena either never existed or has been resolved.
     state.isHeld = true;
@@ -192,7 +191,7 @@ class GestureArenaManager {
   ///  * [sweep]
   ///  * [hold]
   void release(int pointer) {
-    final _GestureArena state = _arenas[pointer];
+    final _GestureArena? state = _arenas[pointer];
     if (state == null)
       return; // This arena either never existed or has been resolved.
     state.isHeld = false;
@@ -205,7 +204,7 @@ class GestureArenaManager {
   ///
   /// This is called by calling [GestureArenaEntry.resolve] on the object returned from [add].
   void _resolve(int pointer, GestureArenaMember member, GestureDisposition disposition) {
-    final _GestureArena state = _arenas[pointer];
+    final _GestureArena? state = _arenas[pointer];
     if (state == null)
       return; // This arena has already resolved.
     assert(_debugLogDiagnostic(pointer, '${ disposition == GestureDisposition.accepted ? "Accepting" : "Rejecting" }: $member'));
@@ -236,7 +235,7 @@ class GestureArenaManager {
       assert(_debugLogDiagnostic(pointer, 'Arena empty.'));
     } else if (state.eagerWinner != null) {
       assert(_debugLogDiagnostic(pointer, 'Eager winner: ${state.eagerWinner}'));
-      _resolveInFavorOf(pointer, state, state.eagerWinner);
+      _resolveInFavorOf(pointer, state, state.eagerWinner!);
     }
   }
 
@@ -265,10 +264,10 @@ class GestureArenaManager {
     member.acceptGesture(pointer);
   }
 
-  bool _debugLogDiagnostic(int pointer, String message, [ _GestureArena state ]) {
+  bool _debugLogDiagnostic(int pointer, String message, [ _GestureArena? state ]) {
     assert(() {
       if (debugPrintGestureArenaDiagnostics) {
-        final int count = state != null ? state.members.length : null;
+        final int? count = state != null ? state.members.length : null;
         final String s = count != 1 ? 's' : '';
         debugPrint('Gesture arena ${pointer.toString().padRight(4)} ❙ $message${ count != null ? " with $count member$s." : ""}');
       }

packages/flutter/lib/src/gestures/binding.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:async';
 import 'dart:collection';
@@ -75,8 +74,8 @@ mixin GestureBinding on BindingBase implements HitTestable, HitTestDispatcher, H
   }
 
   /// The singleton instance of this object.
-  static GestureBinding get instance => _instance;
-  static GestureBinding _instance;
+  static GestureBinding? get instance => _instance;
+  static GestureBinding? _instance;
 
   final Queue<PointerEvent> _pendingPointerEvents = Queue<PointerEvent>();
 
@@ -123,7 +122,7 @@ mixin GestureBinding on BindingBase implements HitTestable, HitTestDispatcher, H
 
   void _handlePointerEvent(PointerEvent event) {
     assert(!locked);
-    HitTestResult hitTestResult;
+    HitTestResult? hitTestResult;
     if (event is PointerDownEvent || event is PointerSignalEvent) {
       assert(!_hitTests.containsKey(event.pointer));
       hitTestResult = HitTestResult();
@@ -172,7 +171,7 @@ mixin GestureBinding on BindingBase implements HitTestable, HitTestDispatcher, H
   /// might throw. The [hitTestResult] argument may only be null for
   /// [PointerHoverEvent], [PointerAddedEvent], or [PointerRemovedEvent] events.
   @override // from HitTestDispatcher
-  void dispatchEvent(PointerEvent event, HitTestResult hitTestResult) {
+  void dispatchEvent(PointerEvent event, HitTestResult? hitTestResult) {
     assert(!locked);
     // No hit test information implies that this is a hover or pointer
     // add/remove event.
@@ -243,12 +242,12 @@ class FlutterErrorDetailsForPointerEventDispatcher extends FlutterErrorDetails {
   /// that will subsequently be reported using [FlutterError.onError].
   const FlutterErrorDetailsForPointerEventDispatcher({
     dynamic exception,
-    StackTrace stack,
-    String library,
-    DiagnosticsNode context,
+    StackTrace? stack,
+    String? library,
+    DiagnosticsNode? context,
     this.event,
     this.hitTestEntry,
-    InformationCollector informationCollector,
+    InformationCollector? informationCollector,
     bool silent = false,
   }) : super(
     exception: exception,
@@ -260,7 +259,7 @@ class FlutterErrorDetailsForPointerEventDispatcher extends FlutterErrorDetails {
   );
 
   /// The pointer event that was being routed when the exception was raised.
-  final PointerEvent event;
+  final PointerEvent? event;
 
   /// The hit test result entry for the object whose handleEvent method threw
   /// the exception. May be null if no hit test entry is associated with the
@@ -268,5 +267,5 @@ class FlutterErrorDetailsForPointerEventDispatcher extends FlutterErrorDetails {
   ///
   /// The target object itself is given by the [HitTestEntry.target] property of
   /// the hitTestEntry object.
-  final HitTestEntry hitTestEntry;
+  final HitTestEntry? hitTestEntry;
 }

packages/flutter/lib/src/gestures/constants.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 // Modeled after Android's ViewConfiguration:
 // https://github.com/android/platform_frameworks_base/blob/master/core/java/android/view/ViewConfiguration.java

packages/flutter/lib/src/gestures/converter.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:ui' as ui show PointerData, PointerChange, PointerSignalKind;
 
@@ -214,7 +213,7 @@ class PointerEventConverter {
             break;
         }
       } else {
-        switch (datum.signalKind) {
+        switch (datum.signalKind!) {
           case ui.PointerSignalKind.scroll:
             final Offset scrollDelta =
                 Offset(datum.scrollDeltaX, datum.scrollDeltaY) / devicePixelRatio;
@@ -238,6 +237,5 @@ class PointerEventConverter {
     }
   }
 
-  static double _toLogicalPixels(double physicalPixels, double devicePixelRatio) =>
-      physicalPixels == null ? null : physicalPixels / devicePixelRatio;
+  static double _toLogicalPixels(double physicalPixels, double devicePixelRatio) => physicalPixels / devicePixelRatio;
 }

packages/flutter/lib/src/gestures/debug.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/foundation.dart';
 

packages/flutter/lib/src/gestures/drag.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'drag_details.dart';
 

packages/flutter/lib/src/gestures/drag_details.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:ui' show Offset;
 
@@ -24,7 +23,7 @@ class DragDownDetails {
   /// The [globalPosition] argument must not be null.
   DragDownDetails({
     this.globalPosition = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
   }) : assert(globalPosition != null),
        localPosition = localPosition ?? globalPosition;
 
@@ -71,7 +70,7 @@ class DragStartDetails {
   DragStartDetails({
     this.sourceTimeStamp,
     this.globalPosition = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
   }) : assert(globalPosition != null),
        localPosition = localPosition ?? globalPosition;
 
@@ -79,7 +78,7 @@ class DragStartDetails {
   /// event.
   ///
   /// Could be null if triggered from proxied events such as accessibility.
-  final Duration sourceTimeStamp;
+  final Duration? sourceTimeStamp;
 
   /// The global position at which the pointer contacted the screen.
   ///
@@ -134,8 +133,8 @@ class DragUpdateDetails {
     this.sourceTimeStamp,
     this.delta = Offset.zero,
     this.primaryDelta,
-    @required this.globalPosition,
-    Offset localPosition,
+    required this.globalPosition,
+    Offset? localPosition,
   }) : assert(delta != null),
        assert(primaryDelta == null
            || (primaryDelta == delta.dx && delta.dy == 0.0)
@@ -146,7 +145,7 @@ class DragUpdateDetails {
   /// event.
   ///
   /// Could be null if triggered from proxied events such as accessibility.
-  final Duration sourceTimeStamp;
+  final Duration? sourceTimeStamp;
 
   /// The amount the pointer has moved in the coordinate space of the event
   /// receiver since the previous update.
@@ -169,7 +168,7 @@ class DragUpdateDetails {
   /// two-dimensional drag (e.g., a pan), then this value is null.
   ///
   /// Defaults to null if not specified in the constructor.
-  final double primaryDelta;
+  final double? primaryDelta;
 
   /// The pointer's global position when it triggered this update.
   ///
@@ -233,7 +232,7 @@ class DragEndDetails {
   /// two-dimensional drag (e.g., a pan), then this value is null.
   ///
   /// Defaults to null if not specified in the constructor.
-  final double primaryVelocity;
+  final double? primaryVelocity;
 
   @override
   String toString() => '${objectRuntimeType(this, 'DragEndDetails')}($velocity)';

packages/flutter/lib/src/gestures/eager.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'arena.dart';
 import 'events.dart';
@@ -17,7 +16,7 @@ class EagerGestureRecognizer extends OneSequenceGestureRecognizer {
   /// Create an eager gesture recognizer.
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
-  EagerGestureRecognizer({ PointerDeviceKind kind }) : super(kind: kind);
+  EagerGestureRecognizer({ PointerDeviceKind? kind }) : super(kind: kind);
 
   @override
   void addAllowedPointer(PointerDownEvent event) {

packages/flutter/lib/src/gestures/events.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:ui' show Offset, PointerDeviceKind;
 
@@ -210,9 +209,9 @@ abstract class PointerEvent with Diagnosticable {
     this.kind = PointerDeviceKind.touch,
     this.device = 0,
     this.position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     this.delta = Offset.zero,
-    Offset localDelta,
+    Offset? localDelta,
     this.buttons = 0,
     this.down = false,
     this.obscured = false,
@@ -459,7 +458,7 @@ abstract class PointerEvent with Diagnosticable {
   ///
   ///  * [transformed], which transforms this event into a different coordinate
   ///    space.
-  final Matrix4 transform;
+  final Matrix4? transform;
 
   /// The original un-transformed [PointerEvent] before any [transform]s were
   /// applied.
@@ -470,7 +469,7 @@ abstract class PointerEvent with Diagnosticable {
   /// event, they all receive the event transformed to their local coordinate
   /// space. The [original] property can be used to determine if all those
   /// transformed events actually originated from the same pointer interaction.
-  final PointerEvent original;
+  final PointerEvent? original;
 
   /// Transforms the event from the global coordinate space into the coordinate
   /// space of an event receiver.
@@ -484,7 +483,7 @@ abstract class PointerEvent with Diagnosticable {
   /// Transforms are not commutative. If this method is called on a
   /// [PointerEvent] that has a non-null [transform] value, that value will be
   /// overridden by the provided `transform`.
-  PointerEvent transformed(Matrix4 transform);
+  PointerEvent transformed(Matrix4? transform);
 
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
@@ -528,7 +527,7 @@ abstract class PointerEvent with Diagnosticable {
   ///
   /// The z-value of `position` is assumed to be 0.0. If `transform` is null,
   /// `position` is returned as-is.
-  static Offset transformPosition(Matrix4 transform, Offset position) {
+  static Offset transformPosition(Matrix4? transform, Offset position) {
     if (transform == null) {
       return position;
     }
@@ -545,10 +544,10 @@ abstract class PointerEvent with Diagnosticable {
   ///
   /// If `transform` is null, `untransformedDelta` is returned.
   static Offset transformDeltaViaPositions({
-    @required Offset untransformedEndPosition,
-    Offset transformedEndPosition,
-    @required Offset untransformedDelta,
-    @required Matrix4 transform,
+    required Offset untransformedEndPosition,
+    Offset? transformedEndPosition,
+    required Offset untransformedDelta,
+    required Matrix4? transform,
   }) {
     if (transform == null) {
       return untransformedDelta;
@@ -592,7 +591,7 @@ class PointerAddedEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     bool obscured = false,
     double pressureMin = 1.0,
     double pressureMax = 1.0,
@@ -602,8 +601,8 @@ class PointerAddedEvent extends PointerEvent {
     double radiusMax = 0.0,
     double orientation = 0.0,
     double tilt = 0.0,
-    Matrix4 transform,
-    PointerAddedEvent original,
+    Matrix4? transform,
+    PointerAddedEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -627,7 +626,7 @@ class PointerAddedEvent extends PointerEvent {
        );
 
   @override
-  PointerAddedEvent transformed(Matrix4 transform) {
+  PointerAddedEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -647,7 +646,7 @@ class PointerAddedEvent extends PointerEvent {
       orientation: orientation,
       tilt: tilt,
       transform: transform,
-      original: original as PointerAddedEvent ?? this,
+      original: original as PointerAddedEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -666,15 +665,15 @@ class PointerRemovedEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     bool obscured = false,
     double pressureMin = 1.0,
     double pressureMax = 1.0,
     double distanceMax = 0.0,
     double radiusMin = 0.0,
     double radiusMax = 0.0,
-    Matrix4 transform,
-    PointerRemovedEvent original,
+    Matrix4? transform,
+    PointerRemovedEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -695,7 +694,7 @@ class PointerRemovedEvent extends PointerEvent {
        );
 
   @override
-  PointerRemovedEvent transformed(Matrix4 transform) {
+  PointerRemovedEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -712,7 +711,7 @@ class PointerRemovedEvent extends PointerEvent {
       radiusMin: radiusMin,
       radiusMax: radiusMax,
       transform: transform,
-      original: original as PointerRemovedEvent ?? this,
+      original: original as PointerRemovedEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -739,9 +738,9 @@ class PointerHoverEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     Offset delta = Offset.zero,
-    Offset localDelta,
+    Offset? localDelta,
     int buttons = 0,
     bool obscured = false,
     double pressureMin = 1.0,
@@ -756,8 +755,8 @@ class PointerHoverEvent extends PointerEvent {
     double orientation = 0.0,
     double tilt = 0.0,
     bool synthesized = false,
-    Matrix4 transform,
-    PointerHoverEvent original,
+    Matrix4? transform,
+    PointerHoverEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -789,7 +788,7 @@ class PointerHoverEvent extends PointerEvent {
        );
 
   @override
-  PointerHoverEvent transformed(Matrix4 transform) {
+  PointerHoverEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -822,7 +821,7 @@ class PointerHoverEvent extends PointerEvent {
       tilt: tilt,
       synthesized: synthesized,
       transform: transform,
-      original: original as PointerHoverEvent ?? this,
+      original: original as PointerHoverEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -849,9 +848,9 @@ class PointerEnterEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     Offset delta = Offset.zero,
-    Offset localDelta,
+    Offset? localDelta,
     int buttons = 0,
     bool obscured = false,
     double pressureMin = 1.0,
@@ -867,8 +866,8 @@ class PointerEnterEvent extends PointerEvent {
     double tilt = 0.0,
     bool down = false,
     bool synthesized = false,
-    Matrix4 transform,
-    PointerEnterEvent original,
+    Matrix4? transform,
+    PointerEnterEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -912,34 +911,34 @@ class PointerEnterEvent extends PointerEvent {
   ///
   /// This is used by the [MouseTracker] to synthesize enter events.
   PointerEnterEvent.fromMouseEvent(PointerEvent event) : this(
-    timeStamp: event?.timeStamp,
-    kind: event?.kind,
-    device: event?.device,
-    position: event?.position,
-    localPosition: event?.localPosition,
-    delta: event?.delta,
-    localDelta: event?.localDelta,
-    buttons: event?.buttons,
-    obscured: event?.obscured,
-    pressureMin: event?.pressureMin,
-    pressureMax: event?.pressureMax,
-    distance: event?.distance,
-    distanceMax: event?.distanceMax,
-    size: event?.size,
-    radiusMajor: event?.radiusMajor,
-    radiusMinor: event?.radiusMinor,
-    radiusMin: event?.radiusMin,
-    radiusMax: event?.radiusMax,
-    orientation: event?.orientation,
-    tilt: event?.tilt,
-    down: event?.down,
-    synthesized: event?.synthesized,
-    transform: event?.transform,
+    timeStamp: event.timeStamp,
+    kind: event.kind,
+    device: event.device,
+    position: event.position,
+    localPosition: event.localPosition,
+    delta: event.delta,
+    localDelta: event.localDelta,
+    buttons: event.buttons,
+    obscured: event.obscured,
+    pressureMin: event.pressureMin,
+    pressureMax: event.pressureMax,
+    distance: event.distance,
+    distanceMax: event.distanceMax,
+    size: event.size,
+    radiusMajor: event.radiusMajor,
+    radiusMinor: event.radiusMinor,
+    radiusMin: event.radiusMin,
+    radiusMax: event.radiusMax,
+    orientation: event.orientation,
+    tilt: event.tilt,
+    down: event.down,
+    synthesized: event.synthesized,
+    transform: event.transform,
     original: null,
   );
 
   @override
-  PointerEnterEvent transformed(Matrix4 transform) {
+  PointerEnterEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -973,7 +972,7 @@ class PointerEnterEvent extends PointerEvent {
       down: down,
       synthesized: synthesized,
       transform: transform,
-      original: original as PointerEnterEvent ?? this,
+      original: original as PointerEnterEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -1000,9 +999,9 @@ class PointerExitEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     Offset delta = Offset.zero,
-    Offset localDelta,
+    Offset? localDelta,
     int buttons = 0,
     bool obscured = false,
     double pressureMin = 1.0,
@@ -1018,8 +1017,8 @@ class PointerExitEvent extends PointerEvent {
     double tilt = 0.0,
     bool down = false,
     bool synthesized = false,
-    Matrix4 transform,
-    PointerExitEvent original,
+    Matrix4? transform,
+    PointerExitEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -1063,34 +1062,34 @@ class PointerExitEvent extends PointerEvent {
   ///
   /// This is used by the [MouseTracker] to synthesize exit events.
   PointerExitEvent.fromMouseEvent(PointerEvent event) : this(
-    timeStamp: event?.timeStamp,
-    kind: event?.kind,
-    device: event?.device,
-    position: event?.position,
-    localPosition: event?.localPosition,
-    delta: event?.delta,
-    localDelta: event?.localDelta,
-    buttons: event?.buttons,
-    obscured: event?.obscured,
-    pressureMin: event?.pressureMin,
-    pressureMax: event?.pressureMax,
-    distance: event?.distance,
-    distanceMax: event?.distanceMax,
-    size: event?.size,
-    radiusMajor: event?.radiusMajor,
-    radiusMinor: event?.radiusMinor,
-    radiusMin: event?.radiusMin,
-    radiusMax: event?.radiusMax,
-    orientation: event?.orientation,
-    tilt: event?.tilt,
-    down: event?.down,
-    synthesized: event?.synthesized,
-    transform: event?.transform,
+    timeStamp: event.timeStamp,
+    kind: event.kind,
+    device: event.device,
+    position: event.position,
+    localPosition: event.localPosition,
+    delta: event.delta,
+    localDelta: event.localDelta,
+    buttons: event.buttons,
+    obscured: event.obscured,
+    pressureMin: event.pressureMin,
+    pressureMax: event.pressureMax,
+    distance: event.distance,
+    distanceMax: event.distanceMax,
+    size: event.size,
+    radiusMajor: event.radiusMajor,
+    radiusMinor: event.radiusMinor,
+    radiusMin: event.radiusMin,
+    radiusMax: event.radiusMax,
+    orientation: event.orientation,
+    tilt: event.tilt,
+    down: event.down,
+    synthesized: event.synthesized,
+    transform: event.transform,
     original: null,
   );
 
   @override
-  PointerExitEvent transformed(Matrix4 transform) {
+  PointerExitEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -1124,7 +1123,7 @@ class PointerExitEvent extends PointerEvent {
       down: down,
       synthesized: synthesized,
       transform: transform,
-      original: original as PointerExitEvent ?? this,
+      original: original as PointerExitEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -1146,7 +1145,7 @@ class PointerDownEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     int buttons = kPrimaryButton,
     bool obscured = false,
     double pressure = 1.0,
@@ -1160,8 +1159,8 @@ class PointerDownEvent extends PointerEvent {
     double radiusMax = 0.0,
     double orientation = 0.0,
     double tilt = 0.0,
-    Matrix4 transform,
-    PointerDownEvent original,
+    Matrix4? transform,
+    PointerDownEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -1191,7 +1190,7 @@ class PointerDownEvent extends PointerEvent {
        );
 
   @override
-  PointerDownEvent transformed(Matrix4 transform) {
+  PointerDownEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -1216,7 +1215,7 @@ class PointerDownEvent extends PointerEvent {
       orientation: orientation,
       tilt: tilt,
       transform: transform,
-      original: original as PointerDownEvent ?? this,
+      original: original as PointerDownEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -1241,9 +1240,9 @@ class PointerMoveEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     Offset delta = Offset.zero,
-    Offset localDelta,
+    Offset? localDelta,
     int buttons = kPrimaryButton,
     bool obscured = false,
     double pressure = 1.0,
@@ -1259,8 +1258,8 @@ class PointerMoveEvent extends PointerEvent {
     double tilt = 0.0,
     int platformData = 0,
     bool synthesized = false,
-    Matrix4 transform,
-    PointerMoveEvent original,
+    Matrix4? transform,
+    PointerMoveEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -1294,7 +1293,7 @@ class PointerMoveEvent extends PointerEvent {
        );
 
   @override
-  PointerMoveEvent transformed(Matrix4 transform) {
+  PointerMoveEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -1330,7 +1329,7 @@ class PointerMoveEvent extends PointerEvent {
       platformData: platformData,
       synthesized: synthesized,
       transform: transform,
-      original: original as PointerMoveEvent ?? this,
+      original: original as PointerMoveEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -1352,7 +1351,7 @@ class PointerUpEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     int buttons = 0,
     bool obscured = false,
     // Allow pressure customization here because PointerUpEvent can contain
@@ -1369,8 +1368,8 @@ class PointerUpEvent extends PointerEvent {
     double radiusMax = 0.0,
     double orientation = 0.0,
     double tilt = 0.0,
-    Matrix4 transform,
-    PointerUpEvent original,
+    Matrix4? transform,
+    PointerUpEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -1400,7 +1399,7 @@ class PointerUpEvent extends PointerEvent {
        );
 
   @override
-  PointerUpEvent transformed(Matrix4 transform) {
+  PointerUpEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -1426,7 +1425,7 @@ class PointerUpEvent extends PointerEvent {
       orientation: orientation,
       tilt: tilt,
       transform: transform,
-      original: original as PointerUpEvent ?? this,
+      original: original as PointerUpEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -1451,9 +1450,9 @@ abstract class PointerSignalEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.mouse,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
-    Matrix4 transform,
-    PointerSignalEvent original,
+    Offset? localPosition,
+    Matrix4? transform,
+    PointerSignalEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -1486,10 +1485,10 @@ class PointerScrollEvent extends PointerSignalEvent {
     PointerDeviceKind kind = PointerDeviceKind.mouse,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     this.scrollDelta = Offset.zero,
-    Matrix4 transform,
-    PointerScrollEvent original,
+    Matrix4? transform,
+    PointerScrollEvent? original,
     int embedderId = 0,
   }) : assert(timeStamp != null),
        assert(kind != null),
@@ -1511,7 +1510,7 @@ class PointerScrollEvent extends PointerSignalEvent {
   final Offset scrollDelta;
 
   @override
-  PointerScrollEvent transformed(Matrix4 transform) {
+  PointerScrollEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -1523,7 +1522,7 @@ class PointerScrollEvent extends PointerSignalEvent {
       localPosition: PointerEvent.transformPosition(transform, position),
       scrollDelta: scrollDelta,
       transform: transform,
-      original: original as PointerScrollEvent ?? this,
+      original: original as PointerScrollEvent? ?? this,
       embedderId: embedderId,
     );
   }
@@ -1551,7 +1550,7 @@ class PointerCancelEvent extends PointerEvent {
     PointerDeviceKind kind = PointerDeviceKind.touch,
     int device = 0,
     Offset position = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     int buttons = 0,
     bool obscured = false,
     double pressureMin = 1.0,
@@ -1565,8 +1564,8 @@ class PointerCancelEvent extends PointerEvent {
     double radiusMax = 0.0,
     double orientation = 0.0,
     double tilt = 0.0,
-    Matrix4 transform,
-    PointerCancelEvent original,
+    Matrix4? transform,
+    PointerCancelEvent? original,
     int embedderId = 0,
   }) : super(
          timeStamp: timeStamp,
@@ -1596,7 +1595,7 @@ class PointerCancelEvent extends PointerEvent {
        );
 
   @override
-  PointerCancelEvent transformed(Matrix4 transform) {
+  PointerCancelEvent transformed(Matrix4? transform) {
     if (transform == null || transform == this.transform) {
       return this;
     }
@@ -1621,7 +1620,7 @@ class PointerCancelEvent extends PointerEvent {
       orientation: orientation,
       tilt: tilt,
       transform: transform,
-      original: original as PointerCancelEvent ?? this,
+      original: original as PointerCancelEvent? ?? this,
       embedderId: embedderId,
     );
   }

packages/flutter/lib/src/gestures/force_press.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:ui' show Offset;
 
@@ -51,9 +50,9 @@ class ForcePressDetails {
   ///
   /// The [globalPosition] argument must not be null.
   ForcePressDetails({
-    @required this.globalPosition,
-    Offset localPosition,
-    @required this.pressure,
+    required this.globalPosition,
+    Offset? localPosition,
+    required this.pressure,
   }) : assert(globalPosition != null),
        assert(pressure != null),
        localPosition = localPosition ?? globalPosition;
@@ -128,8 +127,8 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
     this.startPressure = 0.4,
     this.peakPressure = 0.85,
     this.interpolation = _inverseLerp,
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : assert(startPressure != null),
        assert(peakPressure != null),
        assert(interpolation != null),
@@ -143,7 +142,7 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
   ///
   /// The position of the pointer is provided in the callback's `details`
   /// argument, which is a [ForcePressDetails] object.
-  GestureForcePressStartCallback onStart;
+  GestureForcePressStartCallback? onStart;
 
   /// A pointer is in contact with the screen and is either moving on the plane
   /// of the screen, pressing the screen with varying forces or both
@@ -154,7 +153,7 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
   /// matter what the pressure is during this time period. The position and
   /// pressure of the pointer is provided in the callback's `details` argument,
   /// which is a [ForcePressDetails] object.
-  GestureForcePressUpdateCallback onUpdate;
+  GestureForcePressUpdateCallback? onUpdate;
 
   /// A pointer is in contact with the screen and has just pressed with a force
   /// exceeding the [peakPressure]. This is an arbitrary second level action
@@ -163,13 +162,13 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
   ///
   /// The position of the pointer is provided in the callback's `details`
   /// argument, which is a [ForcePressDetails] object.
-  GestureForcePressPeakCallback onPeak;
+  GestureForcePressPeakCallback? onPeak;
 
   /// A pointer is no longer in contact with the screen.
   ///
   /// The position of the pointer is provided in the callback's `details`
   /// argument, which is a [ForcePressDetails] object.
-  GestureForcePressEndCallback onEnd;
+  GestureForcePressEndCallback? onEnd;
 
   /// The pressure of the press required to initiate a force press.
   ///
@@ -209,8 +208,8 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
   /// ```
   final GestureForceInterpolation interpolation;
 
-  OffsetPair _lastPosition;
-  double _lastPressure;
+  late OffsetPair _lastPosition;
+  late double _lastPressure;
   _ForceState _state = _ForceState.ready;
 
   @override
@@ -264,7 +263,7 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
       if (pressure > startPressure && _state == _ForceState.accepted) {
         _state = _ForceState.started;
         if (onStart != null) {
-          invokeCallback<void>('onStart', () => onStart(ForcePressDetails(
+          invokeCallback<void>('onStart', () => onStart!(ForcePressDetails(
             pressure: pressure,
             globalPosition: _lastPosition.global,
             localPosition: _lastPosition.local,
@@ -275,7 +274,7 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
          (_state == _ForceState.started)) {
         _state = _ForceState.peaked;
         if (onPeak != null) {
-          invokeCallback<void>('onPeak', () => onPeak(ForcePressDetails(
+          invokeCallback<void>('onPeak', () => onPeak!(ForcePressDetails(
             pressure: pressure,
             globalPosition: event.position,
             localPosition: event.localPosition,
@@ -285,7 +284,7 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
       if (onUpdate != null &&  !pressure.isNaN &&
          (_state == _ForceState.started || _state == _ForceState.peaked)) {
         if (onUpdate != null) {
-          invokeCallback<void>('onUpdate', () => onUpdate(ForcePressDetails(
+          invokeCallback<void>('onUpdate', () => onUpdate!(ForcePressDetails(
             pressure: pressure,
             globalPosition: event.position,
             localPosition: event.localPosition,
@@ -302,7 +301,7 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
       _state = _ForceState.accepted;
 
     if (onStart != null && _state == _ForceState.started) {
-      invokeCallback<void>('onStart', () => onStart(ForcePressDetails(
+      invokeCallback<void>('onStart', () => onStart!(ForcePressDetails(
         pressure: _lastPressure,
         globalPosition: _lastPosition.global,
         localPosition: _lastPosition.local,
@@ -319,7 +318,7 @@ class ForcePressGestureRecognizer extends OneSequenceGestureRecognizer {
     }
     if (wasAccepted && onEnd != null) {
       if (onEnd != null) {
-        invokeCallback<void>('onEnd', () => onEnd(ForcePressDetails(
+        invokeCallback<void>('onEnd', () => onEnd!(ForcePressDetails(
           pressure: 0.0,
           globalPosition: _lastPosition.global,
           localPosition: _lastPosition.local,

packages/flutter/lib/src/gestures/hit_test.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/foundation.dart';
 import 'package:vector_math/vector_math_64.dart';
@@ -13,8 +12,7 @@ import 'events.dart';
 abstract class HitTestable {
   // This class is intended to be used as an interface, and should not be
   // extended directly; this constructor prevents instantiation and extension.
-  // ignore: unused_element
-  factory HitTestable._() => null;
+  HitTestable._();
 
   /// Check whether the given position hits this object.
   ///
@@ -27,8 +25,7 @@ abstract class HitTestable {
 abstract class HitTestDispatcher {
   // This class is intended to be used as an interface, and should not be
   // extended directly; this constructor prevents instantiation and extension.
-  // ignore: unused_element
-  factory HitTestDispatcher._() => null;
+  HitTestDispatcher._();
 
   /// Override this method to dispatch events.
   void dispatchEvent(PointerEvent event, HitTestResult result);
@@ -38,8 +35,7 @@ abstract class HitTestDispatcher {
 abstract class HitTestTarget {
   // This class is intended to be used as an interface, and should not be
   // extended directly; this constructor prevents instantiation and extension.
-  // ignore: unused_element
-  factory HitTestTarget._() => null;
+  HitTestTarget._();
 
   /// Override this method to receive events.
   void handleEvent(PointerEvent event, HitTestEntry entry);
@@ -67,8 +63,8 @@ class HitTestEntry {
   ///
   ///  * [BoxHitTestResult.addWithPaintTransform], which is used during hit testing
   ///    to build up this transform.
-  Matrix4 get transform => _transform;
-  Matrix4 _transform;
+  Matrix4? get transform => _transform;
+  Matrix4? _transform;
 }
 
 // A type of data that can be applied to a matrix by left-multiplication.

packages/flutter/lib/src/gestures/long_press.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'arena.dart';
 import 'constants.dart';
@@ -56,7 +55,7 @@ class LongPressStartDetails {
   /// The [globalPosition] argument must not be null.
   const LongPressStartDetails({
     this.globalPosition = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
   }) : assert(globalPosition != null),
        localPosition = localPosition ?? globalPosition;
 
@@ -80,9 +79,9 @@ class LongPressMoveUpdateDetails {
   /// The [globalPosition] and [offsetFromOrigin] arguments must not be null.
   const LongPressMoveUpdateDetails({
     this.globalPosition = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     this.offsetFromOrigin = Offset.zero,
-    Offset localOffsetFromOrigin,
+    Offset? localOffsetFromOrigin,
   }) : assert(globalPosition != null),
        assert(offsetFromOrigin != null),
        localPosition = localPosition ?? globalPosition,
@@ -118,7 +117,7 @@ class LongPressEndDetails {
   /// The [globalPosition] argument must not be null.
   const LongPressEndDetails({
     this.globalPosition = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     this.velocity = Velocity.zero,
   }) : assert(globalPosition != null),
        localPosition = localPosition ?? globalPosition;
@@ -162,10 +161,10 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   /// The [duration] argument can be used to overwrite the default duration
   /// after which the long press will be recognized.
   LongPressGestureRecognizer({
-    Duration duration,
-    double postAcceptSlopTolerance,
-    PointerDeviceKind kind,
-    Object debugOwner,
+    Duration? duration,
+    double? postAcceptSlopTolerance,
+    PointerDeviceKind? kind,
+    Object? debugOwner,
   }) : super(
           deadline: duration ?? kLongPressTimeout,
           postAcceptSlopTolerance: postAcceptSlopTolerance,
@@ -174,10 +173,10 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
         );
 
   bool _longPressAccepted = false;
-  OffsetPair _longPressOrigin;
+  OffsetPair? _longPressOrigin;
   // The buttons sent by `PointerDownEvent`. If a `PointerMoveEvent` comes with a
   // different set of buttons, the gesture is canceled.
-  int _initialButtons;
+  int? _initialButtons;
 
   /// Called when a long press gesture by a primary button has been recognized.
   ///
@@ -186,7 +185,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [onLongPressStart], which has the same timing but has data for the
   ///    press location.
-  GestureLongPressCallback onLongPress;
+  GestureLongPressCallback? onLongPress;
 
   /// Called when a long press gesture by a primary button has been recognized.
   ///
@@ -195,7 +194,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [onLongPress], which has the same timing but without details.
   ///  * [LongPressStartDetails], which is passed as an argument to this callback.
-  GestureLongPressStartCallback onLongPressStart;
+  GestureLongPressStartCallback? onLongPressStart;
 
   /// Called when moving after the long press by a primary button is recognized.
   ///
@@ -204,7 +203,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [LongPressMoveUpdateDetails], which is passed as an argument to this
   ///    callback.
-  GestureLongPressMoveUpdateCallback onLongPressMoveUpdate;
+  GestureLongPressMoveUpdateCallback? onLongPressMoveUpdate;
 
   /// Called when the pointer stops contacting the screen after a long-press
   /// by a primary button.
@@ -214,7 +213,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [onLongPressEnd], which has the same timing but has data for the up
   ///    gesture location.
-  GestureLongPressUpCallback onLongPressUp;
+  GestureLongPressUpCallback? onLongPressUp;
 
   /// Called when the pointer stops contacting the screen after a long-press
   /// by a primary button.
@@ -225,7 +224,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [onLongPressUp], which has the same timing, but without details.
   ///  * [LongPressEndDetails], which is passed as an argument to this
   ///    callback.
-  GestureLongPressEndCallback onLongPressEnd;
+  GestureLongPressEndCallback? onLongPressEnd;
 
   /// Called when a long press gesture by a secondary button has been
   /// recognized.
@@ -235,7 +234,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [kSecondaryButton], the button this callback responds to.
   ///  * [onSecondaryLongPressStart], which has the same timing but has data for
   ///    the press location.
-  GestureLongPressCallback onSecondaryLongPress;
+  GestureLongPressCallback? onSecondaryLongPress;
 
   /// Called when a long press gesture by a secondary button has been recognized.
   ///
@@ -245,7 +244,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [onSecondaryLongPress], which has the same timing but without details.
   ///  * [LongPressStartDetails], which is passed as an argument to this
   ///    callback.
-  GestureLongPressStartCallback onSecondaryLongPressStart;
+  GestureLongPressStartCallback? onSecondaryLongPressStart;
 
   /// Called when moving after the long press by a secondary button is
   /// recognized.
@@ -255,7 +254,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [kSecondaryButton], the button this callback responds to.
   ///  * [LongPressMoveUpdateDetails], which is passed as an argument to this
   ///    callback.
-  GestureLongPressMoveUpdateCallback onSecondaryLongPressMoveUpdate;
+  GestureLongPressMoveUpdateCallback? onSecondaryLongPressMoveUpdate;
 
   /// Called when the pointer stops contacting the screen after a long-press by
   /// a secondary button.
@@ -265,7 +264,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [kSecondaryButton], the button this callback responds to.
   ///  * [onSecondaryLongPressEnd], which has the same timing but has data for
   ///    the up gesture location.
-  GestureLongPressUpCallback onSecondaryLongPressUp;
+  GestureLongPressUpCallback? onSecondaryLongPressUp;
 
   /// Called when the pointer stops contacting the screen after a long-press by
   /// a secondary button.
@@ -276,9 +275,9 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   ///  * [onSecondaryLongPressUp], which has the same timing, but without
   ///    details.
   ///  * [LongPressEndDetails], which is passed as an argument to this callback.
-  GestureLongPressEndCallback onSecondaryLongPressEnd;
+  GestureLongPressEndCallback? onSecondaryLongPressEnd;
 
-  VelocityTracker _velocityTracker;
+  VelocityTracker? _velocityTracker;
 
   @override
   bool isPointerAllowed(PointerDownEvent event) {
@@ -310,7 +309,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
     // Exceeding the deadline puts the gesture in the accepted state.
     resolve(GestureDisposition.accepted);
     _longPressAccepted = true;
-    super.acceptGesture(primaryPointer);
+    super.acceptGesture(primaryPointer!);
     _checkLongPressStart();
   }
 
@@ -319,11 +318,11 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
     if (!event.synthesized) {
       if (event is PointerDownEvent) {
         _velocityTracker = VelocityTracker();
-        _velocityTracker.addPosition(event.timeStamp, event.localPosition);
+        _velocityTracker!.addPosition(event.timeStamp, event.localPosition);
       }
       if (event is PointerMoveEvent) {
         assert(_velocityTracker != null);
-        _velocityTracker.addPosition(event.timeStamp, event.localPosition);
+        _velocityTracker!.addPosition(event.timeStamp, event.localPosition);
       }
     }
 
@@ -344,7 +343,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
     } else if (event is PointerMoveEvent) {
       if (event.buttons != _initialButtons) {
         resolve(GestureDisposition.rejected);
-        stopTrackingPointer(primaryPointer);
+        stopTrackingPointer(primaryPointer!);
       } else if (_longPressAccepted) {
         _checkLongPressMoveUpdate(event);
       }
@@ -356,26 +355,26 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
       case kPrimaryButton:
         if (onLongPressStart != null) {
           final LongPressStartDetails details = LongPressStartDetails(
-            globalPosition: _longPressOrigin.global,
-            localPosition: _longPressOrigin.local,
+            globalPosition: _longPressOrigin!.global,
+            localPosition: _longPressOrigin!.local,
           );
-          invokeCallback<void>('onLongPressStart', () => onLongPressStart(details));
+          invokeCallback<void>('onLongPressStart', () => onLongPressStart!(details));
         }
         if (onLongPress != null) {
-          invokeCallback<void>('onLongPress', onLongPress);
+          invokeCallback<void>('onLongPress', onLongPress!);
         }
         break;
       case kSecondaryButton:
         if (onSecondaryLongPressStart != null) {
           final LongPressStartDetails details = LongPressStartDetails(
-            globalPosition: _longPressOrigin.global,
-            localPosition: _longPressOrigin.local,
+            globalPosition: _longPressOrigin!.global,
+            localPosition: _longPressOrigin!.local,
           );
           invokeCallback<void>(
-              'onSecondaryLongPressStart', () => onSecondaryLongPressStart(details));
+              'onSecondaryLongPressStart', () => onSecondaryLongPressStart!(details));
         }
         if (onSecondaryLongPress != null) {
-          invokeCallback<void>('onSecondaryLongPress', onSecondaryLongPress);
+          invokeCallback<void>('onSecondaryLongPress', onSecondaryLongPress!);
         }
         break;
       default:
@@ -387,20 +386,20 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
     final LongPressMoveUpdateDetails details = LongPressMoveUpdateDetails(
       globalPosition: event.position,
       localPosition: event.localPosition,
-      offsetFromOrigin: event.position - _longPressOrigin.global,
-      localOffsetFromOrigin: event.localPosition - _longPressOrigin.local,
+      offsetFromOrigin: event.position - _longPressOrigin!.global,
+      localOffsetFromOrigin: event.localPosition - _longPressOrigin!.local,
     );
     switch (_initialButtons) {
       case kPrimaryButton:
         if (onLongPressMoveUpdate != null) {
           invokeCallback<void>('onLongPressMoveUpdate',
-            () => onLongPressMoveUpdate(details));
+            () => onLongPressMoveUpdate!(details));
         }
         break;
       case kSecondaryButton:
         if (onSecondaryLongPressMoveUpdate != null) {
           invokeCallback<void>('onSecondaryLongPressMoveUpdate',
-            () => onSecondaryLongPressMoveUpdate(details));
+            () => onSecondaryLongPressMoveUpdate!(details));
         }
         break;
       default:
@@ -409,7 +408,7 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   }
 
   void _checkLongPressEnd(PointerEvent event) {
-    final VelocityEstimate estimate = _velocityTracker.getVelocityEstimate();
+    final VelocityEstimate? estimate = _velocityTracker!.getVelocityEstimate();
     final Velocity velocity = estimate == null
         ? Velocity.zero
         : Velocity(pixelsPerSecond: estimate.pixelsPerSecond);
@@ -423,18 +422,18 @@ class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
     switch (_initialButtons) {
       case kPrimaryButton:
         if (onLongPressEnd != null) {
-          invokeCallback<void>('onLongPressEnd', () => onLongPressEnd(details));
+          invokeCallback<void>('onLongPressEnd', () => onLongPressEnd!(details));
         }
         if (onLongPressUp != null) {
-          invokeCallback<void>('onLongPressUp', onLongPressUp);
+          invokeCallback<void>('onLongPressUp', onLongPressUp!);
         }
         break;
       case kSecondaryButton:
         if (onSecondaryLongPressEnd != null) {
-          invokeCallback<void>('onSecondaryLongPressEnd', () => onSecondaryLongPressEnd(details));
+          invokeCallback<void>('onSecondaryLongPressEnd', () => onSecondaryLongPressEnd!(details));
         }
         if (onSecondaryLongPressUp != null) {
-          invokeCallback<void>('onSecondaryLongPressUp', onSecondaryLongPressUp);
+          invokeCallback<void>('onSecondaryLongPressUp', onSecondaryLongPressUp!);
         }
         break;
       default:

packages/flutter/lib/src/gestures/lsq_solver.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:math' as math;
 import 'dart:typed_data';
@@ -76,7 +75,7 @@ class PolynomialFit {
   /// An indicator of the quality of the fit.
   ///
   /// Larger values indicate greater quality.
-  double confidence;
+  late double confidence;
 }
 
 /// Uses the least-squares algorithm to fit a polynomial to a set of data.
@@ -98,7 +97,9 @@ class LeastSquaresSolver {
   final List<double> w;
 
   /// Fits a polynomial of the given degree to the data points.
-  PolynomialFit solve(int degree) {
+  ///
+  /// When there is not enough data to fit a curve null is returned.
+  PolynomialFit? solve(int degree) {
     if (degree > x.length) // Not enough data to fit a curve.
       return null;
 

packages/flutter/lib/src/gestures/monodrag.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/foundation.dart';
 import 'package:vector_math/vector_math_64.dart';
@@ -62,8 +61,8 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   DragGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
     this.dragStartBehavior = DragStartBehavior.start,
   }) : assert(dragStartBehavior != null),
        super(debugOwner: debugOwner, kind: kind);
@@ -101,7 +100,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [DragDownDetails], which is passed as an argument to this callback.
-  GestureDragDownCallback onDown;
+  GestureDragDownCallback? onDown;
 
   /// A pointer has contacted the screen with a primary button and has begun to
   /// move.
@@ -118,7 +117,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [DragStartDetails], which is passed as an argument to this callback.
-  GestureDragStartCallback onStart;
+  GestureDragStartCallback? onStart;
 
   /// A pointer that is in contact with the screen with a primary button and
   /// moving has moved again.
@@ -130,7 +129,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [DragUpdateDetails], which is passed as an argument to this callback.
-  GestureDragUpdateCallback onUpdate;
+  GestureDragUpdateCallback? onUpdate;
 
   /// A pointer that was previously in contact with the screen with a primary
   /// button and moving is no longer in contact with the screen and was moving
@@ -143,48 +142,48 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [DragEndDetails], which is passed as an argument to this callback.
-  GestureDragEndCallback onEnd;
+  GestureDragEndCallback? onEnd;
 
   /// The pointer that previously triggered [onDown] did not complete.
   ///
   /// See also:
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
-  GestureDragCancelCallback onCancel;
+  GestureDragCancelCallback? onCancel;
 
   /// The minimum distance an input pointer drag must have moved to
   /// to be considered a fling gesture.
   ///
   /// This value is typically compared with the distance traveled along the
   /// scrolling axis. If null then [kTouchSlop] is used.
-  double minFlingDistance;
+  double? minFlingDistance;
 
   /// The minimum velocity for an input pointer drag to be considered fling.
   ///
   /// This value is typically compared with the magnitude of fling gesture's
   /// velocity along the scrolling axis. If null then [kMinFlingVelocity]
   /// is used.
-  double minFlingVelocity;
+  double? minFlingVelocity;
 
   /// Fling velocity magnitudes will be clamped to this value.
   ///
   /// If null then [kMaxFlingVelocity] is used.
-  double maxFlingVelocity;
+  double? maxFlingVelocity;
 
   _DragState _state = _DragState.ready;
-  OffsetPair _initialPosition;
-  OffsetPair _pendingDragOffset;
-  Duration _lastPendingEventTimestamp;
+  late OffsetPair _initialPosition;
+  late OffsetPair _pendingDragOffset;
+  Duration? _lastPendingEventTimestamp;
   // The buttons sent by `PointerDownEvent`. If a `PointerMoveEvent` comes with a
   // different set of buttons, the gesture is canceled.
-  int _initialButtons;
-  Matrix4 _lastTransform;
+  int? _initialButtons;
+  Matrix4? _lastTransform;
 
   /// Distance moved in the global coordinate space of the screen in drag direction.
   ///
   /// If drag is only allowed along a defined axis, this value may be negative to
   /// differentiate the direction of the drag.
-  double _globalDistanceMoved;
+  late double _globalDistanceMoved;
 
   /// Determines if a gesture is a fling or not based on velocity.
   ///
@@ -194,7 +193,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   bool isFlingGesture(VelocityEstimate estimate);
 
   Offset _getDeltaForDetails(Offset delta);
-  double _getPrimaryValueFromOffset(Offset value);
+  double? _getPrimaryValueFromOffset(Offset value);
   bool get _hasSufficientGlobalDistanceToAccept;
 
   final Map<int, VelocityTracker> _velocityTrackers = <int, VelocityTracker>{};
@@ -246,7 +245,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
     assert(_state != _DragState.ready);
     if (!event.synthesized
         && (event is PointerDownEvent || event is PointerMoveEvent)) {
-      final VelocityTracker tracker = _velocityTrackers[event.pointer];
+      final VelocityTracker tracker = _velocityTrackers[event.pointer]!;
       assert(tracker != null);
       tracker.addPosition(event.timeStamp, event.localPosition);
     }
@@ -269,7 +268,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
         _lastPendingEventTimestamp = event.timeStamp;
         _lastTransform = event.transform;
         final Offset movedLocally = _getDeltaForDetails(event.localDelta);
-        final Matrix4 localToGlobalTransform = event.transform == null ? null : Matrix4.tryInvert(event.transform);
+        final Matrix4? localToGlobalTransform = event.transform == null ? null : Matrix4.tryInvert(event.transform!);
         _globalDistanceMoved += PointerEvent.transformDeltaViaPositions(
           transform: localToGlobalTransform,
           untransformedDelta: movedLocally,
@@ -292,8 +291,8 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
     if (_state != _DragState.accepted) {
       _state = _DragState.accepted;
       final OffsetPair delta = _pendingDragOffset;
-      final Duration timestamp = _lastPendingEventTimestamp;
-      final Matrix4 transform = _lastTransform;
+      final Duration timestamp = _lastPendingEventTimestamp!;
+      final Matrix4? transform = _lastTransform;
       Offset localUpdateDelta;
       switch (dragStartBehavior) {
         case DragStartBehavior.start:
@@ -309,7 +308,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       _lastTransform = null;
       _checkStart(timestamp);
       if (localUpdateDelta != Offset.zero && onUpdate != null) {
-        final Matrix4 localToGlobal = transform != null ? Matrix4.tryInvert(transform) : null;
+        final Matrix4? localToGlobal = transform != null ? Matrix4.tryInvert(transform) : null;
         final Offset correctedLocalPosition = _initialPosition.local + localUpdateDelta;
         final Offset globalUpdateDelta = PointerEvent.transformDeltaViaPositions(
           untransformedEndPosition: correctedLocalPosition,
@@ -372,7 +371,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       localPosition: _initialPosition.local,
     );
     if (onDown != null)
-      invokeCallback<void>('onDown', () => onDown(details));
+      invokeCallback<void>('onDown', () => onDown!(details));
   }
 
   void _checkStart(Duration timestamp) {
@@ -383,15 +382,15 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       localPosition: _initialPosition.local,
     );
     if (onStart != null)
-      invokeCallback<void>('onStart', () => onStart(details));
+      invokeCallback<void>('onStart', () => onStart!(details));
   }
 
   void _checkUpdate({
-    Duration sourceTimeStamp,
-    Offset delta,
-    double primaryDelta,
-    Offset globalPosition,
-    Offset localPosition,
+    Duration? sourceTimeStamp,
+    required Offset delta,
+    double? primaryDelta,
+    required Offset globalPosition,
+    Offset? localPosition,
   }) {
     assert(_initialButtons == kPrimaryButton);
     final DragUpdateDetails details = DragUpdateDetails(
@@ -402,7 +401,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       localPosition: localPosition,
     );
     if (onUpdate != null)
-      invokeCallback<void>('onUpdate', () => onUpdate(details));
+      invokeCallback<void>('onUpdate', () => onUpdate!(details));
   }
 
   void _checkEnd(int pointer) {
@@ -410,13 +409,13 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
     if (onEnd == null)
       return;
 
-    final VelocityTracker tracker = _velocityTrackers[pointer];
+    final VelocityTracker tracker = _velocityTrackers[pointer]!;
     assert(tracker != null);
 
     DragEndDetails details;
     String Function() debugReport;
 
-    final VelocityEstimate estimate = tracker.getVelocityEstimate();
+    final VelocityEstimate? estimate = tracker.getVelocityEstimate();
     if (estimate != null && isFlingGesture(estimate)) {
       final Velocity velocity = Velocity(pixelsPerSecond: estimate.pixelsPerSecond)
         .clampMagnitude(minFlingVelocity ?? kMinFlingVelocity, maxFlingVelocity ?? kMaxFlingVelocity);
@@ -438,13 +437,13 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
         return '$estimate; judged to not be a fling.';
       };
     }
-    invokeCallback<void>('onEnd', () => onEnd(details), debugReport: debugReport);
+    invokeCallback<void>('onEnd', () => onEnd!(details), debugReport: debugReport);
   }
 
   void _checkCancel() {
     assert(_initialButtons == kPrimaryButton);
     if (onCancel != null)
-      invokeCallback<void>('onCancel', onCancel);
+      invokeCallback<void>('onCancel', onCancel!);
   }
 
   @override
@@ -474,8 +473,8 @@ class VerticalDragGestureRecognizer extends DragGestureRecognizer {
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   VerticalDragGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   @override
@@ -513,8 +512,8 @@ class HorizontalDragGestureRecognizer extends DragGestureRecognizer {
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   HorizontalDragGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   @override
@@ -548,7 +547,7 @@ class HorizontalDragGestureRecognizer extends DragGestureRecognizer {
 ///    some time has passed.
 class PanGestureRecognizer extends DragGestureRecognizer {
   /// Create a gesture recognizer for tracking movement on a plane.
-  PanGestureRecognizer({ Object debugOwner }) : super(debugOwner: debugOwner);
+  PanGestureRecognizer({ Object? debugOwner }) : super(debugOwner: debugOwner);
 
   @override
   bool isFlingGesture(VelocityEstimate estimate) {
@@ -567,7 +566,7 @@ class PanGestureRecognizer extends DragGestureRecognizer {
   Offset _getDeltaForDetails(Offset delta) => delta;
 
   @override
-  double _getPrimaryValueFromOffset(Offset value) => null;
+  double? _getPrimaryValueFromOffset(Offset value) => null;
 
   @override
   String get debugDescription => 'pan';

packages/flutter/lib/src/gestures/multidrag.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:async';
 import 'dart:ui' show Offset;
@@ -19,7 +18,7 @@ import 'recognizer.dart';
 import 'velocity_tracker.dart';
 
 /// Signature for when [MultiDragGestureRecognizer] recognizes the start of a drag gesture.
-typedef GestureMultiDragStartCallback = Drag Function(Offset position);
+typedef GestureMultiDragStartCallback = Drag? Function(Offset position);
 
 /// Per-pointer state for a [MultiDragGestureRecognizer].
 ///
@@ -36,7 +35,7 @@ abstract class MultiDragPointerState {
   final Offset initialPosition;
 
   final VelocityTracker _velocityTracker = VelocityTracker();
-  Drag _client;
+  Drag? _client;
 
   /// The offset of the pointer from the last position that was reported to the client.
   ///
@@ -44,12 +43,12 @@ abstract class MultiDragPointerState {
   /// distance before this movement will be recognized as a drag. This field
   /// accumulates that movement so that we can report it to the client after
   /// the drag starts.
-  Offset get pendingDelta => _pendingDelta;
-  Offset _pendingDelta = Offset.zero;
+  Offset? get pendingDelta => _pendingDelta;
+  Offset? _pendingDelta = Offset.zero;
 
-  Duration _lastPendingEventTimestamp;
+  Duration? _lastPendingEventTimestamp;
 
-  GestureArenaEntry _arenaEntry;
+  GestureArenaEntry? _arenaEntry;
   void _setArenaEntry(GestureArenaEntry entry) {
     assert(_arenaEntry == null);
     assert(pendingDelta != null);
@@ -61,7 +60,7 @@ abstract class MultiDragPointerState {
   @protected
   @mustCallSuper
   void resolve(GestureDisposition disposition) {
-    _arenaEntry.resolve(disposition);
+    _arenaEntry!.resolve(disposition);
   }
 
   void _move(PointerMoveEvent event) {
@@ -71,14 +70,14 @@ abstract class MultiDragPointerState {
     if (_client != null) {
       assert(pendingDelta == null);
       // Call client last to avoid reentrancy.
-      _client.update(DragUpdateDetails(
+      _client!.update(DragUpdateDetails(
         sourceTimeStamp: event.timeStamp,
         delta: event.delta,
         globalPosition: event.position,
       ));
     } else {
       assert(pendingDelta != null);
-      _pendingDelta += event.delta;
+      _pendingDelta = _pendingDelta! + event.delta;
       _lastPendingEventTimestamp = event.timeStamp;
       checkForResolutionAfterMove();
     }
@@ -119,13 +118,13 @@ abstract class MultiDragPointerState {
     _client = client;
     final DragUpdateDetails details = DragUpdateDetails(
       sourceTimeStamp: _lastPendingEventTimestamp,
-      delta: pendingDelta,
+      delta: pendingDelta!,
       globalPosition: initialPosition,
     );
     _pendingDelta = null;
     _lastPendingEventTimestamp = null;
     // Call client last to avoid reentrancy.
-    _client.update(details);
+    _client!.update(details);
   }
 
   void _up() {
@@ -133,7 +132,7 @@ abstract class MultiDragPointerState {
     if (_client != null) {
       assert(pendingDelta == null);
       final DragEndDetails details = DragEndDetails(velocity: _velocityTracker.getVelocity());
-      final Drag client = _client;
+      final Drag client = _client!;
       _client = null;
       // Call client last to avoid reentrancy.
       client.end(details);
@@ -148,7 +147,7 @@ abstract class MultiDragPointerState {
     assert(_arenaEntry != null);
     if (_client != null) {
       assert(pendingDelta == null);
-      final Drag client = _client;
+      final Drag client = _client!;
       _client = null;
       // Call client last to avoid reentrancy.
       client.cancel();
@@ -195,28 +194,28 @@ abstract class MultiDragPointerState {
 abstract class MultiDragGestureRecognizer<T extends MultiDragPointerState> extends GestureRecognizer {
   /// Initialize the object.
   MultiDragGestureRecognizer({
-    @required Object debugOwner,
-    PointerDeviceKind kind,
+    required Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   /// Called when this class recognizes the start of a drag gesture.
   ///
   /// The remaining notifications for this drag gesture are delivered to the
   /// [Drag] object returned by this callback.
-  GestureMultiDragStartCallback onStart;
+  GestureMultiDragStartCallback? onStart;
 
-  Map<int, T> _pointers = <int, T>{};
+  Map<int, T>? _pointers = <int, T>{};
 
   @override
   void addAllowedPointer(PointerDownEvent event) {
     assert(_pointers != null);
     assert(event.pointer != null);
     assert(event.position != null);
-    assert(!_pointers.containsKey(event.pointer));
+    assert(!_pointers!.containsKey(event.pointer));
     final T state = createNewPointerState(event);
-    _pointers[event.pointer] = state;
-    GestureBinding.instance.pointerRouter.addRoute(event.pointer, _handleEvent);
-    state._setArenaEntry(GestureBinding.instance.gestureArena.add(event.pointer, this));
+    _pointers![event.pointer] = state;
+    GestureBinding.instance!.pointerRouter.addRoute(event.pointer, _handleEvent);
+    state._setArenaEntry(GestureBinding.instance!.gestureArena.add(event.pointer, this));
   }
 
   /// Subclasses should override this method to create per-pointer state
@@ -230,8 +229,8 @@ abstract class MultiDragGestureRecognizer<T extends MultiDragPointerState> exten
     assert(event.pointer != null);
     assert(event.timeStamp != null);
     assert(event.position != null);
-    assert(_pointers.containsKey(event.pointer));
-    final T state = _pointers[event.pointer];
+    assert(_pointers!.containsKey(event.pointer));
+    final T state = _pointers![event.pointer]!;
     if (event is PointerMoveEvent) {
       state._move(event);
       // We might be disposed here.
@@ -256,20 +255,20 @@ abstract class MultiDragGestureRecognizer<T extends MultiDragPointerState> exten
   @override
   void acceptGesture(int pointer) {
     assert(_pointers != null);
-    final T state = _pointers[pointer];
+    final T? state = _pointers![pointer];
     if (state == null)
       return; // We might already have canceled this drag if the up comes before the accept.
     state.accepted((Offset initialPosition) => _startDrag(initialPosition, pointer));
   }
 
-  Drag _startDrag(Offset initialPosition, int pointer) {
+  Drag? _startDrag(Offset initialPosition, int pointer) {
     assert(_pointers != null);
-    final T state = _pointers[pointer];
+    final T state = _pointers![pointer]!;
     assert(state != null);
     assert(state._pendingDelta != null);
-    Drag drag;
+    Drag? drag;
     if (onStart != null)
-      drag = invokeCallback<Drag>('onStart', () => onStart(initialPosition));
+      drag = invokeCallback<Drag?>('onStart', () => onStart!(initialPosition));
     if (drag != null) {
       state._startDrag(drag);
     } else {
@@ -281,8 +280,8 @@ abstract class MultiDragGestureRecognizer<T extends MultiDragPointerState> exten
   @override
   void rejectGesture(int pointer) {
     assert(_pointers != null);
-    if (_pointers.containsKey(pointer)) {
-      final T state = _pointers[pointer];
+    if (_pointers!.containsKey(pointer)) {
+      final T state = _pointers![pointer]!;
       assert(state != null);
       state.rejected();
       _removeState(pointer);
@@ -295,15 +294,15 @@ abstract class MultiDragGestureRecognizer<T extends MultiDragPointerState> exten
       // for the given pointer because dispose() has already removed it.
       return;
     }
-    assert(_pointers.containsKey(pointer));
-    GestureBinding.instance.pointerRouter.removeRoute(pointer, _handleEvent);
-    _pointers.remove(pointer).dispose();
+    assert(_pointers!.containsKey(pointer));
+    GestureBinding.instance!.pointerRouter.removeRoute(pointer, _handleEvent);
+    _pointers!.remove(pointer)!.dispose();
   }
 
   @override
   void dispose() {
-    _pointers.keys.toList().forEach(_removeState);
-    assert(_pointers.isEmpty);
+    _pointers!.keys.toList().forEach(_removeState);
+    assert(_pointers!.isEmpty);
     _pointers = null;
     super.dispose();
   }
@@ -315,7 +314,7 @@ class _ImmediatePointerState extends MultiDragPointerState {
   @override
   void checkForResolutionAfterMove() {
     assert(pendingDelta != null);
-    if (pendingDelta.distance > kTouchSlop)
+    if (pendingDelta!.distance > kTouchSlop)
       resolve(GestureDisposition.accepted);
   }
 
@@ -344,8 +343,8 @@ class _ImmediatePointerState extends MultiDragPointerState {
 class ImmediateMultiDragGestureRecognizer extends MultiDragGestureRecognizer<_ImmediatePointerState> {
   /// Create a gesture recognizer for tracking multiple pointers at once.
   ImmediateMultiDragGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   @override
@@ -364,7 +363,7 @@ class _HorizontalPointerState extends MultiDragPointerState {
   @override
   void checkForResolutionAfterMove() {
     assert(pendingDelta != null);
-    if (pendingDelta.dx.abs() > kTouchSlop)
+    if (pendingDelta!.dx.abs() > kTouchSlop)
       resolve(GestureDisposition.accepted);
   }
 
@@ -393,8 +392,8 @@ class HorizontalMultiDragGestureRecognizer extends MultiDragGestureRecognizer<_H
   /// Create a gesture recognizer for tracking multiple pointers at once
   /// but only if they first move horizontally.
   HorizontalMultiDragGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   @override
@@ -413,7 +412,7 @@ class _VerticalPointerState extends MultiDragPointerState {
   @override
   void checkForResolutionAfterMove() {
     assert(pendingDelta != null);
-    if (pendingDelta.dy.abs() > kTouchSlop)
+    if (pendingDelta!.dy.abs() > kTouchSlop)
       resolve(GestureDisposition.accepted);
   }
 
@@ -442,8 +441,8 @@ class VerticalMultiDragGestureRecognizer extends MultiDragGestureRecognizer<_Ver
   /// Create a gesture recognizer for tracking multiple pointers at once
   /// but only if they first move vertically.
   VerticalMultiDragGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   @override
@@ -462,16 +461,16 @@ class _DelayedPointerState extends MultiDragPointerState {
     _timer = Timer(delay, _delayPassed);
   }
 
-  Timer _timer;
-  GestureMultiDragStartCallback _starter;
+  Timer? _timer;
+  GestureMultiDragStartCallback? _starter;
 
   void _delayPassed() {
     assert(_timer != null);
     assert(pendingDelta != null);
-    assert(pendingDelta.distance <= kTouchSlop);
+    assert(pendingDelta!.distance <= kTouchSlop);
     _timer = null;
     if (_starter != null) {
-      _starter(initialPosition);
+      _starter!(initialPosition);
       _starter = null;
     } else {
       resolve(GestureDisposition.accepted);
@@ -505,7 +504,7 @@ class _DelayedPointerState extends MultiDragPointerState {
       return;
     }
     assert(pendingDelta != null);
-    if (pendingDelta.distance > kTouchSlop) {
+    if (pendingDelta!.distance > kTouchSlop) {
       resolve(GestureDisposition.rejected);
       _ensureTimerStopped();
     }
@@ -545,8 +544,8 @@ class DelayedMultiDragGestureRecognizer extends MultiDragGestureRecognizer<_Dela
   /// can be changed for specific behaviors.
   DelayedMultiDragGestureRecognizer({
     this.delay = kLongPressTimeout,
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : assert(delay != null),
        super(debugOwner: debugOwner, kind: kind);
 

packages/flutter/lib/src/gestures/multitap.dart

@@ -2,11 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:async';
 import 'dart:ui' show Offset;
-import 'package:flutter/foundation.dart' show required;
 import 'package:vector_math/vector_math_64.dart';
 
 import 'arena.dart';
@@ -43,7 +41,7 @@ typedef GestureMultiTapCancelCallback = void Function(int pointer);
 /// CountdownZoned tracks whether the specified duration has elapsed since
 /// creation, honoring [Zone].
 class _CountdownZoned {
-  _CountdownZoned({ @required Duration duration })
+  _CountdownZoned({ required Duration duration })
        : assert(duration != null) {
     Timer(duration, _onTimeout);
   }
@@ -61,9 +59,9 @@ class _CountdownZoned {
 /// larger gesture.
 class _TapTracker {
   _TapTracker({
-    @required PointerDownEvent event,
+    required PointerDownEvent event,
     this.entry,
-    @required Duration doubleTapMinTime,
+    required Duration doubleTapMinTime,
   }) : assert(doubleTapMinTime != null),
        assert(event != null),
        assert(event.buttons != null),
@@ -73,24 +71,24 @@ class _TapTracker {
        _doubleTapMinTimeCountdown = _CountdownZoned(duration: doubleTapMinTime);
 
   final int pointer;
-  final GestureArenaEntry entry;
+  final GestureArenaEntry? entry;
   final Offset _initialGlobalPosition;
   final int initialButtons;
   final _CountdownZoned _doubleTapMinTimeCountdown;
 
   bool _isTrackingPointer = false;
 
-  void startTrackingPointer(PointerRoute route, Matrix4 transform) {
+  void startTrackingPointer(PointerRoute route, Matrix4? transform) {
     if (!_isTrackingPointer) {
       _isTrackingPointer = true;
-      GestureBinding.instance.pointerRouter.addRoute(pointer, route, transform);
+      GestureBinding.instance!.pointerRouter.addRoute(pointer, route, transform);
     }
   }
 
   void stopTrackingPointer(PointerRoute route) {
     if (_isTrackingPointer) {
       _isTrackingPointer = false;
-      GestureBinding.instance.pointerRouter.removeRoute(pointer, route);
+      GestureBinding.instance!.pointerRouter.removeRoute(pointer, route);
     }
   }
 
@@ -119,8 +117,8 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   DoubleTapGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   // Implementation notes:
@@ -152,10 +150,10 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
   /// See also:
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
-  GestureDoubleTapCallback onDoubleTap;
+  GestureDoubleTapCallback? onDoubleTap;
 
-  Timer _doubleTapTimer;
-  _TapTracker _firstTap;
+  Timer? _doubleTapTimer;
+  _TapTracker? _firstTap;
   final Map<int, _TapTracker> _trackers = <int, _TapTracker>{};
 
   @override
@@ -176,10 +174,10 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
   @override
   void addAllowedPointer(PointerEvent event) {
     if (_firstTap != null) {
-      if (!_firstTap.isWithinGlobalTolerance(event, kDoubleTapSlop)) {
+      if (!_firstTap!.isWithinGlobalTolerance(event, kDoubleTapSlop)) {
         // Ignore out-of-bounds second taps.
         return;
-      } else if (!_firstTap.hasElapsedMinTime() || !_firstTap.hasSameButton(event as PointerDownEvent)) {
+      } else if (!_firstTap!.hasElapsedMinTime() || !_firstTap!.hasSameButton(event as PointerDownEvent)) {
         // Restart when the second tap is too close to the first, or when buttons
         // mismatch.
         _reset();
@@ -193,7 +191,7 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
     _stopDoubleTapTimer();
     final _TapTracker tracker = _TapTracker(
       event: event as PointerDownEvent,
-      entry: GestureBinding.instance.gestureArena.add(event.pointer, this),
+      entry: GestureBinding.instance!.gestureArena.add(event.pointer, this),
       doubleTapMinTime: kDoubleTapMinTime,
     );
     _trackers[event.pointer] = tracker;
@@ -201,8 +199,7 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
   }
 
   void _handleEvent(PointerEvent event) {
-    final _TapTracker tracker = _trackers[event.pointer];
-    assert(tracker != null);
+    final _TapTracker tracker = _trackers[event.pointer]!;
     if (event is PointerUpEvent) {
       if (_firstTap == null)
         _registerFirstTap(tracker);
@@ -221,11 +218,11 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
 
   @override
   void rejectGesture(int pointer) {
-    _TapTracker tracker = _trackers[pointer];
+    _TapTracker? tracker = _trackers[pointer];
     // If tracker isn't in the list, check if this is the first tap tracker
     if (tracker == null &&
         _firstTap != null &&
-        _firstTap.pointer == pointer)
+        _firstTap!.pointer == pointer)
       tracker = _firstTap;
     // If tracker is still null, we rejected ourselves already
     if (tracker != null)
@@ -234,7 +231,7 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
 
   void _reject(_TapTracker tracker) {
     _trackers.remove(tracker.pointer);
-    tracker.entry.resolve(GestureDisposition.rejected);
+    tracker.entry!.resolve(GestureDisposition.rejected);
     _freezeTracker(tracker);
     // If the first tap is in progress, and we've run out of taps to track,
     // reset won't have any work to do. But if we're in the second tap, we need
@@ -255,17 +252,17 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
     if (_firstTap != null) {
       // Note, order is important below in order for the resolve -> reject logic
       // to work properly.
-      final _TapTracker tracker = _firstTap;
+      final _TapTracker tracker = _firstTap!;
       _firstTap = null;
       _reject(tracker);
-      GestureBinding.instance.gestureArena.release(tracker.pointer);
+      GestureBinding.instance!.gestureArena.release(tracker.pointer);
     }
     _clearTrackers();
   }
 
   void _registerFirstTap(_TapTracker tracker) {
     _startDoubleTapTimer();
-    GestureBinding.instance.gestureArena.hold(tracker.pointer);
+    GestureBinding.instance!.gestureArena.hold(tracker.pointer);
     // Note, order is important below in order for the clear -> reject logic to
     // work properly.
     _freezeTracker(tracker);
@@ -275,8 +272,8 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
   }
 
   void _registerSecondTap(_TapTracker tracker) {
-    _firstTap.entry.resolve(GestureDisposition.accepted);
-    tracker.entry.resolve(GestureDisposition.accepted);
+    _firstTap!.entry!.resolve(GestureDisposition.accepted);
+    tracker.entry!.resolve(GestureDisposition.accepted);
     _freezeTracker(tracker);
     _trackers.remove(tracker.pointer);
     _checkUp(tracker.initialButtons);
@@ -298,7 +295,7 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
 
   void _stopDoubleTapTimer() {
     if (_doubleTapTimer != null) {
-      _doubleTapTimer.cancel();
+      _doubleTapTimer!.cancel();
       _doubleTapTimer = null;
     }
   }
@@ -306,7 +303,7 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
   void _checkUp(int buttons) {
     assert(buttons == kPrimaryButton);
     if (onDoubleTap != null)
-      invokeCallback<void>('onDoubleTap', onDoubleTap);
+      invokeCallback<void>('onDoubleTap', onDoubleTap!);
   }
 
   @override
@@ -319,13 +316,13 @@ class DoubleTapGestureRecognizer extends GestureRecognizer {
 class _TapGesture extends _TapTracker {
 
   _TapGesture({
-    this.gestureRecognizer,
-    PointerEvent event,
-    Duration longTapDelay,
+    required this.gestureRecognizer,
+    required PointerEvent event,
+    required Duration longTapDelay,
   }) : _lastPosition = OffsetPair.fromEventPosition(event),
        super(
     event: event as PointerDownEvent,
-    entry: GestureBinding.instance.gestureArena.add(event.pointer, gestureRecognizer),
+    entry: GestureBinding.instance!.gestureArena.add(event.pointer, gestureRecognizer),
     doubleTapMinTime: kDoubleTapMinTime,
   ) {
     startTrackingPointer(handleEvent, event.transform);
@@ -340,10 +337,10 @@ class _TapGesture extends _TapTracker {
   final MultiTapGestureRecognizer gestureRecognizer;
 
   bool _wonArena = false;
-  Timer _timer;
+  Timer? _timer;
 
   OffsetPair _lastPosition;
-  OffsetPair _finalPosition;
+  OffsetPair? _finalPosition;
 
   void handleEvent(PointerEvent event) {
     assert(event.pointer == pointer);
@@ -384,12 +381,12 @@ class _TapGesture extends _TapTracker {
     if (_wonArena)
       reject();
     else
-      entry.resolve(GestureDisposition.rejected); // eventually calls reject()
+      entry!.resolve(GestureDisposition.rejected); // eventually calls reject()
   }
 
   void _check() {
     if (_wonArena && _finalPosition != null)
-      gestureRecognizer._dispatchTap(pointer, _finalPosition);
+      gestureRecognizer._dispatchTap(pointer, _finalPosition!);
   }
 }
 
@@ -409,31 +406,31 @@ class MultiTapGestureRecognizer extends GestureRecognizer {
   /// [onLongTapDown] is called immediately after [onTapDown].
   MultiTapGestureRecognizer({
     this.longTapDelay = Duration.zero,
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   /// A pointer that might cause a tap has contacted the screen at a particular
   /// location.
-  GestureMultiTapDownCallback onTapDown;
+  GestureMultiTapDownCallback? onTapDown;
 
   /// A pointer that will trigger a tap has stopped contacting the screen at a
   /// particular location.
-  GestureMultiTapUpCallback onTapUp;
+  GestureMultiTapUpCallback? onTapUp;
 
   /// A tap has occurred.
-  GestureMultiTapCallback onTap;
+  GestureMultiTapCallback? onTap;
 
   /// The pointer that previously triggered [onTapDown] will not end up causing
   /// a tap.
-  GestureMultiTapCancelCallback onTapCancel;
+  GestureMultiTapCancelCallback? onTapCancel;
 
   /// The amount of time between [onTapDown] and [onLongTapDown].
   Duration longTapDelay;
 
   /// A pointer that might cause a tap is still in contact with the screen at a
   /// particular location after [longTapDelay].
-  GestureMultiTapDownCallback onLongTapDown;
+  GestureMultiTapDownCallback? onLongTapDown;
 
   final Map<int, _TapGesture> _gestureMap = <int, _TapGesture>{};
 
@@ -447,7 +444,7 @@ class MultiTapGestureRecognizer extends GestureRecognizer {
     );
     if (onTapDown != null)
       invokeCallback<void>('onTapDown', () {
-        onTapDown(event.pointer, TapDownDetails(
+        onTapDown!(event.pointer, TapDownDetails(
           globalPosition: event.position,
           localPosition: event.localPosition,
           kind: event.kind,
@@ -458,13 +455,13 @@ class MultiTapGestureRecognizer extends GestureRecognizer {
   @override
   void acceptGesture(int pointer) {
     assert(_gestureMap.containsKey(pointer));
-    _gestureMap[pointer].accept();
+    _gestureMap[pointer]!.accept();
   }
 
   @override
   void rejectGesture(int pointer) {
     assert(_gestureMap.containsKey(pointer));
-    _gestureMap[pointer].reject();
+    _gestureMap[pointer]!.reject();
     assert(!_gestureMap.containsKey(pointer));
   }
 
@@ -472,7 +469,7 @@ class MultiTapGestureRecognizer extends GestureRecognizer {
     assert(_gestureMap.containsKey(pointer));
     _gestureMap.remove(pointer);
     if (onTapCancel != null)
-      invokeCallback<void>('onTapCancel', () => onTapCancel(pointer));
+      invokeCallback<void>('onTapCancel', () => onTapCancel!(pointer));
   }
 
   void _dispatchTap(int pointer, OffsetPair position) {
@@ -480,20 +477,20 @@ class MultiTapGestureRecognizer extends GestureRecognizer {
     _gestureMap.remove(pointer);
     if (onTapUp != null)
       invokeCallback<void>('onTapUp', () {
-        onTapUp(pointer, TapUpDetails(
+        onTapUp!(pointer, TapUpDetails(
           localPosition: position.local,
           globalPosition: position.global,
         ));
       });
     if (onTap != null)
-      invokeCallback<void>('onTap', () => onTap(pointer));
+      invokeCallback<void>('onTap', () => onTap!(pointer));
   }
 
   void _dispatchLongTap(int pointer, OffsetPair lastPosition) {
     assert(_gestureMap.containsKey(pointer));
     if (onLongTapDown != null)
       invokeCallback<void>('onLongTapDown', () {
-        onLongTapDown(
+        onLongTapDown!(
           pointer,
           TapDownDetails(
             globalPosition: lastPosition.global,

packages/flutter/lib/src/gestures/pointer_router.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/foundation.dart';
 import 'package:vector_math/vector_math_64.dart';
@@ -14,8 +13,8 @@ typedef PointerRoute = void Function(PointerEvent event);
 
 /// A routing table for [PointerEvent] events.
 class PointerRouter {
-  final Map<int, Map<PointerRoute, Matrix4>> _routeMap = <int, Map<PointerRoute, Matrix4>>{};
-  final Map<PointerRoute, Matrix4> _globalRoutes = <PointerRoute, Matrix4>{};
+  final Map<int, Map<PointerRoute, Matrix4?>> _routeMap = <int, Map<PointerRoute, Matrix4?>>{};
+  final Map<PointerRoute, Matrix4?> _globalRoutes = <PointerRoute, Matrix4?>{};
 
   /// Adds a route to the routing table.
   ///
@@ -24,10 +23,10 @@ class PointerRouter {
   ///
   /// Routes added reentrantly within [PointerRouter.route] will take effect when
   /// routing the next event.
-  void addRoute(int pointer, PointerRoute route, [Matrix4 transform]) {
-    final Map<PointerRoute, Matrix4> routes = _routeMap.putIfAbsent(
+  void addRoute(int pointer, PointerRoute route, [Matrix4? transform]) {
+    final Map<PointerRoute, Matrix4?> routes = _routeMap.putIfAbsent(
       pointer,
-      () => <PointerRoute, Matrix4>{},
+      () => <PointerRoute, Matrix4?>{},
     );
     assert(!routes.containsKey(route));
     routes[route] = transform;
@@ -42,7 +41,7 @@ class PointerRouter {
   /// immediately.
   void removeRoute(int pointer, PointerRoute route) {
     assert(_routeMap.containsKey(pointer));
-    final Map<PointerRoute, Matrix4> routes = _routeMap[pointer];
+    final Map<PointerRoute, Matrix4?> routes = _routeMap[pointer]!;
     assert(routes.containsKey(route));
     routes.remove(route);
     if (routes.isEmpty)
@@ -55,7 +54,7 @@ class PointerRouter {
   ///
   /// Routes added reentrantly within [PointerRouter.route] will take effect when
   /// routing the next event.
-  void addGlobalRoute(PointerRoute route, [Matrix4 transform]) {
+  void addGlobalRoute(PointerRoute route, [Matrix4? transform]) {
     assert(!_globalRoutes.containsKey(route));
     _globalRoutes[route] = transform;
   }
@@ -72,12 +71,12 @@ class PointerRouter {
     _globalRoutes.remove(route);
   }
 
-  void _dispatch(PointerEvent event, PointerRoute route, Matrix4 transform) {
+  void _dispatch(PointerEvent event, PointerRoute route, Matrix4? transform) {
     try {
       event = event.transformed(transform);
       route(event);
     } catch (exception, stack) {
-      InformationCollector collector;
+      InformationCollector? collector;
       assert(() {
         collector = () sync* {
           yield DiagnosticsProperty<PointerRouter>('router', this, level: DiagnosticLevel.debug);
@@ -101,13 +100,13 @@ class PointerRouter {
   /// Routes are called in the order in which they were added to the
   /// PointerRouter object.
   void route(PointerEvent event) {
-    final Map<PointerRoute, Matrix4> routes = _routeMap[event.pointer];
-    final Map<PointerRoute, Matrix4> copiedGlobalRoutes = Map<PointerRoute, Matrix4>.from(_globalRoutes);
+    final Map<PointerRoute, Matrix4?>? routes = _routeMap[event.pointer];
+    final Map<PointerRoute, Matrix4?> copiedGlobalRoutes = Map<PointerRoute, Matrix4?>.from(_globalRoutes);
     if (routes != null) {
       _dispatchEventToRoutes(
         event,
         routes,
-        Map<PointerRoute, Matrix4>.from(routes),
+        Map<PointerRoute, Matrix4?>.from(routes),
       );
     }
     _dispatchEventToRoutes(event, _globalRoutes, copiedGlobalRoutes);
@@ -115,10 +114,10 @@ class PointerRouter {
 
   void _dispatchEventToRoutes(
     PointerEvent event,
-    Map<PointerRoute, Matrix4> referenceRoutes,
-    Map<PointerRoute, Matrix4> copiedRoutes,
+    Map<PointerRoute, Matrix4?> referenceRoutes,
+    Map<PointerRoute, Matrix4?> copiedRoutes,
   ) {
-    copiedRoutes.forEach((PointerRoute route, Matrix4 transform) {
+    copiedRoutes.forEach((PointerRoute route, Matrix4? transform) {
       if (referenceRoutes.containsKey(route)) {
         _dispatch(event, route, transform);
       }

packages/flutter/lib/src/gestures/pointer_signal_resolver.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:flutter/foundation.dart';
 
@@ -27,15 +26,15 @@ bool _isSameEvent(PointerSignalEvent event1, PointerSignalEvent event2) {
 /// at the end of event dispatch. The first callback registered will be the one
 /// that is called.
 class PointerSignalResolver {
-  PointerSignalResolvedCallback _firstRegisteredCallback;
+  PointerSignalResolvedCallback? _firstRegisteredCallback;
 
-  PointerSignalEvent _currentEvent;
+  PointerSignalEvent? _currentEvent;
 
   /// Registers interest in handling [event].
   void register(PointerSignalEvent event, PointerSignalResolvedCallback callback) {
     assert(event != null);
     assert(callback != null);
-    assert(_currentEvent == null || _isSameEvent(_currentEvent, event));
+    assert(_currentEvent == null || _isSameEvent(_currentEvent!, event));
     if (_firstRegisteredCallback != null) {
       return;
     }
@@ -53,11 +52,11 @@ class PointerSignalResolver {
       assert(_currentEvent == null);
       return;
     }
-    assert(_isSameEvent(_currentEvent, event));
+    assert(_isSameEvent(_currentEvent!, event));
     try {
-      _firstRegisteredCallback(_currentEvent);
+      _firstRegisteredCallback!(_currentEvent!);
     } catch (exception, stack) {
-      InformationCollector collector;
+      InformationCollector? collector;
       assert(() {
         collector = () sync* {
           yield DiagnosticsProperty<PointerSignalEvent>('Event', event, style: DiagnosticsTreeStyle.errorProperty);

packages/flutter/lib/src/gestures/recognizer.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:async';
 import 'dart:collection';
@@ -67,17 +66,17 @@ abstract class GestureRecognizer extends GestureArenaMember with DiagnosticableT
   /// by providing the optional [kind] argument. If [kind] is null,
   /// the recognizer will accept pointer events from all device kinds.
   /// {@endtemplate}
-  GestureRecognizer({ this.debugOwner, PointerDeviceKind kind }) : _kindFilter = kind;
+  GestureRecognizer({ this.debugOwner, PointerDeviceKind? kind }) : _kindFilter = kind;
 
   /// The recognizer's owner.
   ///
   /// This is used in the [toString] serialization to report the object for which
   /// this gesture recognizer was created, to aid in debugging.
-  final Object debugOwner;
+  final Object? debugOwner;
 
   /// The kind of device that's allowed to be recognized. If null, events from
   /// all device kinds will be tracked and recognized.
-  final PointerDeviceKind _kindFilter;
+  final PointerDeviceKind? _kindFilter;
 
   /// Holds a mapping between pointer IDs and the kind of devices they are
   /// coming from.
@@ -141,7 +140,7 @@ abstract class GestureRecognizer extends GestureArenaMember with DiagnosticableT
   @protected
   PointerDeviceKind getKindForPointer(int pointer) {
     assert(_pointerToKind.containsKey(pointer));
-    return _pointerToKind[pointer];
+    return _pointerToKind[pointer]!;
   }
 
   /// Releases any resources used by the object.
@@ -167,13 +166,13 @@ abstract class GestureRecognizer extends GestureArenaMember with DiagnosticableT
   /// callback that returns a string describing useful debugging information,
   /// e.g. the arguments passed to the callback.
   @protected
-  T invokeCallback<T>(String name, RecognizerCallback<T> callback, { String debugReport() }) {
+  T? invokeCallback<T>(String name, RecognizerCallback<T> callback, { String Function()? debugReport }) {
     assert(callback != null);
-    T result;
+    T? result;
     try {
       assert(() {
         if (debugPrintRecognizerCallbacksTrace) {
-          final String report = debugReport != null ? debugReport() : null;
+          final String? report = debugReport != null ? debugReport() : null;
           // The 19 in the line below is the width of the prefix used by
           // _debugLogDiagnostic in arena.dart.
           final String prefix = debugPrintGestureArenaDiagnostics ? ' ' * 19 + '❙ ' : '';
@@ -183,7 +182,7 @@ abstract class GestureRecognizer extends GestureArenaMember with DiagnosticableT
       }());
       result = callback();
     } catch (exception, stack) {
-      InformationCollector collector;
+      InformationCollector? collector;
       assert(() {
         collector = () sync* {
           yield StringProperty('Handler', name);
@@ -222,8 +221,8 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   OneSequenceGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   final Map<int, GestureArenaEntry> _entries = <int, GestureArenaEntry>{};
@@ -267,7 +266,7 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   @protected
   @mustCallSuper
   void resolvePointer(int pointer, GestureDisposition disposition) {
-    final GestureArenaEntry entry = _entries[pointer];
+    final GestureArenaEntry? entry = _entries[pointer];
     if (entry != null) {
       entry.resolve(disposition);
       _entries.remove(pointer);
@@ -278,7 +277,7 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   void dispose() {
     resolve(GestureDisposition.rejected);
     for (final int pointer in _trackedPointers)
-      GestureBinding.instance.pointerRouter.removeRoute(pointer, handleEvent);
+      GestureBinding.instance!.pointerRouter.removeRoute(pointer, handleEvent);
     _trackedPointers.clear();
     assert(_entries.isEmpty);
     super.dispose();
@@ -294,10 +293,10 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   /// A recognizer can be assigned to a team only when it is not participating
   /// in the arena. For example, a common time to assign a recognizer to a team
   /// is shortly after creating the recognizer.
-  GestureArenaTeam get team => _team;
-  GestureArenaTeam _team;
+  GestureArenaTeam? get team => _team;
+  GestureArenaTeam? _team;
   /// The [team] can only be set once.
-  set team(GestureArenaTeam value) {
+  set team(GestureArenaTeam? value) {
     assert(value != null);
     assert(_entries.isEmpty);
     assert(_trackedPointers.isEmpty);
@@ -307,8 +306,8 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
 
   GestureArenaEntry _addPointerToArena(int pointer) {
     if (_team != null)
-      return _team.add(pointer, this);
-    return GestureBinding.instance.gestureArena.add(pointer, this);
+      return _team!.add(pointer, this);
+    return GestureBinding.instance!.gestureArena.add(pointer, this);
   }
 
   /// Causes events related to the given pointer ID to be routed to this recognizer.
@@ -321,8 +320,8 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   ///
   /// Use [stopTrackingPointer] to remove the route added by this function.
   @protected
-  void startTrackingPointer(int pointer, [Matrix4 transform]) {
-    GestureBinding.instance.pointerRouter.addRoute(pointer, handleEvent, transform);
+  void startTrackingPointer(int pointer, [Matrix4? transform]) {
+    GestureBinding.instance!.pointerRouter.addRoute(pointer, handleEvent, transform);
     _trackedPointers.add(pointer);
     assert(!_entries.containsValue(pointer));
     _entries[pointer] = _addPointerToArena(pointer);
@@ -337,7 +336,7 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   @protected
   void stopTrackingPointer(int pointer) {
     if (_trackedPointers.contains(pointer)) {
-      GestureBinding.instance.pointerRouter.removeRoute(pointer, handleEvent);
+      GestureBinding.instance!.pointerRouter.removeRoute(pointer, handleEvent);
       _trackedPointers.remove(pointer);
       if (_trackedPointers.isEmpty)
         didStopTrackingLastPointer(pointer);
@@ -391,8 +390,8 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
     this.deadline,
     this.preAcceptSlopTolerance = kTouchSlop,
     this.postAcceptSlopTolerance = kTouchSlop,
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : assert(
          preAcceptSlopTolerance == null || preAcceptSlopTolerance >= 0,
          'The preAcceptSlopTolerance must be positive or null',
@@ -408,7 +407,7 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
   ///
   /// The [didExceedDeadline] will not be called if the primary pointer is
   /// accepted, rejected, or all pointers are up or canceled before [deadline].
-  final Duration deadline;
+  final Duration? deadline;
 
   /// The maximum distance in logical pixels the gesture is allowed to drift
   /// from the initial touch down position before the gesture is accepted.
@@ -417,7 +416,7 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
   ///
   /// Can be null to indicate that the gesture can drift for any distance.
   /// Defaults to 18 logical pixels.
-  final double preAcceptSlopTolerance;
+  final double? preAcceptSlopTolerance;
 
   /// The maximum distance in logical pixels the gesture is allowed to drift
   /// after the gesture has been accepted.
@@ -427,7 +426,7 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
   ///
   /// Can be null to indicate that the gesture can drift for any distance.
   /// Defaults to 18 logical pixels.
-  final double postAcceptSlopTolerance;
+  final double? postAcceptSlopTolerance;
 
   /// The current state of the recognizer.
   ///
@@ -435,15 +434,15 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
   GestureRecognizerState state = GestureRecognizerState.ready;
 
   /// The ID of the primary pointer this recognizer is tracking.
-  int primaryPointer;
+  int? primaryPointer;
 
   /// The location at which the primary pointer contacted the screen.
-  OffsetPair initialPosition;
+  OffsetPair? initialPosition;
 
   // Whether this pointer is accepted by winning the arena or as defined by
   // a subclass calling acceptGesture.
   bool _gestureAccepted = false;
-  Timer _timer;
+  Timer? _timer;
 
   @override
   void addAllowedPointer(PointerDownEvent event) {
@@ -453,7 +452,7 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
       primaryPointer = event.pointer;
       initialPosition = OffsetPair(local: event.localPosition, global: event.position);
       if (deadline != null)
-        _timer = Timer(deadline, () => didExceedDeadlineWithEvent(event));
+        _timer = Timer(deadline!, () => didExceedDeadlineWithEvent(event));
     }
   }
 
@@ -464,15 +463,15 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
       final bool isPreAcceptSlopPastTolerance =
           !_gestureAccepted &&
           preAcceptSlopTolerance != null &&
-          _getGlobalDistance(event) > preAcceptSlopTolerance;
+          _getGlobalDistance(event) > preAcceptSlopTolerance!;
       final bool isPostAcceptSlopPastTolerance =
           _gestureAccepted &&
           postAcceptSlopTolerance != null &&
-          _getGlobalDistance(event) > postAcceptSlopTolerance;
+          _getGlobalDistance(event) > postAcceptSlopTolerance!;
 
       if (event is PointerMoveEvent && (isPreAcceptSlopPastTolerance || isPostAcceptSlopPastTolerance)) {
         resolve(GestureDisposition.rejected);
-        stopTrackingPointer(primaryPointer);
+        stopTrackingPointer(primaryPointer!);
       } else {
         handlePrimaryPointer(event);
       }
@@ -534,13 +533,13 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
 
   void _stopTimer() {
     if (_timer != null) {
-      _timer.cancel();
+      _timer!.cancel();
       _timer = null;
     }
   }
 
   double _getGlobalDistance(PointerEvent event) {
-    final Offset offset = event.position - initialPosition.global;
+    final Offset offset = event.position - initialPosition!.global;
     return offset.distance;
   }
 
@@ -559,8 +558,8 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
 class OffsetPair {
   /// Creates a [OffsetPair] combining a [local] and [global] [Offset].
   const OffsetPair({
-    @required this.local,
-    @required this.global,
+    required this.local,
+    required this.global,
   });
 
   /// Creates a [OffsetPair] from [PointerEvent.localPosition] and

packages/flutter/lib/src/gestures/scale.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:math' as math;
 
@@ -38,7 +37,7 @@ class ScaleStartDetails {
   /// Creates details for [GestureScaleStartCallback].
   ///
   /// The [focalPoint] argument must not be null.
-  ScaleStartDetails({ this.focalPoint = Offset.zero, Offset localFocalPoint, })
+  ScaleStartDetails({ this.focalPoint = Offset.zero, Offset? localFocalPoint, })
     : assert(focalPoint != null), localFocalPoint = localFocalPoint ?? focalPoint;
 
   /// The initial focal point of the pointers in contact with the screen.
@@ -75,7 +74,7 @@ class ScaleUpdateDetails {
   /// argument must be greater than or equal to zero.
   ScaleUpdateDetails({
     this.focalPoint = Offset.zero,
-    Offset localFocalPoint,
+    Offset? localFocalPoint,
     this.scale = 1.0,
     this.horizontalScale = 1.0,
     this.verticalScale = 1.0,
@@ -225,37 +224,37 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   ScaleGestureRecognizer({
-    Object debugOwner,
-    PointerDeviceKind kind,
+    Object? debugOwner,
+    PointerDeviceKind? kind,
   }) : super(debugOwner: debugOwner, kind: kind);
 
   /// The pointers in contact with the screen have established a focal point and
   /// initial scale of 1.0.
-  GestureScaleStartCallback onStart;
+  GestureScaleStartCallback? onStart;
 
   /// The pointers in contact with the screen have indicated a new focal point
   /// and/or scale.
-  GestureScaleUpdateCallback onUpdate;
+  GestureScaleUpdateCallback? onUpdate;
 
   /// The pointers are no longer in contact with the screen.
-  GestureScaleEndCallback onEnd;
+  GestureScaleEndCallback? onEnd;
 
   _ScaleState _state = _ScaleState.ready;
 
-  Matrix4 _lastTransform;
-
-  Offset _initialFocalPoint;
-  Offset _currentFocalPoint;
-  double _initialSpan;
-  double _currentSpan;
-  double _initialHorizontalSpan;
-  double _currentHorizontalSpan;
-  double _initialVerticalSpan;
-  double _currentVerticalSpan;
-  _LineBetweenPointers _initialLine;
-  _LineBetweenPointers _currentLine;
-  Map<int, Offset> _pointerLocations;
-  List<int> _pointerQueue; // A queue to sort pointers in order of entrance
+  Matrix4? _lastTransform;
+
+  late Offset _initialFocalPoint;
+  late Offset _currentFocalPoint;
+  late double _initialSpan;
+  late double _currentSpan;
+  late double _initialHorizontalSpan;
+  late double _currentHorizontalSpan;
+  late double _initialVerticalSpan;
+  late double _currentVerticalSpan;
+  _LineBetweenPointers? _initialLine;
+  _LineBetweenPointers? _currentLine;
+  late Map<int, Offset> _pointerLocations;
+  late List<int> _pointerQueue; // A queue to sort pointers in order of entrance
   final Map<int, VelocityTracker> _velocityTrackers = <int, VelocityTracker>{};
 
   double get _scaleFactor => _initialSpan > 0.0 ? _currentSpan / _initialSpan : 1.0;
@@ -268,15 +267,15 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     if (_initialLine == null || _currentLine == null) {
       return 0.0;
     }
-    final double fx = _initialLine.pointerStartLocation.dx;
-    final double fy = _initialLine.pointerStartLocation.dy;
-    final double sx = _initialLine.pointerEndLocation.dx;
-    final double sy = _initialLine.pointerEndLocation.dy;
+    final double fx = _initialLine!.pointerStartLocation.dx;
+    final double fy = _initialLine!.pointerStartLocation.dy;
+    final double sx = _initialLine!.pointerEndLocation.dx;
+    final double sy = _initialLine!.pointerEndLocation.dy;
 
-    final double nfx = _currentLine.pointerStartLocation.dx;
-    final double nfy = _currentLine.pointerStartLocation.dy;
-    final double nsx = _currentLine.pointerEndLocation.dx;
-    final double nsy = _currentLine.pointerEndLocation.dy;
+    final double nfx = _currentLine!.pointerStartLocation.dx;
+    final double nfy = _currentLine!.pointerStartLocation.dy;
+    final double nsx = _currentLine!.pointerEndLocation.dx;
+    final double nsy = _currentLine!.pointerEndLocation.dy;
 
     final double angle1 = math.atan2(fy - sy, fx - sx);
     final double angle2 = math.atan2(nfy - nsy, nfx - nsx);
@@ -307,8 +306,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     bool didChangeConfiguration = false;
     bool shouldStartIfAccepted = false;
     if (event is PointerMoveEvent) {
-      final VelocityTracker tracker = _velocityTrackers[event.pointer];
-      assert(tracker != null);
+      final VelocityTracker tracker = _velocityTrackers[event.pointer]!;
       if (!event.synthesized)
         tracker.addPosition(event.timeStamp, event.position);
       _pointerLocations[event.pointer] = event.position;
@@ -341,7 +339,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     // Compute the focal point
     Offset focalPoint = Offset.zero;
     for (final int pointer in _pointerLocations.keys)
-      focalPoint += _pointerLocations[pointer];
+      focalPoint += _pointerLocations[pointer]!;
     _currentFocalPoint = count > 0 ? focalPoint / count.toDouble() : Offset.zero;
 
     // Span is the average deviation from focal point. Horizontal and vertical
@@ -351,9 +349,9 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     double totalHorizontalDeviation = 0.0;
     double totalVerticalDeviation = 0.0;
     for (final int pointer in _pointerLocations.keys) {
-      totalDeviation += (_currentFocalPoint - _pointerLocations[pointer]).distance;
-      totalHorizontalDeviation += (_currentFocalPoint.dx - _pointerLocations[pointer].dx).abs();
-      totalVerticalDeviation += (_currentFocalPoint.dy - _pointerLocations[pointer].dy).abs();
+      totalDeviation += (_currentFocalPoint - _pointerLocations[pointer]!).distance;
+      totalHorizontalDeviation += (_currentFocalPoint.dx - _pointerLocations[pointer]!.dx).abs();
+      totalVerticalDeviation += (_currentFocalPoint.dy - _pointerLocations[pointer]!.dy).abs();
     }
     _currentSpan = count > 0 ? totalDeviation / count : 0.0;
     _currentHorizontalSpan = count > 0 ? totalHorizontalDeviation / count : 0.0;
@@ -369,22 +367,22 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     if (count < 2) {
       _initialLine = _currentLine;
     } else if (_initialLine != null &&
-      _initialLine.pointerStartId == _pointerQueue[0] &&
-      _initialLine.pointerEndId == _pointerQueue[1]) {
+      _initialLine!.pointerStartId == _pointerQueue[0] &&
+      _initialLine!.pointerEndId == _pointerQueue[1]) {
       /// Rotation updated, set the [_currentLine]
       _currentLine = _LineBetweenPointers(
         pointerStartId: _pointerQueue[0],
-        pointerStartLocation: _pointerLocations[_pointerQueue[0]],
+        pointerStartLocation: _pointerLocations[_pointerQueue[0]]!,
         pointerEndId: _pointerQueue[1],
-        pointerEndLocation: _pointerLocations[_pointerQueue[1]],
+        pointerEndLocation: _pointerLocations[_pointerQueue[1]]!,
       );
     } else {
       /// A new rotation process is on the way, set the [_initialLine]
       _initialLine = _LineBetweenPointers(
         pointerStartId: _pointerQueue[0],
-        pointerStartLocation: _pointerLocations[_pointerQueue[0]],
+        pointerStartLocation: _pointerLocations[_pointerQueue[0]]!,
         pointerEndId: _pointerQueue[1],
-        pointerEndLocation: _pointerLocations[_pointerQueue[1]],
+        pointerEndLocation: _pointerLocations[_pointerQueue[1]]!,
       );
       _currentLine = null;
     }
@@ -398,17 +396,16 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     _initialVerticalSpan = _currentVerticalSpan;
     if (_state == _ScaleState.started) {
       if (onEnd != null) {
-        final VelocityTracker tracker = _velocityTrackers[pointer];
-        assert(tracker != null);
+        final VelocityTracker tracker = _velocityTrackers[pointer]!;
 
         Velocity velocity = tracker.getVelocity();
         if (_isFlingGesture(velocity)) {
           final Offset pixelsPerSecond = velocity.pixelsPerSecond;
           if (pixelsPerSecond.distanceSquared > kMaxFlingVelocity * kMaxFlingVelocity)
             velocity = Velocity(pixelsPerSecond: (pixelsPerSecond / pixelsPerSecond.distance) * kMaxFlingVelocity);
-          invokeCallback<void>('onEnd', () => onEnd(ScaleEndDetails(velocity: velocity)));
+          invokeCallback<void>('onEnd', () => onEnd!(ScaleEndDetails(velocity: velocity)));
         } else {
-          invokeCallback<void>('onEnd', () => onEnd(ScaleEndDetails(velocity: Velocity.zero)));
+          invokeCallback<void>('onEnd', () => onEnd!(ScaleEndDetails(velocity: Velocity.zero)));
         }
       }
       _state = _ScaleState.accepted;
@@ -437,7 +434,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
 
     if (_state == _ScaleState.started && onUpdate != null)
       invokeCallback<void>('onUpdate', () {
-        onUpdate(ScaleUpdateDetails(
+        onUpdate!(ScaleUpdateDetails(
           scale: _scaleFactor,
           horizontalScale: _horizontalScaleFactor,
           verticalScale: _verticalScaleFactor,
@@ -452,7 +449,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     assert(_state == _ScaleState.started);
     if (onStart != null)
       invokeCallback<void>('onStart', () {
-        onStart(ScaleStartDetails(
+        onStart!(ScaleStartDetails(
           focalPoint: _currentFocalPoint,
           localFocalPoint: PointerEvent.transformPosition(_lastTransform, _currentFocalPoint),
         ));

packages/flutter/lib/src/gestures/tap.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'package:vector_math/vector_math_64.dart' show Matrix4;
 import 'package:flutter/foundation.dart';
@@ -24,7 +23,7 @@ class TapDownDetails {
   /// The [globalPosition] argument must not be null.
   TapDownDetails({
     this.globalPosition = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
     this.kind,
   }) : assert(globalPosition != null),
        localPosition = localPosition ?? globalPosition;
@@ -33,7 +32,7 @@ class TapDownDetails {
   final Offset globalPosition;
 
   /// The kind of the device that initiated the event.
-  final PointerDeviceKind kind;
+  final PointerDeviceKind? kind;
 
   /// The local position at which the pointer contacted the screen.
   final Offset localPosition;
@@ -61,7 +60,7 @@ class TapUpDetails {
   /// The [globalPosition] argument must not be null.
   TapUpDetails({
     this.globalPosition = Offset.zero,
-    Offset localPosition,
+    Offset? localPosition,
   }) : assert(globalPosition != null),
        localPosition = localPosition ?? globalPosition;
 
@@ -133,14 +132,14 @@ typedef GestureTapCancelCallback = void Function();
 ///    any buttons.
 abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer {
   /// Creates a tap gesture recognizer.
-  BaseTapGestureRecognizer({ Object debugOwner })
+  BaseTapGestureRecognizer({ Object? debugOwner })
     : super(deadline: kPressTimeout , debugOwner: debugOwner);
 
   bool _sentTapDown = false;
   bool _wonArenaForPrimaryPointer = false;
 
-  PointerDownEvent _down;
-  PointerUpEvent _up;
+  PointerDownEvent? _down;
+  PointerUpEvent? _up;
 
   /// A pointer has contacted the screen, which might be the start of a tap.
   ///
@@ -153,7 +152,7 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
   /// If this recognizer doesn't win the arena, [handleTapCancel] is called next.
   /// Otherwise, [handleTapUp] is called next.
   @protected
-  void handleTapDown({ PointerDownEvent down });
+  void handleTapDown({ required PointerDownEvent down });
 
   /// A pointer has stopped contacting the screen, which is recognized as a tap.
   ///
@@ -166,7 +165,7 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
   /// If this recognizer doesn't win the arena, [handleTapCancel] is called
   /// instead.
   @protected
-  void handleTapUp({ PointerDownEvent down, PointerUpEvent up });
+  void handleTapUp({ required PointerDownEvent down, required PointerUpEvent up });
 
   /// A pointer that previously triggered [handleTapDown] will not end up
   /// causing a tap.
@@ -182,7 +181,7 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
   ///
   /// If this recognizer wins the arena, [handleTapUp] is called instead.
   @protected
-  void handleTapCancel({ PointerDownEvent down, PointerCancelEvent cancel, String reason });
+  void handleTapCancel({ required PointerDownEvent down, PointerCancelEvent? cancel, required String reason });
 
   @override
   void addAllowedPointer(PointerDownEvent event) {
@@ -205,7 +204,7 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
 
   @override
   @protected
-  void startTrackingPointer(int pointer, [Matrix4 transform]) {
+  void startTrackingPointer(int pointer, [Matrix4? transform]) {
     // The recognizer should never track any pointers when `_down` is null,
     // because calling `_checkDown` in this state will throw exception.
     assert(_down != null);
@@ -223,9 +222,9 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
         _checkCancel(event, '');
       }
       _reset();
-    } else if (event.buttons != _down.buttons) {
+    } else if (event.buttons != _down!.buttons) {
       resolve(GestureDisposition.rejected);
-      stopTrackingPointer(primaryPointer);
+      stopTrackingPointer(primaryPointer!);
     }
   }
 
@@ -273,7 +272,7 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
     if (_sentTapDown) {
       return;
     }
-    handleTapDown(down: _down);
+    handleTapDown(down: _down!);
     _sentTapDown = true;
   }
 
@@ -281,12 +280,12 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
     if (!_wonArenaForPrimaryPointer || _up == null) {
       return;
     }
-    handleTapUp(down: _down, up: _up);
+    handleTapUp(down: _down!, up: _up!);
     _reset();
   }
 
-  void _checkCancel(PointerCancelEvent event, String note) {
-    handleTapCancel(down: _down, cancel: event, reason: note);
+  void _checkCancel(PointerCancelEvent? event, String note) {
+    handleTapCancel(down: _down!, cancel: event, reason: note);
   }
 
   void _reset() {
@@ -332,7 +331,7 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
 ///  * [MultiTapGestureRecognizer]
 class TapGestureRecognizer extends BaseTapGestureRecognizer {
   /// Creates a tap gesture recognizer.
-  TapGestureRecognizer({ Object debugOwner }) : super(debugOwner: debugOwner);
+  TapGestureRecognizer({ Object? debugOwner }) : super(debugOwner: debugOwner);
 
   /// A pointer has contacted the screen at a particular location with a primary
   /// button, which might be the start of a tap.
@@ -349,7 +348,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   ///  * [onSecondaryTapDown], a similar callback but for a secondary button.
   ///  * [TapDownDetails], which is passed as an argument to this callback.
   ///  * [GestureDetector.onTapDown], which exposes this callback.
-  GestureTapDownCallback onTapDown;
+  GestureTapDownCallback? onTapDown;
 
   /// A pointer has stopped contacting the screen at a particular location,
   /// which is recognized as a tap of a primary button.
@@ -365,7 +364,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   ///  * [onSecondaryTapUp], a similar callback but for a secondary button.
   ///  * [TapUpDetails], which is passed as an argument to this callback.
   ///  * [GestureDetector.onTapUp], which exposes this callback.
-  GestureTapUpCallback onTapUp;
+  GestureTapUpCallback? onTapUp;
 
   /// A pointer has stopped contacting the screen, which is recognized as a tap
   /// of a primary button.
@@ -380,7 +379,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [onTapUp], which has the same timing but with details.
   ///  * [GestureDetector.onTap], which exposes this callback.
-  GestureTapCallback onTap;
+  GestureTapCallback? onTap;
 
   /// A pointer that previously triggered [onTapDown] will not end up causing
   /// a tap.
@@ -396,7 +395,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [onSecondaryTapCancel], a similar callback but for a secondary button.
   ///  * [GestureDetector.onTapCancel], which exposes this callback.
-  GestureTapCancelCallback onTapCancel;
+  GestureTapCancelCallback? onTapCancel;
 
   /// A pointer has stopped contacting the screen, which is recognized as a tap
   /// of a secondary button.
@@ -412,7 +411,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   ///  * [kSecondaryButton], the button this callback responds to.
   ///  * [onSecondaryTapUp], which has the same timing but with details.
   ///  * [GestureDetector.onSecondaryTap], which exposes this callback.
-  GestureTapCallback onSecondaryTap;
+  GestureTapCallback? onSecondaryTap;
 
   /// A pointer has contacted the screen at a particular location with a
   /// secondary button, which might be the start of a secondary tap.
@@ -429,7 +428,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   ///  * [onTapDown], a similar callback but for a primary button.
   ///  * [TapDownDetails], which is passed as an argument to this callback.
   ///  * [GestureDetector.onSecondaryTapDown], which exposes this callback.
-  GestureTapDownCallback onSecondaryTapDown;
+  GestureTapDownCallback? onSecondaryTapDown;
 
   /// A pointer has stopped contacting the screen at a particular location,
   /// which is recognized as a tap of a secondary button.
@@ -448,7 +447,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   ///  * [onTapUp], a similar callback but for a primary button.
   ///  * [TapUpDetails], which is passed as an argument to this callback.
   ///  * [GestureDetector.onSecondaryTapUp], which exposes this callback.
-  GestureTapUpCallback onSecondaryTapUp;
+  GestureTapUpCallback? onSecondaryTapUp;
 
   /// A pointer that previously triggered [onSecondaryTapDown] will not end up
   /// causing a tap.
@@ -463,7 +462,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   ///  * [kSecondaryButton], the button this callback responds to.
   ///  * [onTapCancel], a similar callback but for a primary button.
   ///  * [GestureDetector.onTapCancel], which exposes this callback.
-  GestureTapCancelCallback onSecondaryTapCancel;
+  GestureTapCancelCallback? onSecondaryTapCancel;
 
   @override
   bool isPointerAllowed(PointerDownEvent event) {
@@ -490,7 +489,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
 
   @protected
   @override
-  void handleTapDown({PointerDownEvent down}) {
+  void handleTapDown({required PointerDownEvent down}) {
     final TapDownDetails details = TapDownDetails(
       globalPosition: down.position,
       localPosition: down.localPosition,
@@ -499,11 +498,11 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
     switch (down.buttons) {
       case kPrimaryButton:
         if (onTapDown != null)
-          invokeCallback<void>('onTapDown', () => onTapDown(details));
+          invokeCallback<void>('onTapDown', () => onTapDown!(details));
         break;
       case kSecondaryButton:
         if (onSecondaryTapDown != null)
-          invokeCallback<void>('onSecondaryTapDown', () => onSecondaryTapDown(details));
+          invokeCallback<void>('onSecondaryTapDown', () => onSecondaryTapDown!(details));
         break;
       default:
     }
@@ -511,7 +510,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
 
   @protected
   @override
-  void handleTapUp({PointerDownEvent down, PointerUpEvent up}) {
+  void handleTapUp({ required PointerDownEvent down, required PointerUpEvent up}) {
     final TapUpDetails details = TapUpDetails(
       globalPosition: up.position,
       localPosition: up.localPosition,
@@ -519,15 +518,15 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
     switch (down.buttons) {
       case kPrimaryButton:
         if (onTapUp != null)
-          invokeCallback<void>('onTapUp', () => onTapUp(details));
+          invokeCallback<void>('onTapUp', () => onTapUp!(details));
         if (onTap != null)
-          invokeCallback<void>('onTap', onTap);
+          invokeCallback<void>('onTap', onTap!);
         break;
       case kSecondaryButton:
         if (onSecondaryTapUp != null)
-          invokeCallback<void>('onSecondaryTapUp', () => onSecondaryTapUp(details));
+          invokeCallback<void>('onSecondaryTapUp', () => onSecondaryTapUp!(details));
         if (onSecondaryTap != null)
-          invokeCallback<void>('onSecondaryTap', () => onSecondaryTap());
+          invokeCallback<void>('onSecondaryTap', () => onSecondaryTap!());
         break;
       default:
     }
@@ -535,16 +534,16 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
 
   @protected
   @override
-  void handleTapCancel({PointerDownEvent down, PointerCancelEvent cancel, String reason}) {
+  void handleTapCancel({ required PointerDownEvent down, PointerCancelEvent? cancel, required String reason }) {
     final String note = reason == '' ? reason : '$reason ';
     switch (down.buttons) {
       case kPrimaryButton:
         if (onTapCancel != null)
-          invokeCallback<void>('${note}onTapCancel', onTapCancel);
+          invokeCallback<void>('${note}onTapCancel', onTapCancel!);
         break;
       case kSecondaryButton:
         if (onSecondaryTapCancel != null)
-          invokeCallback<void>('${note}onSecondaryTapCancel', onSecondaryTapCancel);
+          invokeCallback<void>('${note}onSecondaryTapCancel', onSecondaryTapCancel!);
         break;
       default:
     }

packages/flutter/lib/src/gestures/team.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'arena.dart';
 import 'binding.dart';
@@ -27,8 +26,8 @@ class _CombiningGestureArenaMember extends GestureArenaMember {
   final int _pointer;
 
   bool _resolved = false;
-  GestureArenaMember _winner;
-  GestureArenaEntry _entry;
+  GestureArenaMember? _winner;
+  GestureArenaEntry? _entry;
 
   @override
   void acceptGesture(int pointer) {
@@ -40,7 +39,7 @@ class _CombiningGestureArenaMember extends GestureArenaMember {
       if (member != _winner)
         member.rejectGesture(pointer);
     }
-    _winner.acceptGesture(pointer);
+    _winner!.acceptGesture(pointer);
   }
 
   @override
@@ -54,7 +53,7 @@ class _CombiningGestureArenaMember extends GestureArenaMember {
   void _close() {
     assert(!_resolved);
     _resolved = true;
-    final _CombiningGestureArenaMember combiner = _owner._combiners.remove(_pointer);
+    final _CombiningGestureArenaMember? combiner = _owner._combiners.remove(_pointer);
     assert(combiner == this);
   }
 
@@ -62,7 +61,7 @@ class _CombiningGestureArenaMember extends GestureArenaMember {
     assert(!_resolved);
     assert(_pointer == pointer);
     _members.add(member);
-    _entry ??= GestureBinding.instance.gestureArena.add(pointer, this);
+    _entry ??= GestureBinding.instance!.gestureArena.add(pointer, this);
     return _CombiningGestureArenaEntry(this, member);
   }
 
@@ -73,11 +72,11 @@ class _CombiningGestureArenaMember extends GestureArenaMember {
       _members.remove(member);
       member.rejectGesture(_pointer);
       if (_members.isEmpty)
-        _entry.resolve(disposition);
+        _entry!.resolve(disposition);
     } else {
       assert(disposition == GestureDisposition.accepted);
       _winner ??= _owner.captain ?? member;
-      _entry.resolve(disposition);
+      _entry!.resolve(disposition);
     }
   }
 }
@@ -131,7 +130,7 @@ class GestureArenaTeam {
   /// If not null, when any one of the [GestureArenaTeam] members claims victory
   /// the captain accepts the gesture.
   /// If null, the member that claims a victory accepts the gesture.
-  GestureArenaMember captain;
+  GestureArenaMember? captain;
 
   /// Adds a new member to the arena on behalf of this team.
   ///

packages/flutter/lib/src/gestures/velocity_tracker.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
 
 import 'dart:ui' show Offset;
 
@@ -19,7 +18,7 @@ class Velocity {
   ///
   /// The [pixelsPerSecond] argument must not be null.
   const Velocity({
-    @required this.pixelsPerSecond,
+    required this.pixelsPerSecond,
   }) : assert(pixelsPerSecond != null);
 
   /// A velocity that isn't moving at all.
@@ -95,10 +94,10 @@ class VelocityEstimate {
   ///
   /// [pixelsPerSecond], [confidence], [duration], and [offset] must not be null.
   const VelocityEstimate({
-    @required this.pixelsPerSecond,
-    @required this.confidence,
-    @required this.duration,
-    @required this.offset,
+    required this.pixelsPerSecond,
+    required this.confidence,
+    required this.duration,
+    required this.offset,
   }) : assert(pixelsPerSecond != null),
        assert(confidence != null),
        assert(duration != null),
@@ -154,7 +153,7 @@ class VelocityTracker {
   static const int _minSampleSize = 3;
 
   // Circular buffer; current sample at _index.
-  final List<_PointAtTime> _samples = List<_PointAtTime>(_historySize);
+  final List<_PointAtTime?> _samples = List<_PointAtTime?>.filled(_historySize, null, growable: false);
   int _index = 0;
 
   /// Adds a position as the given time to the tracker.
@@ -171,7 +170,7 @@ class VelocityTracker {
   /// Information is added using [addPosition].
   ///
   /// Returns null if there is no data on which to base an estimate.
-  VelocityEstimate getVelocityEstimate() {
+  VelocityEstimate? getVelocityEstimate() {
     final List<double> x = <double>[];
     final List<double> y = <double>[];
     final List<double> w = <double>[];
@@ -179,7 +178,7 @@ class VelocityTracker {
     int sampleCount = 0;
     int index = _index;
 
-    final _PointAtTime newestSample = _samples[index];
+    final _PointAtTime? newestSample = _samples[index];
     if (newestSample == null)
       return null;
 
@@ -189,7 +188,7 @@ class VelocityTracker {
     // Starting with the most recent PointAtTime sample, iterate backwards while
     // the samples represent continuous motion.
     do {
-      final _PointAtTime sample = _samples[index];
+      final _PointAtTime? sample = _samples[index];
       if (sample == null)
         break;
 
@@ -212,10 +211,10 @@ class VelocityTracker {
 
     if (sampleCount >= _minSampleSize) {
       final LeastSquaresSolver xSolver = LeastSquaresSolver(time, x, w);
-      final PolynomialFit xFit = xSolver.solve(2);
+      final PolynomialFit? xFit = xSolver.solve(2);
       if (xFit != null) {
         final LeastSquaresSolver ySolver = LeastSquaresSolver(time, y, w);
-        final PolynomialFit yFit = ySolver.solve(2);
+        final PolynomialFit? yFit = ySolver.solve(2);
         if (yFit != null) {
           return VelocityEstimate( // convert from pixels/ms to pixels/s
             pixelsPerSecond: Offset(xFit.coefficients[1] * 1000, yFit.coefficients[1] * 1000),
@@ -245,7 +244,7 @@ class VelocityTracker {
   /// Returns [Velocity.zero] if there is no data from which to compute an
   /// estimate or if the estimated velocity is zero.
   Velocity getVelocity() {
-    final VelocityEstimate estimate = getVelocityEstimate();
+    final VelocityEstimate? estimate = getVelocityEstimate();
     if (estimate == null || estimate.pixelsPerSecond == Offset.zero)
       return Velocity.zero;
     return Velocity(pixelsPerSecond: estimate.pixelsPerSecond);