migrate some cupertino files to nullsafety (#66024)

* migrate some cupertino files to nullsafety * address review comments * address review comments

migrate some cupertino files to nullsafety (#66024)
* migrate some cupertino files to nullsafety * address review comments * address review comments
45fa60eb · Alexandre Ardhuin · GitHub · 71c1f6c3 · 45fa60eb · 45fa60eb
Unverified Commit 45fa60eb authored Sep 23, 2020 by Alexandre Ardhuin Committed by GitHub Sep 23, 2020
10 changed files
--- a/packages/flutter/lib/src/cupertino/activity_indicator.dart
+++ b/packages/flutter/lib/src/cupertino/activity_indicator.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
 import 'dart:math' as math;
 import 'package:flutter/widgets.dart';
@@ -28,7 +26,7 @@ const Color _kActiveTickColor = CupertinoDynamicColor.withBrightness(
 class CupertinoActivityIndicator extends StatefulWidget {
  /// Creates an iOS-style activity indicator that spins clockwise.
  const CupertinoActivityIndicator({
-    Key key,
+    Key? key,
    this.animating = true,
    this.radius = _kDefaultIndicatorRadius,
  })  : assert(animating != null),
@@ -44,7 +42,7 @@ class CupertinoActivityIndicator extends StatefulWidget {
  /// will be shown) and 1.0 (all ticks will be shown) inclusive. Defaults
  /// to 1.0.
  const CupertinoActivityIndicator.partiallyRevealed({
-    Key key,
+    Key? key,
    this.radius = _kDefaultIndicatorRadius,
    this.progress = 1.0,
  })  : assert(radius != null),
@@ -80,7 +78,7 @@ class CupertinoActivityIndicator extends StatefulWidget {
 class _CupertinoActivityIndicatorState extends State<CupertinoActivityIndicator>
    with SingleTickerProviderStateMixin {
-  AnimationController _controller;
+  late AnimationController _controller;
  @override
  void initState() {
@@ -121,7 +119,7 @@ class _CupertinoActivityIndicatorState extends State<CupertinoActivityIndicator>
        painter: _CupertinoActivityIndicatorPainter(
          position: _controller,
          activeColor:
-              CupertinoDynamicColor.resolve(_kActiveTickColor, context),
+              CupertinoDynamicColor.resolve(_kActiveTickColor, context)!,
          radius: widget.radius,
          progress: widget.progress,
        ),
@@ -150,10 +148,10 @@ const int _partiallyRevealedAlpha = 147;
 class _CupertinoActivityIndicatorPainter extends CustomPainter {
  _CupertinoActivityIndicatorPainter({
-    @required this.position,
+    required this.position,
-    @required this.activeColor,
+    required this.activeColor,
-    @required this.radius,
+    required this.radius,
-    @required this.progress,
+    required this.progress,
  })  : tickFundamentalRRect = RRect.fromLTRBXY(
          -radius / _kDefaultIndicatorRadius,
          -radius / 3.0,

--- a/packages/flutter/lib/src/cupertino/button.dart
+++ b/packages/flutter/lib/src/cupertino/button.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
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
@@ -34,15 +32,15 @@ const EdgeInsets _kBackgroundButtonPadding = EdgeInsets.symmetric(
 class CupertinoButton extends StatefulWidget {
  /// Creates an iOS-style button.
  const CupertinoButton({
-    Key key,
+    Key? key,
-    @required this.child,
+    required this.child,
    this.padding,
    this.color,
    this.disabledColor = CupertinoColors.quaternarySystemFill,
    this.minSize = kMinInteractiveDimensionCupertino,
    this.pressedOpacity = 0.4,
    this.borderRadius = const BorderRadius.all(Radius.circular(8.0)),
-    @required this.onPressed,
+    required this.onPressed,
  }) : assert(pressedOpacity == null || (pressedOpacity >= 0.0 && pressedOpacity <= 1.0)),
       assert(disabledColor != null),
       _filled = false,
@@ -55,14 +53,14 @@ class CupertinoButton extends StatefulWidget {
  /// To specify a custom background color, use the [color] argument of the
  /// default constructor.
  const CupertinoButton.filled({
-    Key key,
+    Key? key,
-    @required this.child,
+    required this.child,
    this.padding,
    this.disabledColor = CupertinoColors.quaternarySystemFill,
    this.minSize = kMinInteractiveDimensionCupertino,
    this.pressedOpacity = 0.4,
    this.borderRadius = const BorderRadius.all(Radius.circular(8.0)),
-    @required this.onPressed,
+    required this.onPressed,
  }) : assert(pressedOpacity == null || (pressedOpacity >= 0.0 && pressedOpacity <= 1.0)),
       assert(disabledColor != null),
       color = null,
@@ -77,7 +75,7 @@ class CupertinoButton extends StatefulWidget {
  /// The amount of space to surround the child inside the bounds of the button.
  ///
  /// Defaults to 16.0 pixels.
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
  /// The color of the button's background.
  ///
@@ -85,7 +83,7 @@ class CupertinoButton extends StatefulWidget {
  ///
  /// Defaults to the [CupertinoTheme]'s `primaryColor` when the
  /// [CupertinoButton.filled] constructor is used.
-  final Color color;
+  final Color? color;
  /// The color of the button's background when the button is disabled.
  ///
@@ -98,25 +96,25 @@ class CupertinoButton extends StatefulWidget {
  /// The callback that is called when the button is tapped or otherwise activated.
  ///
  /// If this is set to null, the button will be disabled.
-  final VoidCallback onPressed;
+  final VoidCallback? onPressed;
  /// Minimum size of the button.
  ///
  /// Defaults to kMinInteractiveDimensionCupertino which the iOS Human
  /// Interface Guidelines recommends as the minimum tappable area.
-  final double minSize;
+  final double? minSize;
  /// The opacity that the button will fade to when it is pressed.
  /// The button will have an opacity of 1.0 when it is not pressed.
  ///
  /// This defaults to 0.4. If null, opacity will not change on pressed if using
  /// your own custom effects is desired.
-  final double pressedOpacity;
+  final double? pressedOpacity;
  /// The radius of the button's corners when it has a background color.
  ///
  /// Defaults to round corners of 8 logical pixels.
-  final BorderRadius borderRadius;
+  final BorderRadius? borderRadius;
  final bool _filled;
@@ -140,8 +138,8 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
  static const Duration kFadeInDuration = Duration(milliseconds: 100);
  final Tween<double> _opacityTween = Tween<double>(begin: 1.0);
-  AnimationController _animationController;
+  late AnimationController _animationController;
-  Animation<double> _opacityAnimation;
+  late Animation<double> _opacityAnimation;
  @override
  void initState() {
@@ -170,7 +168,6 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
  @override
  void dispose() {
    _animationController.dispose();
-    _animationController = null;
    super.dispose();
  }
@@ -214,12 +211,12 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
  Widget build(BuildContext context) {
    final bool enabled = widget.enabled;
    final CupertinoThemeData themeData = CupertinoTheme.of(context);
-    final Color primaryColor = themeData.primaryColor;
+    final Color? primaryColor = themeData.primaryColor;
-    final Color backgroundColor = widget.color == null
+    final Color? backgroundColor = widget.color == null
      ? (widget._filled ? primaryColor : null)
      : CupertinoDynamicColor.resolve(widget.color, context);
-    final Color foregroundColor = backgroundColor != null
+    final Color? foregroundColor = backgroundColor != null
      ? themeData.primaryContrastingColor
      : enabled
        ? primaryColor
@@ -239,9 +236,9 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
          constraints: widget.minSize == null
            ? const BoxConstraints()
            : BoxConstraints(
-              minWidth: widget.minSize,
+                minWidth: widget.minSize!,
-              minHeight: widget.minSize,
+                minHeight: widget.minSize!,
-            ),
+              ),
          child: FadeTransition(
            opacity: _opacityAnimation,
            child: DecoratedBox(

--- a/packages/flutter/lib/src/cupertino/constants.dart
+++ b/packages/flutter/lib/src/cupertino/constants.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 minimum dimension of any interactive region according to the iOS Human
 /// Interface Guidelines.
 ///

--- a/packages/flutter/lib/src/cupertino/icons.dart
+++ b/packages/flutter/lib/src/cupertino/icons.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
 import 'package:flutter/widgets.dart';
 /// Identifiers for the supported Cupertino icons.

--- a/packages/flutter/lib/src/cupertino/picker.dart
+++ b/packages/flutter/lib/src/cupertino/picker.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
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
@@ -70,7 +68,7 @@ class CupertinoPicker extends StatefulWidget {
  /// will loop the list back to the beginning.  If set to false, the list will
  /// stop scrolling when you reach the end or the beginning.
  CupertinoPicker({
-    Key key,
+    Key? key,
    this.diameterRatio = _kDefaultDiameterRatio,
    this.backgroundColor,
    this.offAxisFraction = 0.0,
@@ -78,9 +76,9 @@ class CupertinoPicker extends StatefulWidget {
    this.magnification = 1.0,
    this.scrollController,
    this.squeeze = _kSqueeze,
-    @required this.itemExtent,
+    required this.itemExtent,
-    @required this.onSelectedItemChanged,
+    required this.onSelectedItemChanged,
-    @required List<Widget> children,
+    required List<Widget> children,
    bool looping = false,
  }) : assert(children != null),
       assert(diameterRatio != null),
@@ -113,7 +111,7 @@ class CupertinoPicker extends StatefulWidget {
  /// (i.e. the picker is going to have a completely transparent background), to match
  /// the native UIPicker and UIDatePicker.
  CupertinoPicker.builder({
-    Key key,
+    Key? key,
    this.diameterRatio = _kDefaultDiameterRatio,
    this.backgroundColor,
    this.offAxisFraction = 0.0,
@@ -121,10 +119,10 @@ class CupertinoPicker extends StatefulWidget {
    this.magnification = 1.0,
    this.scrollController,
    this.squeeze = _kSqueeze,
-    @required this.itemExtent,
+    required this.itemExtent,
-    @required this.onSelectedItemChanged,
+    required this.onSelectedItemChanged,
-    @required IndexedWidgetBuilder itemBuilder,
+    required NullableIndexedWidgetBuilder itemBuilder,
-    int childCount,
+    int? childCount,
  }) : assert(itemBuilder != null),
       assert(diameterRatio != null),
       assert(diameterRatio > 0.0, RenderListWheelViewport.diameterRatioZeroMessage),
@@ -153,7 +151,7 @@ class CupertinoPicker extends StatefulWidget {
  ///
  /// Any alpha value less 255 (fully opaque) will cause the removal of the
  /// wheel list edge fade gradient from rendering of the widget.
-  final Color backgroundColor;
+  final Color? backgroundColor;
  /// {@macro flutter.rendering.wheelList.offAxisFraction}
  final double offAxisFraction;
@@ -168,7 +166,7 @@ class CupertinoPicker extends StatefulWidget {
  /// to set the initial item.
  ///
  /// If null, an implicit one will be created internally.
-  final FixedExtentScrollController scrollController;
+  final FixedExtentScrollController? scrollController;
  /// The uniform height of all children.
  ///
@@ -198,8 +196,8 @@ class CupertinoPicker extends StatefulWidget {
 }
 class _CupertinoPickerState extends State<CupertinoPicker> {
-  int _lastHapticIndex;
+  int? _lastHapticIndex;
-  FixedExtentScrollController _controller;
+  FixedExtentScrollController? _controller;
  @override
  void initState() {
@@ -255,7 +253,7 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
  /// Draws the magnifier borders.
  Widget _buildMagnifierScreen() {
-    final Color resolvedBorderColor = CupertinoDynamicColor.resolve(_kHighlighterBorder, context);
+    final Color resolvedBorderColor = CupertinoDynamicColor.resolve(_kHighlighterBorder, context)!;
    return IgnorePointer(
      child: Center(
@@ -276,7 +274,7 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
  @override
  Widget build(BuildContext context) {
-    final Color resolvedBackgroundColor = CupertinoDynamicColor.resolve(widget.backgroundColor, context);
+    final Color? resolvedBackgroundColor = CupertinoDynamicColor.resolve(widget.backgroundColor, context);
    final Widget result = DefaultTextStyle(
      style: CupertinoTheme.of(context).textTheme.pickerTextStyle,
@@ -284,7 +282,7 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
        children: <Widget>[
          Positioned.fill(
            child: _CupertinoPickerSemantics(
-              scrollController: widget.scrollController ?? _controller,
+              scrollController: widget.scrollController ?? _controller!,
              child: ListWheelScrollView.useDelegate(
                controller: widget.scrollController ?? _controller,
                physics: const FixedExtentScrollPhysics(),
@@ -321,38 +319,45 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
 // scroll controller.
 class _CupertinoPickerSemantics extends SingleChildRenderObjectWidget {
  const _CupertinoPickerSemantics({
-    Key key,
+    Key? key,
-    Widget child,
+    Widget? child,
-    @required this.scrollController,
+    required this.scrollController,
  }) : super(key: key, child: child);
  final FixedExtentScrollController scrollController;
  @override
-  RenderObject createRenderObject(BuildContext context) => _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context));
+  RenderObject createRenderObject(BuildContext context) {
+    assert(debugCheckHasDirectionality(context));
+    return _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context)!);
+  }
  @override
  void updateRenderObject(BuildContext context, covariant _RenderCupertinoPickerSemantics renderObject) {
+    assert(debugCheckHasDirectionality(context));
    renderObject
-      ..textDirection = Directionality.of(context)
+      ..textDirection = Directionality.of(context)!
      ..controller = scrollController;
  }
 }
 class _RenderCupertinoPickerSemantics extends RenderProxyBox {
  _RenderCupertinoPickerSemantics(FixedExtentScrollController controller, this._textDirection) {
-    this.controller = controller;
+    _updateController(null, controller);
  }
  FixedExtentScrollController get controller => _controller;
-  FixedExtentScrollController _controller;
+  late FixedExtentScrollController _controller;
-  set controller(FixedExtentScrollController value) {
+  set controller(FixedExtentScrollController value) => _updateController(_controller, value);
-    if (value == _controller)
+  // This method exists to allow controller to be non-null. It is only called with a null oldValue from construtor.
+  void _updateController(FixedExtentScrollController? oldValue, FixedExtentScrollController value) {
+    if (value == oldValue)
      return;
-    if (_controller != null)
+    if (oldValue != null)
-      _controller.removeListener(_handleScrollUpdate);
+      oldValue.removeListener(_handleScrollUpdate);
    else
-      _currentIndex = value.initialItem ?? 0;
+      _currentIndex = value.initialItem;
    value.addListener(_handleScrollUpdate);
    _controller = value;
  }
@@ -399,15 +404,15 @@ class _RenderCupertinoPickerSemantics extends RenderProxyBox {
    final Map<int, SemanticsNode> indexedChildren = <int, SemanticsNode>{};
    scrollable.visitChildren((SemanticsNode child) {
      assert(child.indexInParent != null);
-      indexedChildren[child.indexInParent] = child;
+      indexedChildren[child.indexInParent!] = child;
      return true;
    });
    if (indexedChildren[_currentIndex] == null) {
      return node.updateWith(config: config);
    }
-    config.value = indexedChildren[_currentIndex].label;
+    config.value = indexedChildren[_currentIndex]!.label;
-    final SemanticsNode previousChild = indexedChildren[_currentIndex - 1];
+    final SemanticsNode? previousChild = indexedChildren[_currentIndex - 1];
-    final SemanticsNode nextChild = indexedChildren[_currentIndex + 1];
+    final SemanticsNode? nextChild = indexedChildren[_currentIndex + 1];
    if (nextChild != null) {
      config.increasedValue = nextChild.label;
      config.onIncrease = _handleIncrease;

--- a/packages/flutter/lib/src/cupertino/refresh.dart
+++ b/packages/flutter/lib/src/cupertino/refresh.dart
@@ -2,8 +2,7 @@
 // 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:math';
 import 'package:flutter/rendering.dart';
@@ -18,10 +17,10 @@ const double _kActivityIndicatorMargin = 16.0;
 class _CupertinoSliverRefresh extends SingleChildRenderObjectWidget {
  const _CupertinoSliverRefresh({
-    Key key,
+    Key? key,
    this.refreshIndicatorLayoutExtent = 0.0,
    this.hasLayoutExtent = false,
-    Widget child,
+    Widget? child,
  }) : assert(refreshIndicatorLayoutExtent != null),
       assert(refreshIndicatorLayoutExtent >= 0.0),
       assert(hasLayoutExtent != null),
@@ -61,9 +60,9 @@ class _CupertinoSliverRefresh extends SingleChildRenderObjectWidget {
 class _RenderCupertinoSliverRefresh extends RenderSliver
    with RenderObjectWithChildMixin<RenderBox> {
  _RenderCupertinoSliverRefresh({
-    @required double refreshIndicatorExtent,
+    required double refreshIndicatorExtent,
-    @required bool hasLayoutExtent,
+    required bool hasLayoutExtent,
-    RenderBox child,
+    RenderBox? child,
  }) : assert(refreshIndicatorExtent != null),
       assert(refreshIndicatorExtent >= 0.0),
       assert(hasLayoutExtent != null),
@@ -135,7 +134,7 @@ class _RenderCupertinoSliverRefresh extends RenderSliver
    // Layout the child giving it the space of the currently dragged overscroll
    // which may or may not include a sliver layout extent space that it will
    // keep after the user lets go during the refresh process.
-    child.layout(
+    child!.layout(
      constraints.asBoxConstraints(
        maxExtent: layoutExtent
            // Plus only the overscrolled portion immediately preceding this
@@ -153,11 +152,11 @@ class _RenderCupertinoSliverRefresh extends RenderSliver
          // layoutExtent may be zero. Check layoutExtent also since even
          // with a layoutExtent, the indicator builder may decide to not
          // build anything.
-          max(child.size.height, layoutExtent) - constraints.scrollOffset,
+          max(child!.size.height, layoutExtent) - constraints.scrollOffset,
          0.0,
        ),
        maxPaintExtent: max(
-          max(child.size.height, layoutExtent) - constraints.scrollOffset,
+          max(child!.size.height, layoutExtent) - constraints.scrollOffset,
          0.0,
        ),
        layoutExtent: max(layoutExtent - constraints.scrollOffset, 0.0),
@@ -171,8 +170,8 @@ class _RenderCupertinoSliverRefresh extends RenderSliver
  @override
  void paint(PaintingContext paintContext, Offset offset) {
    if (constraints.overlap < 0.0 ||
-        constraints.scrollOffset + child.size.height > 0) {
+        constraints.scrollOffset + child!.size.height > 0) {
-      paintContext.paintChild(child, offset);
+      paintContext.paintChild(child!, offset);
    }
  }
@@ -345,7 +344,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
  /// The [onRefresh] argument will be called when pulled far enough to trigger
  /// a refresh.
  const CupertinoSliverRefreshControl({
-    Key key,
+    Key? key,
    this.refreshTriggerPullDistance = _defaultRefreshTriggerPullDistance,
    this.refreshIndicatorExtent = _defaultRefreshIndicatorExtent,
    this.builder = buildRefreshIndicator,
@@ -389,7 +388,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
  ///
  /// Will not be called when the available space is zero such as before any
  /// overscroll.
-  final RefreshControlIndicatorBuilder builder;
+  final RefreshControlIndicatorBuilder? builder;
  /// Callback invoked when pulled by [refreshTriggerPullDistance].
  ///
@@ -399,7 +398,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
  /// Can be null, in which case a single frame of [RefreshIndicatorMode.armed]
  /// state will be drawn before going immediately to the [RefreshIndicatorMode.done]
  /// where the sliver will start retracting.
-  final RefreshCallback onRefresh;
+  final RefreshCallback? onRefresh;
  static const double _defaultRefreshTriggerPullDistance = 100.0;
  static const double _defaultRefreshIndicatorExtent = 60.0;
@@ -408,7 +407,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
  /// state that gets passed into the [builder] function. Used for testing.
  @visibleForTesting
  static RefreshIndicatorMode state(BuildContext context) {
-    final _CupertinoSliverRefreshControlState state = context.findAncestorStateOfType<_CupertinoSliverRefreshControlState>();
+    final _CupertinoSliverRefreshControlState state = context.findAncestorStateOfType<_CupertinoSliverRefreshControlState>()!;
    return state.refreshState;
  }
@@ -483,9 +482,9 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
  // original `refreshTriggerPullDistance` is left.
  static const double _inactiveResetOverscrollFraction = 0.1;
-  RefreshIndicatorMode refreshState;
+  late RefreshIndicatorMode refreshState;
  // [Future] returned by the widget's `onRefresh`.
-  Future<void> refreshTask;
+  Future<void>? refreshTask;
  // The amount of space available from the inner indicator box's perspective.
  //
  // The value is the sum of the sliver's layout extent and the overscroll
@@ -512,10 +511,10 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
      nextState = RefreshIndicatorMode.done;
      // Either schedule the RenderSliver to re-layout on the next frame
      // when not currently in a frame or schedule it on the next frame.
-      if (SchedulerBinding.instance.schedulerPhase == SchedulerPhase.idle) {
+      if (SchedulerBinding.instance!.schedulerPhase == SchedulerPhase.idle) {
        setState(() => hasSliverLayoutExtent = false);
      } else {
-        SchedulerBinding.instance.addPostFrameCallback((Duration timestamp) {
+        SchedulerBinding.instance!.addPostFrameCallback((Duration timestamp) {
          setState(() => hasSliverLayoutExtent = false);
        });
      }
@@ -541,8 +540,8 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
            // Call onRefresh after this frame finished since the function is
            // user supplied and we're always here in the middle of the sliver's
            // performLayout.
-            SchedulerBinding.instance.addPostFrameCallback((Duration timestamp) {
+            SchedulerBinding.instance!.addPostFrameCallback((Duration timestamp) {
-              refreshTask = widget.onRefresh()..whenComplete(() {
+              refreshTask = widget.onRefresh!()..whenComplete(() {
                if (mounted) {
                  setState(() => refreshTask = null);
                  // Trigger one more transition because by this time, BoxConstraint's
@@ -557,9 +556,6 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
          }
          return RefreshIndicatorMode.armed;
        }
-        // Don't continue here. We can never possibly call onRefresh and
-        // progress to the next state in one [computeNextState] call.
-        break;
      case RefreshIndicatorMode.armed:
        if (refreshState == RefreshIndicatorMode.armed && refreshTask == null) {
          goToDone();
@@ -610,7 +606,7 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
          latestIndicatorBoxExtent = constraints.maxHeight;
          refreshState = transitionNextState();
          if (widget.builder != null && latestIndicatorBoxExtent > 0) {
-            return widget.builder(
+            return widget.builder!(
              context,
              refreshState,
              latestIndicatorBoxExtent,

--- a/packages/flutter/lib/src/cupertino/slider.dart
+++ b/packages/flutter/lib/src/cupertino/slider.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
 import 'dart:math' as math;
 import 'dart:ui' show lerpDouble;
@@ -55,9 +53,9 @@ class CupertinoSlider extends StatefulWidget {
  /// * [onChangeEnd] is called when the user is done selecting a new value for
  ///   the slider.
  const CupertinoSlider({
-    Key key,
+    Key? key,
-    @required this.value,
+    required this.value,
-    @required this.onChanged,
+    required this.onChanged,
    this.onChangeStart,
    this.onChangeEnd,
    this.min = 0.0,
@@ -110,7 +108,7 @@ class CupertinoSlider extends StatefulWidget {
  ///    changing the value.
  ///  * [onChangeEnd] for a callback that is called when the user stops
  ///    changing the value.
-  final ValueChanged<double> onChanged;
+  final ValueChanged<double>? onChanged;
  /// Called when the user starts selecting a new value for the slider.
  ///
@@ -145,7 +143,7 @@ class CupertinoSlider extends StatefulWidget {
  ///
  ///  * [onChangeEnd] for a callback that is called when the value change is
  ///    complete.
-  final ValueChanged<double> onChangeStart;
+  final ValueChanged<double>? onChangeStart;
  /// Called when the user is done selecting a new value for the slider.
  ///
@@ -177,7 +175,7 @@ class CupertinoSlider extends StatefulWidget {
  ///
  ///  * [onChangeStart] for a callback that is called when a value change
  ///    begins.
-  final ValueChanged<double> onChangeEnd;
+  final ValueChanged<double>? onChangeEnd;
  /// The minimum value the user can select.
  ///
@@ -192,12 +190,12 @@ class CupertinoSlider extends StatefulWidget {
  /// The number of discrete divisions.
  ///
  /// If null, the slider is continuous.
-  final int divisions;
+  final int? divisions;
  /// The color to use for the portion of the slider that has been selected.
  ///
  /// Defaults to the [CupertinoTheme]'s primary color if null.
-  final Color activeColor;
+  final Color? activeColor;
  /// The color to use for the thumb of the slider.
  ///
@@ -221,20 +219,20 @@ class CupertinoSlider extends StatefulWidget {
 class _CupertinoSliderState extends State<CupertinoSlider> with TickerProviderStateMixin {
  void _handleChanged(double value) {
    assert(widget.onChanged != null);
-    final double lerpValue = lerpDouble(widget.min, widget.max, value);
+    final double lerpValue = lerpDouble(widget.min, widget.max, value)!;
    if (lerpValue != widget.value) {
-      widget.onChanged(lerpValue);
+      widget.onChanged!(lerpValue);
    }
  }
  void _handleDragStart(double value) {
    assert(widget.onChangeStart != null);
-    widget.onChangeStart(lerpDouble(widget.min, widget.max, value));
+    widget.onChangeStart!(lerpDouble(widget.min, widget.max, value)!);
  }
  void _handleDragEnd(double value) {
    assert(widget.onChangeEnd != null);
-    widget.onChangeEnd(lerpDouble(widget.min, widget.max, value));
+    widget.onChangeEnd!(lerpDouble(widget.min, widget.max, value)!);
  }
  @override
@@ -245,7 +243,7 @@ class _CupertinoSliderState extends State<CupertinoSlider> with TickerProviderSt
      activeColor: CupertinoDynamicColor.resolve(
        widget.activeColor ?? CupertinoTheme.of(context).primaryColor,
        context,
-      ),
+      )!,
      thumbColor: widget.thumbColor,
      onChanged: widget.onChanged != null ? _handleChanged : null,
      onChangeStart: widget.onChangeStart != null ? _handleDragStart : null,
@@ -257,54 +255,56 @@ class _CupertinoSliderState extends State<CupertinoSlider> with TickerProviderSt
 class _CupertinoSliderRenderObjectWidget extends LeafRenderObjectWidget {
  const _CupertinoSliderRenderObjectWidget({
-    Key key,
+    Key? key,
-    this.value,
+    required this.value,
    this.divisions,
-    this.activeColor,
+    required this.activeColor,
-    this.thumbColor,
+    required this.thumbColor,
    this.onChanged,
    this.onChangeStart,
    this.onChangeEnd,
-    this.vsync,
+    required this.vsync,
  }) : super(key: key);
  final double value;
-  final int divisions;
+  final int? divisions;
  final Color activeColor;
  final Color thumbColor;
-  final ValueChanged<double> onChanged;
+  final ValueChanged<double>? onChanged;
-  final ValueChanged<double> onChangeStart;
+  final ValueChanged<double>? onChangeStart;
-  final ValueChanged<double> onChangeEnd;
+  final ValueChanged<double>? onChangeEnd;
  final TickerProvider vsync;
  @override
  _RenderCupertinoSlider createRenderObject(BuildContext context) {
+    assert(debugCheckHasDirectionality(context));
    return _RenderCupertinoSlider(
      value: value,
      divisions: divisions,
      activeColor: activeColor,
-      thumbColor: CupertinoDynamicColor.resolve(thumbColor, context),
+      thumbColor: CupertinoDynamicColor.resolve(thumbColor, context)!,
-      trackColor: CupertinoDynamicColor.resolve(CupertinoColors.systemFill, context),
+      trackColor: CupertinoDynamicColor.resolve(CupertinoColors.systemFill, context)!,
      onChanged: onChanged,
      onChangeStart: onChangeStart,
      onChangeEnd: onChangeEnd,
      vsync: vsync,
-      textDirection: Directionality.of(context),
+      textDirection: Directionality.of(context)!,
    );
  }
  @override
  void updateRenderObject(BuildContext context, _RenderCupertinoSlider renderObject) {
+    assert(debugCheckHasDirectionality(context));
    renderObject
      ..value = value
      ..divisions = divisions
      ..activeColor = activeColor
-      ..thumbColor = CupertinoDynamicColor.resolve(thumbColor, context)
+      ..thumbColor = CupertinoDynamicColor.resolve(thumbColor, context)!
-      ..trackColor = CupertinoDynamicColor.resolve(CupertinoColors.systemFill, context)
+      ..trackColor = CupertinoDynamicColor.resolve(CupertinoColors.systemFill, context)!
      ..onChanged = onChanged
      ..onChangeStart = onChangeStart
      ..onChangeEnd = onChangeEnd
-      ..textDirection = Directionality.of(context);
+      ..textDirection = Directionality.of(context)!;
    // Ticker provider cannot change since there's a 1:1 relationship between
    // the _SliderRenderObjectWidget object and the _SliderState object.
  }
@@ -319,16 +319,16 @@ const double _kAdjustmentUnit = 0.1; // Matches iOS implementation of material s
 class _RenderCupertinoSlider extends RenderConstrainedBox {
  _RenderCupertinoSlider({
-    @required double value,
+    required double value,
-    int divisions,
+    int? divisions,
-    Color activeColor,
+    required Color activeColor,
-    Color thumbColor,
+    required Color thumbColor,
-    Color trackColor,
+    required Color trackColor,
-    ValueChanged<double> onChanged,
+    ValueChanged<double>? onChanged,
    this.onChangeStart,
    this.onChangeEnd,
-    TickerProvider vsync,
+    required TickerProvider vsync,
-    @required TextDirection textDirection,
+    required TextDirection textDirection,
  }) : assert(value != null && value >= 0.0 && value <= 1.0),
       assert(textDirection != null),
       _value = value,
@@ -364,9 +364,9 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
    markNeedsSemanticsUpdate();
  }
-  int get divisions => _divisions;
+  int? get divisions => _divisions;
-  int _divisions;
+  int? _divisions;
-  set divisions(int value) {
+  set divisions(int? value) {
    if (value == _divisions)
      return;
    _divisions = value;
@@ -400,9 +400,9 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
    markNeedsPaint();
  }
-  ValueChanged<double> get onChanged => _onChanged;
+  ValueChanged<double>? get onChanged => _onChanged;
-  ValueChanged<double> _onChanged;
+  ValueChanged<double>? _onChanged;
-  set onChanged(ValueChanged<double> value) {
+  set onChanged(ValueChanged<double>? value) {
    if (value == _onChanged)
      return;
    final bool wasInteractive = isInteractive;
@@ -411,8 +411,8 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
      markNeedsSemanticsUpdate();
  }
-  ValueChanged<double> onChangeStart;
+  ValueChanged<double>? onChangeStart;
-  ValueChanged<double> onChangeEnd;
+  ValueChanged<double>? onChangeEnd;
  TextDirection get textDirection => _textDirection;
  TextDirection _textDirection;
@@ -424,15 +424,15 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
    markNeedsPaint();
  }
-  AnimationController _position;
+  late AnimationController _position;
-  HorizontalDragGestureRecognizer _drag;
+  late HorizontalDragGestureRecognizer _drag;
  double _currentDragValue = 0.0;
  double get _discretizedCurrentDragValue {
-    double dragValue = _currentDragValue.clamp(0.0, 1.0) as double;
+    double dragValue = _currentDragValue.clamp(0.0, 1.0);
    if (divisions != null)
-      dragValue = (dragValue * divisions).round() / divisions;
+      dragValue = (dragValue * divisions!).round() / divisions!;
    return dragValue;
  }
@@ -448,7 +448,7 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
        visualPosition = _value;
        break;
    }
-    return lerpDouble(_trackLeft + CupertinoThumbPainter.radius, _trackRight - CupertinoThumbPainter.radius, visualPosition);
+    return lerpDouble(_trackLeft + CupertinoThumbPainter.radius, _trackRight - CupertinoThumbPainter.radius, visualPosition)!;
  }
  bool get isInteractive => onChanged != null;
@@ -458,7 +458,7 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
  void _handleDragUpdate(DragUpdateDetails details) {
    if (isInteractive) {
      final double extent = math.max(_kPadding, size.width - 2.0 * (_kPadding + CupertinoThumbPainter.radius));
-      final double valueDelta = details.primaryDelta / extent;
+      final double valueDelta = details.primaryDelta! / extent;
      switch (textDirection) {
        case TextDirection.rtl:
          _currentDragValue -= valueDelta;
@@ -467,7 +467,7 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
          _currentDragValue += valueDelta;
          break;
      }
-      onChanged(_discretizedCurrentDragValue);
+      onChanged!(_discretizedCurrentDragValue);
    }
  }
@@ -476,16 +476,16 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
  void _startInteraction(Offset globalPosition) {
    if (isInteractive) {
      if (onChangeStart != null) {
-        onChangeStart(_discretizedCurrentDragValue);
+        onChangeStart!(_discretizedCurrentDragValue);
      }
      _currentDragValue = _value;
-      onChanged(_discretizedCurrentDragValue);
+      onChanged!(_discretizedCurrentDragValue);
    }
  }
  void _endInteraction() {
    if (onChangeEnd != null) {
-      onChangeEnd(_discretizedCurrentDragValue);
+      onChangeEnd!(_discretizedCurrentDragValue);
    }
    _currentDragValue = 0.0;
  }
@@ -558,15 +558,15 @@ class _RenderCupertinoSlider extends RenderConstrainedBox {
    }
  }
-  double get _semanticActionUnit => divisions != null ? 1.0 / divisions : _kAdjustmentUnit;
+  double get _semanticActionUnit => divisions != null ? 1.0 / divisions! : _kAdjustmentUnit;
  void _increaseAction() {
    if (isInteractive)
-      onChanged((value + _semanticActionUnit).clamp(0.0, 1.0) as double);
+      onChanged!((value + _semanticActionUnit).clamp(0.0, 1.0));
  }
  void _decreaseAction() {
    if (isInteractive)
-      onChanged((value - _semanticActionUnit).clamp(0.0, 1.0) as double);
+      onChanged!((value - _semanticActionUnit).clamp(0.0, 1.0));
  }
 }
--- a/packages/flutter/lib/src/cupertino/thumb_painter.dart
+++ b/packages/flutter/lib/src/cupertino/thumb_painter.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
 import 'package:flutter/painting.dart';
 import 'colors.dart';

--- a/packages/flutter/lib/src/widgets/list_wheel_scroll_view.dart
+++ b/packages/flutter/lib/src/widgets/list_wheel_scroll_view.dart
@@ -163,7 +163,7 @@ class ListWheelChildBuilderDelegate extends ListWheelChildDelegate {
  }) : assert(builder != null);
  /// Called lazily to build children.
-  final IndexedWidgetBuilder builder;
+  final NullableIndexedWidgetBuilder builder;
  /// {@template flutter.widgets.wheelList.childCount}
  /// If non-null, [childCount] is the maximum number of children that can be

--- a/packages/flutter/lib/src/widgets/navigator.dart
+++ b/packages/flutter/lib/src/widgets/navigator.dart
@@ -633,7 +633,7 @@ class HeroControllerScope extends InheritedWidget {
  /// Creates a widget to host the input [controller].
  const HeroControllerScope({
    Key? key,
-    this.controller,
+    required HeroController this.controller,
    required Widget child,
  }) : assert(controller != null),
       super(key: key, child: child);