migrate animation to nullsafety (#62485)

* migrate animation to nullsafety

* nullable tween

* fix generic type issue

* address review comment

packages/flutter/lib/animation.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 animation system.
 ///
 /// To use, import `package:flutter/animation.dart`.

packages/flutter/lib/src/animation/animation.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';
 

packages/flutter/lib/src/animation/curves.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:ui';
@@ -526,7 +525,7 @@ abstract class Curve2D extends ParametricCurve<Offset> {
     assert(x != null);
     double start = 0.0;
     double end = 1.0;
-    double mid;
+    late double mid;
     double offsetToOrigin(double pos) => x - transform(pos).dx;
     // Use a binary search to find the inverse point within 1e-6, or 100
     // subdivisions, whichever comes first.
@@ -626,8 +625,8 @@ class CatmullRomSpline extends Curve2D {
   CatmullRomSpline(
       List<Offset> controlPoints, {
         double tension = 0.0,
-        Offset startHandle,
-        Offset endHandle,
+        Offset? startHandle,
+        Offset? endHandle,
       }) : assert(controlPoints != null),
            assert(tension != null),
            assert(tension <= 1.0, 'tension $tension must not be greater than 1.0.'),
@@ -646,8 +645,8 @@ class CatmullRomSpline extends Curve2D {
   CatmullRomSpline.precompute(
       List<Offset> controlPoints, {
         double tension = 0.0,
-        Offset startHandle,
-        Offset endHandle,
+        Offset? startHandle,
+        Offset? endHandle,
       }) : assert(controlPoints != null),
            assert(tension != null),
            assert(tension <= 1.0, 'tension $tension must not be greater than 1.0.'),
@@ -663,8 +662,8 @@ class CatmullRomSpline extends Curve2D {
   static List<List<Offset>> _computeSegments(
       List<Offset> controlPoints,
       double tension, {
-      Offset startHandle,
-      Offset endHandle,
+      Offset? startHandle,
+      Offset? endHandle,
     }) {
     // If not specified, select the first and last control points (which are
     // handles: they are not intersected by the resulting curve) so that they
@@ -713,17 +712,17 @@ class CatmullRomSpline extends Curve2D {
   final List<List<Offset>> _cubicSegments;
 
   // This is non-empty only if the _cubicSegments are being computed lazily.
-  final List<Offset> _controlPoints;
-  final Offset _startHandle;
-  final Offset _endHandle;
-  final double _tension;
+  final List<Offset>? _controlPoints;
+  final Offset? _startHandle;
+  final Offset? _endHandle;
+  final double? _tension;
 
   void _initializeIfNeeded() {
     if (_cubicSegments.isNotEmpty) {
       return;
     }
     _cubicSegments.addAll(
-      _computeSegments(_controlPoints, _tension, startHandle: _startHandle, endHandle: _endHandle),
+      _computeSegments(_controlPoints!, _tension!, startHandle: _startHandle, endHandle: _endHandle),
     );
   }
 
@@ -905,9 +904,9 @@ class CatmullRomCurve extends Curve {
   /// In release mode, this function can be used to decide if a proposed
   /// modification to the curve will result in a valid curve.
   static bool validateControlPoints(
-      List<Offset> controlPoints, {
+      List<Offset>? controlPoints, {
       double tension = 0.0,
-      List<String> reasons,
+      List<String>? reasons,
     }) {
     assert(tension != null);
     if (controlPoints == null) {
@@ -937,7 +936,7 @@ class CatmullRomCurve extends Curve {
           (controlPoints[i].dx <= 0.0 || controlPoints[i].dx >= 1.0)) {
         assert(() {
           reasons?.add('Control points must have X values between 0.0 and 1.0, exclusive. '
-              'Point $i has an x value (${controlPoints[i].dx}) which is outside the range.');
+              'Point $i has an x value (${controlPoints![i].dx}) which is outside the range.');
           return true;
         }());
         return false;
@@ -946,7 +945,7 @@ class CatmullRomCurve extends Curve {
         assert(() {
           reasons?.add('Each X coordinate must be greater than the preceding X coordinate '
               '(i.e. must be monotonically increasing in X). Point $i has an x value of '
-              '${controlPoints[i].dx}, which is not greater than $lastX');
+              '${controlPoints![i].dx}, which is not greater than $lastX');
           return true;
         }());
         return false;
@@ -1053,7 +1052,7 @@ class CatmullRomCurve extends Curve {
 
     // Now interpolate between the found sample and the next one.
     final double t2 = (t - startValue.dx) / (endValue.dx - startValue.dx);
-    return lerpDouble(startValue.dy, endValue.dy, t2);
+    return lerpDouble(startValue.dy, endValue.dy, t2)!;
   }
 }
 

packages/flutter/lib/src/animation/listener_helpers.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';
 
@@ -121,7 +120,7 @@ mixin AnimationLocalListenersMixin {
   void notifyListeners() {
     final List<VoidCallback> localListeners = List<VoidCallback>.from(_listeners);
     for (final VoidCallback listener in localListeners) {
-      InformationCollector collector;
+      InformationCollector? collector;
       assert(() {
         collector = () sync* {
           yield DiagnosticsProperty<AnimationLocalListenersMixin>(
@@ -199,7 +198,7 @@ mixin AnimationLocalStatusListenersMixin {
         if (_statusListeners.contains(listener))
           listener(status);
       } catch (exception, stack) {
-        InformationCollector collector;
+        InformationCollector? collector;
         assert(() {
           collector = () sync* {
             yield DiagnosticsProperty<AnimationLocalStatusListenersMixin>(

packages/flutter/lib/src/animation/tween.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 Color, Size, Rect;
 
@@ -215,19 +214,22 @@ class Tween<T extends dynamic> extends Animatable<T> {
   /// The [begin] and [end] properties must be non-null before the tween is
   /// first used, but the arguments can be null if the values are going to be
   /// filled in later.
-  Tween({ this.begin, this.end });
+  Tween({
+    this.begin,
+    this.end,
+  });
 
   /// The value this variable has at the beginning of the animation.
   ///
   /// See the constructor for details about whether this property may be null
   /// (it varies from subclass to subclass).
-  T begin;
+  T? begin;
 
   /// The value this variable has at the end of the animation.
   ///
   /// See the constructor for details about whether this property may be null
   /// (it varies from subclass to subclass).
-  T end;
+  T? end;
 
   /// Returns the value this variable has at the given animation clock value.
   ///
@@ -256,9 +258,9 @@ class Tween<T extends dynamic> extends Animatable<T> {
   @override
   T transform(double t) {
     if (t == 0.0)
-      return begin;
+      return begin as T;
     if (t == 1.0)
-      return end;
+      return end as T;
     return lerp(t);
   }
 
@@ -290,7 +292,7 @@ class ReverseTween<T> extends Tween<T> {
 /// [Color.lerp].
 ///
 /// See [Tween] for a discussion on how to use interpolation objects.
-class ColorTween extends Tween<Color> {
+class ColorTween extends Tween<Color?> {
   /// Creates a [Color] tween.
   ///
   /// The [begin] and [end] properties may be null; the null value
@@ -300,11 +302,11 @@ class ColorTween extends Tween<Color> {
   /// or [end] if you want the effect of fading in or out of transparent.
   /// Instead prefer null. [Colors.transparent] refers to black transparent and
   /// thus will fade out of or into black which is likely unwanted.
-  ColorTween({ Color begin, Color end }) : super(begin: begin, end: end);
+  ColorTween({ Color? begin, Color? end }) : super(begin: begin, end: end);
 
   /// Returns the value this variable has at the given animation clock value.
   @override
-  Color lerp(double t) => Color.lerp(begin, end, t);
+  Color? lerp(double t) => Color.lerp(begin, end, t);
 }
 
 /// An interpolation between two sizes.
@@ -313,16 +315,16 @@ class ColorTween extends Tween<Color> {
 /// [Size.lerp].
 ///
 /// See [Tween] for a discussion on how to use interpolation objects.
-class SizeTween extends Tween<Size> {
+class SizeTween extends Tween<Size?> {
   /// Creates a [Size] tween.
   ///
   /// The [begin] and [end] properties may be null; the null value
   /// is treated as an empty size.
-  SizeTween({ Size begin, Size end }) : super(begin: begin, end: end);
+  SizeTween({ Size? begin, Size? end }) : super(begin: begin, end: end);
 
   /// Returns the value this variable has at the given animation clock value.
   @override
-  Size lerp(double t) => Size.lerp(begin, end, t);
+  Size? lerp(double t) => Size.lerp(begin, end, t);
 }
 
 /// An interpolation between two rectangles.
@@ -331,16 +333,16 @@ class SizeTween extends Tween<Size> {
 /// [Rect.lerp].
 ///
 /// See [Tween] for a discussion on how to use interpolation objects.
-class RectTween extends Tween<Rect> {
+class RectTween extends Tween<Rect?> {
   /// Creates a [Rect] tween.
   ///
   /// The [begin] and [end] properties may be null; the null value
   /// is treated as an empty rect at the top left corner.
-  RectTween({ Rect begin, Rect end }) : super(begin: begin, end: end);
+  RectTween({ Rect? begin, Rect? end }) : super(begin: begin, end: end);
 
   /// Returns the value this variable has at the given animation clock value.
   @override
-  Rect lerp(double t) => Rect.lerp(begin, end, t);
+  Rect? lerp(double t) => Rect.lerp(begin, end, t);
 }
 
 /// An interpolation between two integers that rounds.
@@ -360,12 +362,12 @@ class IntTween extends Tween<int> {
   /// The [begin] and [end] properties must be non-null before the tween is
   /// first used, but the arguments can be null if the values are going to be
   /// filled in later.
-  IntTween({ int begin, int end }) : super(begin: begin, end: end);
+  IntTween({ int? begin, int? end }) : super(begin: begin, end: end);
 
   // The inherited lerp() function doesn't work with ints because it multiplies
   // the begin and end types by a double, and int * double returns a double.
   @override
-  int lerp(double t) => (begin + (end - begin) * t).round();
+  int lerp(double t) => (begin! + (end! - begin!) * t).round();
 }
 
 /// An interpolation between two integers that floors.
@@ -385,12 +387,12 @@ class StepTween extends Tween<int> {
   /// The [begin] and [end] properties must be non-null before the tween is
   /// first used, but the arguments can be null if the values are going to be
   /// filled in later.
-  StepTween({ int begin, int end }) : super(begin: begin, end: end);
+  StepTween({ int? begin, int? end }) : super(begin: begin, end: end);
 
   // The inherited lerp() function doesn't work with ints because it multiplies
   // the begin and end types by a double, and int * double returns a double.
   @override
-  int lerp(double t) => (begin + (end - begin) * t).floor();
+  int lerp(double t) => (begin! + (end! - begin!) * t).floor();
 }
 
 /// A tween with a constant value.
@@ -400,7 +402,7 @@ class ConstantTween<T> extends Tween<T> {
 
   /// This tween doesn't interpolate, it always returns the same value.
   @override
-  T lerp(double t) => begin;
+  T lerp(double t) => begin as T;
 
   @override
   String toString() => '${objectRuntimeType(this, 'ReverseTween')}(value: $begin)';
@@ -436,7 +438,7 @@ class CurveTween extends Animatable<double> {
   /// Creates a curve tween.
   ///
   /// The [curve] argument must not be null.
-  CurveTween({ @required this.curve })
+  CurveTween({ required this.curve })
     : assert(curve != null);
 
   /// The curve to use when transforming the value of the animation.

packages/flutter/lib/src/animation/tween_sequence.dart

@@ -2,9 +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 'animation.dart';
 import 'tween.dart';
@@ -90,8 +87,7 @@ class TweenSequence<T> extends Animatable<T> {
         return _evaluateAt(t, index);
     }
     // Should be unreachable.
-    assert(false, 'TweenSequence.evaluate() could not find an interval for $t');
-    return null;
+    throw StateError('TweenSequence.evaluate() could not find an interval for $t');
   }
 
   @override
@@ -128,8 +124,8 @@ class TweenSequenceItem<T> {
   ///
   /// The [tween] must not be null and [weight] must be greater than 0.0.
   const TweenSequenceItem({
-    @required this.tween,
-    @required this.weight,
+    required this.tween,
+    required this.weight,
   }) : assert(tween != null),
        assert(weight != null),
        assert(weight > 0.0);