Allow for customizable ModalRoute barrierTween (#48345)

packages/flutter/lib/src/widgets/routes.dart

@@ -998,7 +998,7 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   ///
   /// If [barrierDismissible] is false, then tapping the barrier has no effect.
   ///
-  /// If this getter would ever start returning a different color,
+  /// If this getter would ever start returning a different value,
   /// [changedInternalState] should be invoked so that the change can take
   /// effect.
   ///
@@ -1062,7 +1062,7 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   /// For example, when a dialog is on the screen, the page below the dialog is
   /// usually darkened by the modal barrier.
   ///
-  /// If this getter would ever start returning a different color,
+  /// If this getter would ever start returning a different label,
   /// [changedInternalState] should be invoked so that the change can take
   /// effect.
   ///
@@ -1073,6 +1073,32 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   ///  * [ModalBarrier], the widget that implements this feature.
   String get barrierLabel;
 
+  /// The curve that is used for animating the modal barrier in and out.
+  ///
+  /// The modal barrier is the scrim that is rendered behind each route, which
+  /// generally prevents the user from interacting with the route below the
+  /// current route, and normally partially obscures such routes.
+  ///
+  /// For example, when a dialog is on the screen, the page below the dialog is
+  /// usually darkened by the modal barrier.
+  ///
+  /// While the route is animating into position, the color is animated from
+  /// transparent to the specified [barrierColor].
+  ///
+  /// If this getter would ever start returning a different curve,
+  /// [changedInternalState] should be invoked so that the change can take
+  /// effect.
+  ///
+  /// It defaults to [Curves.ease].
+  ///
+  /// See also:
+  ///
+  ///  * [barrierColor], which determines the color that the modal transitions
+  ///    to.
+  ///  * [Curves] for a collection of common curves.
+  ///  * [AnimatedModalBarrier], the widget that implements this feature.
+  Curve get barrierCurve => Curves.ease;
+
   /// Whether the route should remain in memory when it is inactive.
   ///
   /// If this is true, then the route is maintained, so that any futures it is
@@ -1282,30 +1308,28 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   final GlobalKey _subtreeKey = GlobalKey();
   final PageStorageBucket _storageBucket = PageStorageBucket();
 
-  static final Animatable<double> _easeCurveTween = CurveTween(curve: Curves.ease);
-
   // one of the builders
   OverlayEntry _modalBarrier;
   Widget _buildModalBarrier(BuildContext context) {
     Widget barrier;
-    if (barrierColor != null && !offstage) { // changedInternalState is called if these update
+    if (barrierColor != null && !offstage) { // changedInternalState is called if barrierColor or offstage updates
       assert(barrierColor != _kTransparent);
       final Animation<Color> color = animation.drive(
         ColorTween(
           begin: _kTransparent,
-          end: barrierColor, // changedInternalState is called if this updates
+          end: barrierColor, // changedInternalState is called if barrierColor updates
-        ).chain(_easeCurveTween),
+        ).chain(CurveTween(curve: barrierCurve)), // changedInternalState is called if barrierCurve updates
       );
       barrier = AnimatedModalBarrier(
         color: color,
-        dismissible: barrierDismissible, // changedInternalState is called if this updates
+        dismissible: barrierDismissible, // changedInternalState is called if barrierDismissible updates
-        semanticsLabel: barrierLabel, // changedInternalState is called if this updates
+        semanticsLabel: barrierLabel, // changedInternalState is called if barrierLabel updates
         barrierSemanticsDismissible: semanticsDismissible,
       );
     } else {
       barrier = ModalBarrier(
-        dismissible: barrierDismissible, // changedInternalState is called if this updates
+        dismissible: barrierDismissible, // changedInternalState is called if barrierDismissible updates
-        semanticsLabel: barrierLabel, // changedInternalState is called if this updates
+        semanticsLabel: barrierLabel, // changedInternalState is called if barrierLabel updates
         barrierSemanticsDismissible: semanticsDismissible,
       );
     }
@@ -1316,7 +1340,7 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
       );
     }
     return IgnorePointer(
-      ignoring: animation.status == AnimationStatus.reverse || // changedInternalState is called when this updates
+      ignoring: animation.status == AnimationStatus.reverse || // changedInternalState is called when animation.status updates
                 animation.status == AnimationStatus.dismissed, // dismissed is possible when doing a manual pop gesture
       child: barrier,
     );

packages/flutter/test/widgets/routes_test.dart

@@ -1034,6 +1034,133 @@ void main() {
       expect(find.byKey(containerKey), findsNothing);
     });
   });
+
+  group('ModalRoute', () {
+    testWidgets('default barrierCurve', (WidgetTester tester) async {
+      await tester.pumpWidget(MaterialApp(
+        home: Material(
+          child: Builder(
+            builder: (BuildContext context) {
+              return Center(
+                child: RaisedButton(
+                  child: const Text('X'),
+                  onPressed: () {
+                    Navigator.of(context).push<void>(
+                      _TestDialogRouteWithCustomBarrierCurve<void>(
+                        child: const Text('Hello World'),
+                      )
+                    );
+                  },
+                ),
+              );
+            }
+          ),
+        ),
+      ));
+
+      final CurveTween _defaultBarrierTween = CurveTween(curve: Curves.ease);
+      int _getExpectedBarrierTweenAlphaValue(double t) {
+        return Color.getAlphaFromOpacity(_defaultBarrierTween.transform(t));
+      }
+
+      await tester.tap(find.text('X'));
+      await tester.pump();
+      final Finder animatedModalBarrier = find.byType(AnimatedModalBarrier);
+      expect(animatedModalBarrier, findsOneWidget);
+
+      Animation<Color> modalBarrierAnimation;
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(modalBarrierAnimation.value, Colors.transparent);
+
+      await tester.pump(const Duration(milliseconds: 25));
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(
+        modalBarrierAnimation.value.alpha,
+        closeTo(_getExpectedBarrierTweenAlphaValue(0.25), 1.0),
+      );
+
+      await tester.pump(const Duration(milliseconds: 25));
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(
+        modalBarrierAnimation.value.alpha,
+        closeTo(_getExpectedBarrierTweenAlphaValue(0.50), 1.0),
+      );
+
+      await tester.pump(const Duration(milliseconds: 25));
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(
+        modalBarrierAnimation.value.alpha,
+        closeTo(_getExpectedBarrierTweenAlphaValue(0.75), 1.0),
+      );
+
+      await tester.pumpAndSettle();
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(modalBarrierAnimation.value, Colors.black);
+    });
+
+    testWidgets('custom barrierCurve', (WidgetTester tester) async {
+      await tester.pumpWidget(MaterialApp(
+        home: Material(
+          child: Builder(
+            builder: (BuildContext context) {
+              return Center(
+                child: RaisedButton(
+                  child: const Text('X'),
+                  onPressed: () {
+                    Navigator.of(context).push<void>(
+                      _TestDialogRouteWithCustomBarrierCurve<void>(
+                        child: const Text('Hello World'),
+                        barrierCurve: Curves.linear,
+                      ),
+                    );
+                  },
+                ),
+              );
+            },
+          ),
+        ),
+      ));
+
+      final CurveTween _customBarrierTween = CurveTween(curve: Curves.linear);
+      int _getExpectedBarrierTweenAlphaValue(double t) {
+        return Color.getAlphaFromOpacity(_customBarrierTween.transform(t));
+      }
+
+      await tester.tap(find.text('X'));
+      await tester.pump();
+      final Finder animatedModalBarrier = find.byType(AnimatedModalBarrier);
+      expect(animatedModalBarrier, findsOneWidget);
+
+      Animation<Color> modalBarrierAnimation;
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(modalBarrierAnimation.value, Colors.transparent);
+
+      await tester.pump(const Duration(milliseconds: 25));
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(
+        modalBarrierAnimation.value.alpha,
+        closeTo(_getExpectedBarrierTweenAlphaValue(0.25), 1.0),
+      );
+
+      await tester.pump(const Duration(milliseconds: 25));
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(
+        modalBarrierAnimation.value.alpha,
+        closeTo(_getExpectedBarrierTweenAlphaValue(0.50), 1.0),
+      );
+
+      await tester.pump(const Duration(milliseconds: 25));
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(
+        modalBarrierAnimation.value.alpha,
+        closeTo(_getExpectedBarrierTweenAlphaValue(0.75), 1.0),
+      );
+
+      await tester.pumpAndSettle();
+      modalBarrierAnimation = tester.widget<AnimatedModalBarrier>(animatedModalBarrier).color;
+      expect(modalBarrierAnimation.value, Colors.black);
+    });
+  });
 }
 
 double _getOpacity(GlobalKey key, WidgetTester tester) {
@@ -1089,3 +1216,43 @@ class DialogObserver extends NavigatorObserver {
     super.didPush(route, previousRoute);
   }
 }
+
+class _TestDialogRouteWithCustomBarrierCurve<T> extends PopupRoute<T> {
+  _TestDialogRouteWithCustomBarrierCurve({
+    @required Widget child,
+    Curve barrierCurve,
+  }) : _barrierCurve = barrierCurve,
+       _child = child;
+
+  final Widget _child;
+
+  @override
+  bool get barrierDismissible => true;
+
+  @override
+  String get barrierLabel => null;
+
+  @override
+  Color get barrierColor => Colors.black; // easier value to test against
+
+  @override
+  Curve get barrierCurve {
+    if (_barrierCurve == null) {
+      return super.barrierCurve;
+    }
+    return _barrierCurve;
+  }
+  final Curve _barrierCurve;
+
+  @override
+  Duration get transitionDuration => const Duration(milliseconds: 100); // easier value to test against
+
+  @override
+  Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+    return Semantics(
+      child: _child,
+      scopesRoute: true,
+      explicitChildNodes: true,
+    );
+  }
+}