Fix bug with back gesture with a persistent bottom sheet showing. (#6498)

Fixes https://github.com/flutter/flutter/issues/6451

packages/flutter/lib/src/material/page.dart

@@ -131,7 +131,7 @@ class _CupertinoBackGestureController extends NavigationGestureController {
   }
 
   @override
-  void dragEnd(double velocity) {
+  bool dragEnd(double velocity) {
     if (velocity.abs() >= _kMinFlingVelocity) {
       controller.fling(velocity: -velocity);
     } else if (controller.value <= 0.5) {
@@ -141,8 +141,11 @@ class _CupertinoBackGestureController extends NavigationGestureController {
     }
 
     // Don't end the gesture until the transition completes.
+    final AnimationStatus status = controller.status;
     handleStatusChanged(controller.status);
     controller?.addStatusListener(handleStatusChanged);
+
+    return (status == AnimationStatus.reverse || status == AnimationStatus.dismissed);
   }
 
   void handleStatusChanged(AnimationStatus status) {

packages/flutter/lib/src/material/scaffold.dart

@@ -820,12 +820,16 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   }
 
   void _handleDragEnd(DragEndDetails details) {
-    _backGestureController?.dragEnd(details.velocity.pixelsPerSecond.dx / context.size.width);
+    final bool willPop = _backGestureController?.dragEnd(details.velocity.pixelsPerSecond.dx / context.size.width) ?? false;
+    if (willPop)
+      _currentBottomSheet?.close();
     _backGestureController = null;
   }
 
   void _handleDragCancel() {
-    _backGestureController?.dragEnd(0.0);
+    final bool willPop = _backGestureController?.dragEnd(0.0) ?? false;
+    if (willPop)
+      _currentBottomSheet?.close();
     _backGestureController = null;
   }
 

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

@@ -218,7 +218,10 @@ abstract class NavigationGestureController {
 
   /// The drag gesture has ended with a horizontal motion of
   /// [fractionalVelocity] as a fraction of screen width per second.
-  void dragEnd(double fractionalVelocity);
+  ///
+  /// Returns true if the gesture will complete (i.e. a back gesture will
+  /// result in a pop).
+  bool dragEnd(double fractionalVelocity);
 }
 
 /// Signature for the [Navigator.popUntil] predicate argument.

packages/flutter/test/widget/page_transitions_test.dart

@@ -13,6 +13,38 @@ class TestOverlayRoute extends OverlayRoute<Null> {
   Widget _build(BuildContext context) => new Text('Overlay');
 }
 
+class PersistentBottomSheetTest extends StatefulWidget {
+  PersistentBottomSheetTest({ Key key }) : super(key: key);
+
+  @override
+  PersistentBottomSheetTestState createState() => new PersistentBottomSheetTestState();
+}
+
+class PersistentBottomSheetTestState extends State<PersistentBottomSheetTest> {
+  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
+
+  bool setStateCalled = false;
+
+  void showBottomSheet() {
+    _scaffoldKey.currentState.showBottomSheet/*<Null>*/((BuildContext context) {
+      return new Text('bottomSheet');
+    })
+    .closed.then((_) {
+      setState(() {
+        setStateCalled = true;
+      });
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new Scaffold(
+      key: _scaffoldKey,
+      body: new Text('Sheet')
+    );
+  }
+}
+
 void main() {
   testWidgets('Check onstage/offstage handling around transitions', (WidgetTester tester) async {
     GlobalKey containerKey1 = new GlobalKey();
@@ -332,4 +364,46 @@ void main() {
     expect(find.text('Home'), isOnstage);
     expect(find.text('Settings'), findsNothing);
   });
+
+  // Tests bug https://github.com/flutter/flutter/issues/6451
+  testWidgets('Check back gesture with a persistent bottom sheet showing', (WidgetTester tester) async {
+    GlobalKey containerKey1 = new GlobalKey();
+    GlobalKey containerKey2 = new GlobalKey();
+    final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
+      '/': (_) => new Scaffold(key: containerKey1, body: new Text('Home')),
+      '/sheet': (_) => new PersistentBottomSheetTest(key: containerKey2),
+    };
+
+    await tester.pumpWidget(new MaterialApp(
+      routes: routes,
+      theme: new ThemeData(platform: TargetPlatform.iOS),
+    ));
+
+    Navigator.pushNamed(containerKey1.currentContext, '/sheet');
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.text('Home'), findsNothing);
+    expect(find.text('Sheet'), isOnstage);
+
+    // Show the bottom sheet.
+    PersistentBottomSheetTestState sheet = containerKey2.currentState;
+    sheet.showBottomSheet();
+
+    await tester.pump(const Duration(seconds: 1));
+
+    // Drag from left edge to invoke the gesture.
+    TestGesture gesture = await tester.startGesture(new Point(5.0, 100.0));
+    await gesture.moveBy(new Offset(500.0, 0.0));
+    await gesture.up();
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.text('Home'), isOnstage);
+    expect(find.text('Sheet'), findsNothing);
+
+    // Sheet called setState and didn't crash.
+    expect(sheet.setStateCalled, isTrue);
+  });
 }