Use slide-from-right transition only when doing back gesture

This eliminates all the issues around the horizontal page transition. I also improved the back gesture animation, and added fling support.

BUG=https://github.com/flutter/flutter/issues/5678
BUG=https://github.com/flutter/flutter/issues/5622

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

@@ -8,6 +8,8 @@ import 'package:flutter/widgets.dart';
 import 'material.dart';
 import 'theme.dart';
 
+const double _kMinFlingVelocity = 1.0;  // screen width per second
+
 // Used for Android and Fuchsia.
 class _MountainViewPageTransition extends AnimatedWidget {
   static final FractionalOffsetTween _kTween = new FractionalOffsetTween(
@@ -48,14 +50,13 @@ class _CupertinoPageTransition extends AnimatedWidget {
 
   _CupertinoPageTransition({
     Key key,
-    Curve curve,
     Animation<double> animation,
     this.child
   }) : super(
     key: key,
     animation: _kTween.animate(new CurvedAnimation(
       parent: animation,
-      curve: new _CupertinoTransitionCurve(curve)
+      curve: new _CupertinoTransitionCurve(null)
     )
   ));
 
@@ -142,18 +143,23 @@ class _CupertinoBackGestureController extends NavigationGestureController {
   }
 
   @override
-  void dragEnd() {
-    if (controller.value <= 0.5) {
-      navigator.pop();
+  void dragEnd(double velocity) {
+    if (velocity.abs() >= _kMinFlingVelocity) {
+      controller.fling(velocity: -velocity);
+    } else if (controller.value <= 0.5) {
+      controller.fling(velocity: -1.0);
     } else {
-      controller.forward();
+      controller.fling(velocity: 1.0);
     }
+
     // Don't end the gesture until the transition completes.
     handleStatusChanged(controller.status);
     controller?.addStatusListener(handleStatusChanged);
   }
 
   void handleStatusChanged(AnimationStatus status) {
+    if (status == AnimationStatus.dismissed)
+      navigator.pop();
     if (status == AnimationStatus.dismissed || status == AnimationStatus.completed)
       dispose();
   }
@@ -164,9 +170,6 @@ class _CupertinoBackGestureController extends NavigationGestureController {
 /// The entrance transition for the page slides the page upwards and fades it
 /// in. The exit transition is the same, but in reverse.
 ///
-/// [MaterialApp] creates material page routes for entries in the
-/// [MaterialApp.routes] map.
-///
 /// By default, when a modal route is replaced by another, the previous route
 /// remains in memory. To free all the resources when this is not necessary, set
 /// [maintainState] to false.
@@ -235,33 +238,18 @@ class MaterialPageRoute<T> extends PageRoute<T> {
 
   @override
   Widget buildTransitions(BuildContext context, Animation<double> animation, Animation<double> forwardAnimation, Widget child) {
-    // TODO(mpcomplete): This hack prevents the previousRoute from animating
-    // when we pop(). Remove once we fix this bug:
-    // https://github.com/flutter/flutter/issues/5577
-    bool userGesture = Navigator.of(context).userGestureInProgress;
-    if (!userGesture)
-      forwardAnimation = kAlwaysDismissedAnimation;
-
-    ThemeData theme = Theme.of(context);
-    switch (theme.platform) {
-      case TargetPlatform.fuchsia:
-      case TargetPlatform.android:
-        return new _MountainViewPageTransition(
-          animation: animation,
-          child: child
-        );
-      case TargetPlatform.iOS:
-        return new _CupertinoPageTransition(
-          // Use a linear curve when controlled by a user gesture. This ensures
-          // the animation tracks the user's finger 1:1.
-          // See https://github.com/flutter/flutter/issues/5664
-          curve: userGesture ? null : Curves.fastOutSlowIn,
-          animation: new AnimationMean(left: animation, right: forwardAnimation),
-          child: child
-        );
+    if (Theme.of(context).platform == TargetPlatform.iOS &&
+        Navigator.of(context).userGestureInProgress) {
+      return new _CupertinoPageTransition(
+        animation: new AnimationMean(left: animation, right: forwardAnimation),
+        child: child
+      );
+    } else {
+      return new _MountainViewPageTransition(
+        animation: animation,
+        child: child
+      );
     }
-
-    return null;
   }
 
   @override

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

@@ -710,12 +710,13 @@ class ScaffoldState extends State<Scaffold> {
   }
 
   void _handleDragEnd(DragEndDetails details) {
-    _backGestureController?.dragEnd();
+    final RenderBox box = context.findRenderObject();
+    _backGestureController?.dragEnd(details.velocity.pixelsPerSecond.dx / box.size.width);
     _backGestureController = null;
   }
 
   void _handleDragCancel() {
-    _backGestureController?.dragEnd();
+    _backGestureController?.dragEnd(0.0);
     _backGestureController = null;
   }
 

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

@@ -176,8 +176,9 @@ abstract class NavigationGestureController {
   // drag should be 0.0 to 1.0.
   void dragUpdate(double fractionalDelta);
 
-  // The drag gesture has ended.
-  void dragEnd();
+  // The drag gesture has ended with a horizontal motion of
+  // [fractionalVelocity] as a fraction of screen width per second.
+  void dragEnd(double fractionalVelocity);
 
   @protected
   NavigatorState get navigator => _navigator;

packages/flutter/test/widget/page_transitions_test.dart

@@ -155,4 +155,135 @@ void main() {
     expect(find.text('Home'), findsNothing);
     expect(find.text('Settings'), isOnstage);
   });
+
+  testWidgets('Check page transition positioning on iOS', (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')),
+      '/settings': (_) => new Scaffold(key: containerKey2, body: new Text('Settings')),
+    };
+
+    await tester.pumpWidget(new MaterialApp(
+      routes: routes,
+      theme: new ThemeData(platform: TargetPlatform.iOS),
+    ));
+
+    Navigator.pushNamed(containerKey1.currentContext, '/settings');
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 16));
+
+    expect(find.text('Home'), isOnstage);
+    expect(find.text('Settings'), isOnstage);
+
+    // Home page is staying in place.
+    Point homeOffset = tester.getTopLeft(find.text('Home'));
+    expect(homeOffset.x, 0.0);
+    expect(homeOffset.y, 0.0);
+
+    // Settings page is sliding up from the bottom.
+    Point settingsOffset = tester.getTopLeft(find.text('Settings'));
+    expect(settingsOffset.x, 0.0);
+    expect(settingsOffset.y, greaterThan(0.0));
+
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.text('Home'), findsNothing);
+    expect(find.text('Settings'), isOnstage);
+
+    // Settings page is in position.
+    settingsOffset = tester.getTopLeft(find.text('Settings'));
+    expect(settingsOffset.x, 0.0);
+    expect(settingsOffset.y, 0.0);
+
+    Navigator.pop(containerKey1.currentContext);
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 16));
+
+    // Home page is staying in place.
+    homeOffset = tester.getTopLeft(find.text('Home'));
+    expect(homeOffset.x, 0.0);
+    expect(homeOffset.y, 0.0);
+
+    // Settings page is sliding down off the bottom.
+    settingsOffset = tester.getTopLeft(find.text('Settings'));
+    expect(settingsOffset.x, 0.0);
+    expect(settingsOffset.y, greaterThan(0.0));
+
+    await tester.pump(const Duration(seconds: 1));
+  });
+
+  testWidgets('Check back gesture disables Heroes', (WidgetTester tester) async {
+    GlobalKey containerKey1 = new GlobalKey();
+    GlobalKey containerKey2 = new GlobalKey();
+    const String kHeroTag = 'hero';
+    final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
+      '/': (_) => new Scaffold(
+        key: containerKey1,
+        body: new Container(
+          decoration: new BoxDecoration(backgroundColor: const Color(0xff00ffff)),
+          child: new Hero(
+            tag: kHeroTag,
+            child: new Text('Home')
+          )
+        )
+      ),
+      '/settings': (_) => new Scaffold(
+        key: containerKey2,
+        body: new Container(
+          padding: const EdgeInsets.all(100.0),
+          decoration: new BoxDecoration(backgroundColor: const Color(0xffff00ff)),
+          child: new Hero(
+            tag: kHeroTag,
+            child: new Text('Settings')
+          )
+        )
+      ),
+    };
+
+    await tester.pumpWidget(new MaterialApp(
+      routes: routes,
+      theme: new ThemeData(platform: TargetPlatform.iOS),
+    ));
+
+    Navigator.pushNamed(containerKey1.currentContext, '/settings');
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 16));
+
+    expect(find.text('Settings'), isOnstage);
+
+    // Settings text is heroing to its new location
+    Point settingsOffset = tester.getTopLeft(find.text('Settings'));
+    expect(settingsOffset.x, greaterThan(0.0));
+    expect(settingsOffset.x, lessThan(100.0));
+    expect(settingsOffset.y, greaterThan(0.0));
+    expect(settingsOffset.y, lessThan(100.0));
+
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.text('Home'), findsNothing);
+    expect(find.text('Settings'), isOnstage);
+
+    // Drag from left edge to invoke the gesture.
+    TestGesture gesture = await tester.startGesture(new Point(5.0, 100.0));
+    await gesture.moveBy(new Offset(50.0, 0.0));
+    await tester.pump();
+
+    // Home is now visible.
+    expect(find.text('Home'), isOnstage);
+    expect(find.text('Settings'), isOnstage);
+
+    // Home page is sliding in from the left, no heroes.
+    Point homeOffset = tester.getTopLeft(find.text('Home'));
+    expect(homeOffset.x, lessThan(0.0));
+    expect(homeOffset.y, 0.0);
+
+    // Settings page is sliding off to the right, no heroes.
+    settingsOffset = tester.getTopLeft(find.text('Settings'));
+    expect(settingsOffset.x, greaterThan(100.0));
+    expect(settingsOffset.y, 100.0);
+  });
 }