Make Cupertino page transition linear during drag (#9167)

* Just realized the creation order is the other way around

* Works

* Revert previous partial solution

* Change pumpAndSettle to pump for gestures

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

@@ -36,15 +36,19 @@ class CupertinoPageTransition extends AnimatedWidget {
     // one.
     @required Animation<double> outgoingRouteAnimation,
     @required this.child,
+    // Perform incoming transition linearly. Use to precisely track back gesture drags.
+    bool linearTransition,
   }) :
-      incomingPositionAnimation = _kRightMiddleTween.animate(
-        new CurvedAnimation(
-          parent: incomingRouteAnimation,
-          curve: Curves.easeOut,
-          reverseCurve: Curves.easeIn,
-        )
-      ),
-      outgoingPositionAnimation = _kMiddleLeftTween.animate(
+      _incomingPositionAnimation = linearTransition
+        ? _kRightMiddleTween.animate(incomingRouteAnimation)
+        : _kRightMiddleTween.animate(
+            new CurvedAnimation(
+              parent: incomingRouteAnimation,
+              curve: Curves.easeOut,
+              reverseCurve: Curves.easeIn,
+            )
+          ),
+      _outgoingPositionAnimation = _kMiddleLeftTween.animate(
         new CurvedAnimation(
           parent: outgoingRouteAnimation,
           curve: Curves.easeOut,
@@ -60,19 +64,20 @@ class CupertinoPageTransition extends AnimatedWidget {
       );
 
   // When this page is coming in to cover another page.
-  final Animation<FractionalOffset> incomingPositionAnimation;
+  final Animation<FractionalOffset> _incomingPositionAnimation;
   // When this page is becoming covered by another page.
-  final Animation<FractionalOffset> outgoingPositionAnimation;
+  final Animation<FractionalOffset> _outgoingPositionAnimation;
   final Widget child;
 
+
   @override
   Widget build(BuildContext context) {
     // TODO(ianh): tell the transform to be un-transformed for hit testing
     // but not while being controlled by a gesture.
     return new SlideTransition(
-      position: outgoingPositionAnimation,
+      position: _outgoingPositionAnimation,
       child: new SlideTransition(
-        position: incomingPositionAnimation,
+        position: _incomingPositionAnimation,
         child: new PhysicalModel(
           shape: BoxShape.rectangle,
           color: _kBackgroundColor,

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

@@ -176,6 +176,9 @@ class MaterialPageRoute<T> extends PageRoute<T> {
           incomingRouteAnimation: animation,
           outgoingRouteAnimation: forwardAnimation,
           child: child,
+          // In the middle of a back gesture drag, let the transition be linear to match finger
+          // motions.
+          linearTransition: _backGestureController != null,
         );
     } else {
       return new _MountainViewPageTransition(

packages/flutter/test/material/page_test.dart

@@ -209,7 +209,7 @@ void main() {
     // Drag from left edge to invoke the gesture.
     final TestGesture gesture = await tester.startGesture(const Point(5.0, 100.0));
     await gesture.moveBy(const Offset(400.0, 0.0));
-    await tester.pumpAndSettle();
+    await tester.pump();
 
     expect(find.text('Page 1'), findsNothing);
     expect(find.text('Page 2'), isOnstage);
@@ -240,11 +240,24 @@ void main() {
     // Drag from left edge to invoke the gesture.
     final TestGesture gesture = await tester.startGesture(const Point(5.0, 100.0));
     await gesture.moveBy(const Offset(400.0, 0.0));
-    await tester.pumpAndSettle();
+    await tester.pump();
 
     // Page 1 is now visible.
     expect(find.text('Page 1'), isOnstage);
     expect(find.text('Page 2'), isOnstage);
+
+    // The route widget position needs to track the finger position very exactly.
+    expect(tester.getTopLeft(find.text('Page 2')), const Point(400.0, 0.0));
+
+    await gesture.moveBy(const Offset(-200.0, 0.0));
+    await tester.pump();
+
+    expect(tester.getTopLeft(find.text('Page 2')), const Point(200.0, 0.0));
+
+    await gesture.moveBy(const Offset(-100.0, 200.0));
+    await tester.pump();
+
+    expect(tester.getTopLeft(find.text('Page 2')), const Point(100.0, 0.0));
   });
 
   testWidgets('test no back gesture on iOS fullscreen dialogs', (WidgetTester tester) async {
@@ -269,7 +282,7 @@ void main() {
     // Drag from left edge to invoke the gesture.
     final TestGesture gesture = await tester.startGesture(const Point(5.0, 100.0));
     await gesture.moveBy(const Offset(400.0, 0.0));
-    await tester.pumpAndSettle();
+    await tester.pump();
 
     expect(find.text('Page 1'), findsNothing);
     expect(find.text('Page 2'), isOnstage);