Make Cupertino page transition elevation animated too (#9166)

* Make Cupertino page transition elevation animated too

* Rename and change physical model to a decorated box

* Tests

* Add a comment

* still need to handle null in the tween somewhere

* nits

* Tweens evaluate to the actual begin/end instances. Let them be non-null

* Rename no decoration to none

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

@@ -6,7 +6,6 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
 const double _kMinFlingVelocity = 1.0;  // screen width per second.
-const Color _kBackgroundColor = const Color(0xFFEFEFF4); // iOS 10 background color.
 
 // Fractional offset from offscreen to the right to fully on screen.
 final FractionalOffsetTween _kRightMiddleTween = new FractionalOffsetTween(
@@ -26,6 +25,20 @@ final FractionalOffsetTween _kBottomUpTween = new FractionalOffsetTween(
   end: FractionalOffset.topLeft,
 );
 
+// BoxDecoration from no shadow to page shadow mimicking iOS page transitions.
+final DecorationTween _kShadowTween = new DecorationTween(
+  begin: BoxDecoration.none, // No shadow initially.
+  end: const BoxDecoration(
+    boxShadow: const <BoxShadow>[
+      const BoxShadow(
+        blurRadius: 10.0,
+        spreadRadius: 4.0,
+        color: const Color(0x38000000),
+      ),
+    ],
+  ),
+);
+
 /// Provides the native iOS page transition animation.
 ///
 /// The page slides in from the right and exits in reverse. It also shifts to the left in
@@ -34,51 +47,50 @@ class CupertinoPageTransition extends StatelessWidget {
   CupertinoPageTransition({
     Key key,
     // Linear route animation from 0.0 to 1.0 when this screen is being pushed.
-    @required Animation<double> incomingRouteAnimation,
+    @required Animation<double> primaryRouteAnimation,
     // Linear route animation from 0.0 to 1.0 when another screen is being pushed on top of this
     // one.
-    @required Animation<double> outgoingRouteAnimation,
+    @required Animation<double> secondaryRouteAnimation,
     @required this.child,
-    // Perform incoming transition linearly. Use to precisely track back gesture drags.
+    // Perform primary transition linearly. Use to precisely track back gesture drags.
     bool linearTransition,
   }) :
-      _incomingPositionAnimation = linearTransition
-        ? _kRightMiddleTween.animate(incomingRouteAnimation)
+      _primaryPositionAnimation = linearTransition
+        ? _kRightMiddleTween.animate(primaryRouteAnimation)
         : _kRightMiddleTween.animate(
             new CurvedAnimation(
-              parent: incomingRouteAnimation,
+              parent: primaryRouteAnimation,
               curve: Curves.easeOut,
               reverseCurve: Curves.easeIn,
             )
           ),
-      _outgoingPositionAnimation = _kMiddleLeftTween.animate(
+      _secondaryPositionAnimation = _kMiddleLeftTween.animate(
         new CurvedAnimation(
-          parent: outgoingRouteAnimation,
+          parent: secondaryRouteAnimation,
           curve: Curves.easeOut,
           reverseCurve: Curves.easeIn,
         )
       ),
+      _primaryShadowAnimation = _kShadowTween.animate(primaryRouteAnimation),
       super(key: key);
 
   // When this page is coming in to cover another page.
-  final Animation<FractionalOffset> _incomingPositionAnimation;
+  final Animation<FractionalOffset> _primaryPositionAnimation;
   // When this page is becoming covered by another page.
-  final Animation<FractionalOffset> _outgoingPositionAnimation;
+  final Animation<FractionalOffset> _secondaryPositionAnimation;
+  final Animation<Decoration> _primaryShadowAnimation;
   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: _secondaryPositionAnimation,
       child: new SlideTransition(
-        position: _incomingPositionAnimation,
-        child: new PhysicalModel(
-          shape: BoxShape.rectangle,
-          color: _kBackgroundColor,
-          elevation: 32,
+        position: _primaryPositionAnimation,
+        child: new DecoratedBoxTransition(
+          decoration: _primaryShadowAnimation,
           child: child,
         ),
       ),

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

@@ -174,8 +174,8 @@ class MaterialPageRoute<T> extends PageRoute<T> {
         );
       else
         return new CupertinoPageTransition(
-          incomingRouteAnimation: animation,
-          outgoingRouteAnimation: secondaryAnimation,
+          primaryRouteAnimation: animation,
+          secondaryRouteAnimation: secondaryAnimation,
           child: child,
           // In the middle of a back gesture drag, let the transition be linear to match finger
           // motions.

packages/flutter/lib/src/painting/box_painter.dart

@@ -1078,6 +1078,9 @@ class BoxDecoration extends Decoration {
     this.shape: BoxShape.rectangle
   });
 
+  /// A [BoxDecoration] with no decorating properties.
+  static const BoxDecoration none = const BoxDecoration();
+
   @override
   bool debugAssertIsValid() {
     assert(shape != BoxShape.circle ||

packages/flutter/test/material/page_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -63,13 +64,17 @@ void main() {
   });
 
   testWidgets('test iOS page transition', (WidgetTester tester) async {
+    final Key page2Key = new UniqueKey();
     await tester.pumpWidget(
       new MaterialApp(
         theme: new ThemeData(platform: TargetPlatform.iOS),
         home: new Material(child: const Text('Page 1')),
         routes: <String, WidgetBuilder>{
           '/next': (BuildContext context) {
-            return new Material(child: const Text('Page 2'));
+            return new Material(
+              key: page2Key,
+              child: const Text('Page 2'),
+            );
           },
         },
       )
@@ -79,10 +84,13 @@ void main() {
 
     tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
     await tester.pump();
-    await tester.pump(const Duration(milliseconds: 100));
+    await tester.pump(const Duration(milliseconds: 150));
 
     Offset widget1TransientTopLeft = tester.getTopLeft(find.text('Page 1'));
     Offset widget2TopLeft = tester.getTopLeft(find.text('Page 2'));
+    DecoratedBox box = tester.element(find.byKey(page2Key)).ancestorWidgetOfExactType(DecoratedBox);
+    BoxDecoration decoration = box.decoration;
+    BoxShadow shadow = decoration.boxShadow[0];
 
     // Page 1 is moving to the left.
     expect(widget1TransientTopLeft.dx < widget1InitialTopLeft.dx, true);
@@ -92,6 +100,9 @@ void main() {
     expect(widget1InitialTopLeft.dy == widget2TopLeft.dy, true);
     // Page 2 is coming in from the right.
     expect(widget2TopLeft.dx > widget1InitialTopLeft.dx, true);
+    // The shadow should be exactly half its maximum extent.
+    expect(shadow.blurRadius, 5.0);
+    expect(shadow.spreadRadius, 2.0);
 
     await tester.pumpAndSettle();
 
@@ -102,6 +113,9 @@ void main() {
     tester.state<NavigatorState>(find.byType(Navigator)).pop();
     await tester.pump();
     await tester.pump(const Duration(milliseconds: 100));
+    box = tester.element(find.byKey(page2Key)).ancestorWidgetOfExactType(DecoratedBox);
+    decoration = box.decoration;
+    shadow = decoration.boxShadow[0];
 
     widget1TransientTopLeft = tester.getTopLeft(find.text('Page 1'));
     widget2TopLeft = tester.getTopLeft(find.text('Page 2'));
@@ -114,6 +128,9 @@ void main() {
     expect(widget1InitialTopLeft.dy == widget2TopLeft.dy, true);
     // Page 2 is leaving towards the right.
     expect(widget2TopLeft.dx > widget1InitialTopLeft.dx, true);
+    // The shadow should be exactly 2/3 of its maximum extent.
+    expect(shadow.blurRadius, closeTo(6.6, 0.1));
+    expect(shadow.spreadRadius, closeTo(2.6, 0.1));
 
     await tester.pumpAndSettle();
 

packages/flutter/test/widgets/transitions_test.dart

@@ -15,7 +15,7 @@ void main() {
     expect(widget.toString, isNot(throwsException));
   });
 
-  group('ContainerTransition test', () {
+  group('DecoratedBoxTransition test', () {
     final DecorationTween decorationTween = new DecorationTween(
       begin: new BoxDecoration(
         backgroundColor: const Color(0xFFFFFFFF),