Change Cupertino page transition box shadow to a simple custom gradient (#9673)

Creates another Decoration for drawing outside the decorated box with a gradient to emulate the shadow.

Lets the cupertino transition page's background be transparent.

Fixes #9321

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

@@ -25,20 +25,118 @@ 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),
-      ),
-    ],
+// Custom decoration from no shadow to page shadow mimicking iOS page 
+// transitions using gradients.
+final DecorationTween _kGradientShadowTween = new DecorationTween(
+  begin: _CupertinoEdgeShadowDecoration.none, // No decoration initially.
+  end: const _CupertinoEdgeShadowDecoration(
+    edgeGradient: const LinearGradient(
+      // Spans 5% of the page.
+      begin: const FractionalOffset(0.95, 0.0),
+      end: FractionalOffset.topRight,
+      // Eyeballed gradient used to mimic a drop shadow on the left side only.
+      colors: const <Color>[
+        const Color(0x00000000), 
+        const Color(0x04000000),
+        const Color(0x12000000),
+        const Color(0x38000000)
+      ],
+      stops: const <double>[0.0, 0.3, 0.6, 1.0],
+    ), 
   ),
 );
 
+/// A custom [Decoration] used to paint an extra shadow on the left edge of the 
+/// box it's decorating. It's like a [BoxDecoration] with only a gradient except 
+/// it paints to the left of the box instead of behind the box.
+class _CupertinoEdgeShadowDecoration extends Decoration {
+  const _CupertinoEdgeShadowDecoration({ this.edgeGradient });
+
+  /// A Decoration with no decorating properties.
+  static const _CupertinoEdgeShadowDecoration none = 
+      const _CupertinoEdgeShadowDecoration();
+
+  /// A gradient to draw to the left of the box being decorated. 
+  /// FractionalOffsets are relative to the original box translated one box
+  /// width to the left.
+  final LinearGradient edgeGradient;
+
+  /// Linearly interpolate between two edge shadow decorations decorations.
+  ///
+  /// See also [Decoration.lerp].
+  static _CupertinoEdgeShadowDecoration lerp(
+    _CupertinoEdgeShadowDecoration a, 
+    _CupertinoEdgeShadowDecoration b, 
+    double t
+  ) {
+    if (a == null && b == null)
+      return null;
+    return new _CupertinoEdgeShadowDecoration(
+      edgeGradient: LinearGradient.lerp(a?.edgeGradient, b?.edgeGradient, t),
+    );
+  }
+
+  @override
+  _CupertinoEdgeShadowDecoration lerpFrom(Decoration a, double t) {
+    if (a is! _CupertinoEdgeShadowDecoration)
+      return _CupertinoEdgeShadowDecoration.lerp(null, this, t);
+    return _CupertinoEdgeShadowDecoration.lerp(a, this, t);
+  }
+
+  @override
+  _CupertinoEdgeShadowDecoration lerpTo(Decoration b, double t) {
+    if (b is! _CupertinoEdgeShadowDecoration)
+      return _CupertinoEdgeShadowDecoration.lerp(this, null, t);
+    return _CupertinoEdgeShadowDecoration.lerp(this, b, t);
+  }
+  
+  @override
+  _CupertinoEdgeShadowPainter createBoxPainter([VoidCallback onChanged]) {
+    return new _CupertinoEdgeShadowPainter(this, onChanged);
+  }
+
+  @override
+  bool operator ==(dynamic other) {
+    if (identical(this, other))
+      return true;
+    if (other.runtimeType != _CupertinoEdgeShadowDecoration)
+      return false;
+    final _CupertinoEdgeShadowDecoration typedOther = other;
+    return edgeGradient == typedOther.edgeGradient;
+  }
+
+  @override
+  int get hashCode {
+    return edgeGradient.hashCode;
+  }
+}
+
+/// A [BoxPainter] used to draw the page transition shadow using gradients.
+class _CupertinoEdgeShadowPainter extends BoxPainter {
+  _CupertinoEdgeShadowPainter(
+    @required this._decoration, 
+    VoidCallback onChange
+  ) : assert(_decoration != null),
+      super(onChange);
+
+  final _CupertinoEdgeShadowDecoration _decoration;
+
+  @override
+  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
+    final LinearGradient gradient = _decoration.edgeGradient;
+    if (gradient == null)
+      return;
+    // The drawable space for the gradient is a rect with the same size as 
+    // its parent box one box width to the left of the box.
+    final Rect rect = 
+        (offset & configuration.size).translate(-configuration.size.width, 0.0);
+    final Paint paint = new Paint()
+      ..shader = gradient.createShader(rect);
+
+    canvas.drawRect(rect, paint);
+  }
+}
+
 /// 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
@@ -71,7 +169,12 @@ class CupertinoPageTransition extends StatelessWidget {
           reverseCurve: Curves.easeIn,
         )
       ),
-      _primaryShadowAnimation = _kShadowTween.animate(primaryRouteAnimation),
+      _primaryShadowAnimation = _kGradientShadowTween.animate(
+        new CurvedAnimation(
+          parent: primaryRouteAnimation,
+          curve: Curves.easeOut,
+        )
+      ),
       super(key: key);
 
   // When this page is coming in to cover another page.

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

@@ -1129,9 +1129,6 @@ 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

@@ -4,7 +4,9 @@
 
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
-import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_test/flutter_test.dart' hide TypeMatcher;
+
+import '../rendering/mock_canvas.dart';
 
 void main() {
   testWidgets('test Android page transition', (WidgetTester tester) async {
@@ -77,14 +79,14 @@ void main() {
     final Offset widget1InitialTopLeft = tester.getTopLeft(find.text('Page 1'));
 
     tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
+
     await tester.pump();
     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];
+    final RenderDecoratedBox box = tester.element(find.byKey(page2Key))
+        .ancestorRenderObjectOfType(const TypeMatcher<RenderDecoratedBox>());
 
     // Page 1 is moving to the left.
     expect(widget1TransientTopLeft.dx < widget1InitialTopLeft.dx, true);
@@ -94,9 +96,14 @@ 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);
+    // The shadow should be drawn to one screen width to the left of where 
+    // the page 2 box is. `paints` tests relative to the painter's given canvas
+    // rather than relative to the screen so assert that it's one screen
+    // width to the left of 0 offset box rect and nothing is drawn inside the 
+    // box's rect.
+    expect(box, paints..rect(
+      rect: new Rect.fromLTWH(-800.0, 0.0, 800.0, 600.0)
+    ));
 
     await tester.pumpAndSettle();
 
@@ -107,9 +114,6 @@ 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'));
@@ -122,9 +126,6 @@ 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();