Allowing more flexibility to the active opacity of the Cupertino Button (#9071)

* Allowing overrides to the activeOpacity of the button

* adding tests

* fix

* fixing animation

* fixes

* moar fixes

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

diff --git a/packages/flutter/lib/src/cupertino/button.dart b/packages/flutter/lib/src/cupertino/button.dart
index 7e6b99e..2e10e0e 100644
--- a/packages/flutter/lib/src/cupertino/button.dart
+++ b/packages/flutter/lib/src/cupertino/button.dart
@@ -47,8 +47,11 @@
     this.padding,
     this.color,
     this.minSize: 44.0,
+    this.pressedOpacity: 0.1,
     @required this.onPressed,
-  });
+  }) {
+    assert(pressedOpacity >= 0.0 && pressedOpacity <= 1.0);
+  }
 
   /// The widget below this widget in the tree.
   ///
@@ -80,6 +83,12 @@
   /// * <https://developer.apple.com/ios/human-interface-guidelines/visual-design/layout/>
   final double minSize;
 
+  /// The opacity that the button will fade to when it is pressed.
+  /// The button will have an opacity of 1.0 when it is not pressed.
+  ///
+  /// This defaults to 0.1.
+  final double pressedOpacity;
+
   /// Whether the button is enabled or disabled. Buttons are disabled by default. To
   /// enable a button, set its [onPressed] property to a non-null value.
   bool get enabled => onPressed != null;
@@ -99,17 +108,26 @@
   // Eyeballed values. Feel free to tweak.
   static const Duration kFadeOutDuration = const Duration(milliseconds: 10);
   static const Duration kFadeInDuration = const Duration(milliseconds: 350);
+  Tween<double> _opacityTween;
 
   AnimationController _animationController;
 
+  void _setTween() {
+    _opacityTween = new Tween<double>(
+      begin: 1.0,
+      end: config.pressedOpacity,
+    );
+  }
+
   @override
   void initState() {
     super.initState();
     _animationController = new AnimationController(
       duration: const Duration(milliseconds: 200),
-      value: 1.0,
+      value: 0.0,
       vsync: this,
     );
+    _setTween();
   }
 
   @override
@@ -119,16 +137,22 @@
     super.dispose();
   }
 
+  @override
+  void didUpdateConfig(CupertinoButton old) {
+    super.didUpdateConfig(old);
+    _setTween();
+  }
+
   void _handleTapDown(PointerDownEvent event) {
-    _animationController.animateTo(0.1, duration: kFadeOutDuration);
+    _animationController.animateTo(1.0, duration: kFadeOutDuration);
   }
 
   void _handleTapUp(PointerUpEvent event) {
-    _animationController.animateTo(1.0, duration: kFadeInDuration);
+    _animationController.animateTo(0.0, duration: kFadeInDuration);
   }
 
   void _handleTapCancel(PointerCancelEvent event) {
-    _animationController.animateTo(1.0, duration: kFadeInDuration);
+    _animationController.animateTo(0.0, duration: kFadeInDuration);
   }
 
   @override
@@ -148,10 +172,10 @@
             minHeight: config.minSize,
           ),
           child: new FadeTransition(
-            opacity: new CurvedAnimation(
+            opacity: _opacityTween.animate(new CurvedAnimation(
               parent: _animationController,
               curve: Curves.decelerate,
-            ),
+            )),
             child: new DecoratedBox(
               decoration: new BoxDecoration(
                 borderRadius: const BorderRadius.all(const Radius.circular(8.0)),

packages/flutter/test/cupertino/button_test.dart

diff --git a/packages/flutter/test/cupertino/button_test.dart b/packages/flutter/test/cupertino/button_test.dart
index cb21fc6..ba8e731 100644
--- a/packages/flutter/test/cupertino/button_test.dart
+++ b/packages/flutter/test/cupertino/button_test.dart
@@ -125,4 +125,44 @@
     // Still doesn't animate.
     expect(SchedulerBinding.instance.transientCallbackCount, equals(0));
   });
+
+  testWidgets('pressedOpacity defaults to 0.1', (WidgetTester tester) async {
+    await tester.pumpWidget(new Center(child: new CupertinoButton(
+      child: new Text('Tap me'),
+      onPressed: () { },
+    )));
+
+    // Keep a "down" gesture on the button
+    final Point center = tester.getCenter(find.byType(CupertinoButton));
+    await tester.startGesture(center);
+    await tester.pumpAndSettle();
+
+    // Check opacity
+    final Opacity opacity = tester.widget(find.descendant(
+      of: find.byType(CupertinoButton),
+      matching: find.byType(Opacity),
+    ));
+    expect(opacity.opacity, 0.1);
+  });
+
+  testWidgets('pressedOpacity parameter', (WidgetTester tester) async {
+    final double pressedOpacity = 0.5;
+    await tester.pumpWidget(new Center(child: new CupertinoButton(
+      pressedOpacity: pressedOpacity,
+      child: new Text('Tap me'),
+      onPressed: () { },
+    )));
+
+    // Keep a "down" gesture on the button
+    final Point center = tester.getCenter(find.byType(CupertinoButton));
+    await tester.startGesture(center);
+    await tester.pumpAndSettle();
+
+    // Check opacity
+    final Opacity opacity = tester.widget(find.descendant(
+      of: find.byType(CupertinoButton),
+      matching: find.byType(Opacity),
+    ));
+    expect(opacity.opacity, pressedOpacity);
+  });
 }