🔋 Enhance cupertino button fade in and fade out (#87139)

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

@@ -148,8 +148,8 @@ class CupertinoButton extends StatefulWidget {
 
 class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProviderStateMixin {
   // Eyeballed values. Feel free to tweak.
-  static const Duration kFadeOutDuration = Duration(milliseconds: 10);
-  static const Duration kFadeInDuration = Duration(milliseconds: 100);
+  static const Duration kFadeOutDuration = Duration(milliseconds: 120);
+  static const Duration kFadeInDuration = Duration(milliseconds: 180);
   final Tween<double> _opacityTween = Tween<double>(begin: 1.0);
 
   late AnimationController _animationController;
@@ -213,8 +213,8 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
       return;
     final bool wasHeldDown = _buttonHeldDown;
     final TickerFuture ticker = _buttonHeldDown
-        ? _animationController.animateTo(1.0, duration: kFadeOutDuration)
-        : _animationController.animateTo(0.0, duration: kFadeInDuration);
+        ? _animationController.animateTo(1.0, duration: kFadeOutDuration, curve: Curves.easeInOutCubicEmphasized)
+        : _animationController.animateTo(0.0, duration: kFadeInDuration, curve: Curves.easeOutCubic);
     ticker.then<void>((void value) {
       if (mounted && wasHeldDown != _buttonHeldDown)
         _animate();

packages/flutter/test/cupertino/button_test.dart

@@ -205,23 +205,27 @@ void main() {
     await tester.pump();
     FadeTransition transition = tester.firstWidget(find.byType(FadeTransition));
 
-    await tester.pump(const Duration(milliseconds: 25));
+    await tester.pump(const Duration(milliseconds: 50));
     transition = tester.firstWidget(find.byType(FadeTransition));
-    expect(transition.opacity.value, moreOrLessEquals(0.4, epsilon: 0.001));
+    expect(transition.opacity.value, moreOrLessEquals(0.403, epsilon: 0.001));
 
-    await tester.pump(const Duration(milliseconds: 25));
+    await tester.pump(const Duration(milliseconds: 100));
     transition = tester.firstWidget(find.byType(FadeTransition));
-    expect(transition.opacity.value, moreOrLessEquals(0.437, epsilon: 0.001));
+    expect(transition.opacity.value, moreOrLessEquals(0.400, epsilon: 0.001));
 
-    await tester.pump(const Duration(milliseconds: 25));
+    await tester.pump(const Duration(milliseconds: 50));
     transition = tester.firstWidget(find.byType(FadeTransition));
-    expect(transition.opacity.value, moreOrLessEquals(0.55, epsilon: 0.001));
+    expect(transition.opacity.value, moreOrLessEquals(0.650, epsilon: 0.001));
 
-    await tester.pump(const Duration(milliseconds: 25));
+    await tester.pump(const Duration(milliseconds: 50));
     transition = tester.firstWidget(find.byType(FadeTransition));
-    expect(transition.opacity.value, moreOrLessEquals(0.737, epsilon: 0.001));
+    expect(transition.opacity.value, moreOrLessEquals(0.894, epsilon: 0.001));
 
-    await tester.pump(const Duration(milliseconds: 25));
+    await tester.pump(const Duration(milliseconds: 50));
+    transition = tester.firstWidget(find.byType(FadeTransition));
+    expect(transition.opacity.value, moreOrLessEquals(0.988, epsilon: 0.001));
+
+    await tester.pump(const Duration(milliseconds: 50));
     transition = tester.firstWidget(find.byType(FadeTransition));
     expect(transition.opacity.value, moreOrLessEquals(1.0, epsilon: 0.001));
   });