Add transparent color to Cupertino colors (#150149)

The Cupertino library doesn't have the transparent color constant that
the Material library has.

Added to increase code readability.

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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

@@ -76,6 +76,13 @@ abstract final class CupertinoColors {
   ///  * [white], opaque white in the [CupertinoColors] palette.
   static const Color black = Color(0xFF000000);
 
+  /// A fully-transparent color, completely invisible.
+  ///
+  /// See also:
+  ///
+  ///  * [material.Colors.transparent], the same color, in the Material Design palette.
+  static const Color transparent = Color(0x00000000);
+
   /// Used in iOS 10 for light background fills such as the chat bubble background.
   ///
   /// This is SystemLightGrayColor in the iOS palette.

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

@@ -854,7 +854,7 @@ class _CupertinoEdgeShadowDecoration extends Decoration {
       // Eyeballed gradient used to mimic a drop shadow on the start side only.
       <Color>[
         Color(0x04000000),
-        Color(0x00000000),
+        CupertinoColors.transparent,
       ],
     ),
   );

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

@@ -465,7 +465,7 @@ class _RenderCupertinoTextSelectionToolbarShape extends RenderShiftedBox {
         ..shader = ui.Gradient.linear(
           Offset.zero,
           const Offset(10.0, 10.0),
-          const <Color>[Color(0x00000000), Color(0xFFFF00FF), Color(0xFFFF00FF), Color(0x00000000)],
+          const <Color>[CupertinoColors.transparent, Color(0xFFFF00FF), Color(0xFFFF00FF), CupertinoColors.transparent],
           const <double>[0.25, 0.25, 0.75, 0.75],
           TileMode.repeated,
         )

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

@@ -132,9 +132,9 @@ class _CupertinoTextSelectionToolbarButtonState extends State<CupertinoTextSelec
     final Widget child = CupertinoButton(
       color: isPressed
         ? _kToolbarPressedColor.resolveFrom(context)
-        : const Color(0x00000000),
+        : CupertinoColors.transparent,
       borderRadius: null,
-      disabledColor: const Color(0x00000000),
+      disabledColor: CupertinoColors.transparent,
       // This CupertinoButton does not actually handle the onPressed callback,
       // this is only here to correctly enable/disable the button (see
       // GestureDetector comment below).

packages/flutter/test/cupertino/context_menu_test.dart

@@ -283,7 +283,7 @@ void main() {
       final BoxDecoration? boxDecoration = (tester.firstWidget(decoyChildDescendant) as Container).decoration as BoxDecoration?;
       const List<Color?> expectedColors = <Color?>[null, Color(0x00000000)];
 
-      // `Color(0x00000000)` -> Is `Colors.transparent`.
+      // `Color(0x00000000)` -> Is `CupertinoColors.transparent`.
       // `null`              -> Default when no color argument is given in `BoxDecoration`.
       // Any other color won't preserve the child's property.
       expect(expectedColors, contains(boxDecoration?.color));

packages/flutter/test/cupertino/route_test.dart

@@ -1489,11 +1489,11 @@ void main() {
       final RenderBox box = tester.firstRenderObject<RenderBox>(find.byType(CustomPaint));
 
       // Animation starts with effectively no shadow
-      expect(box, paintsShadowRect(dx: 795, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 795, color: CupertinoColors.transparent));
-      expect(box, paintsShadowRect(dx: 785, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 785, color: CupertinoColors.transparent));
-      expect(box, paintsShadowRect(dx: 775, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 775, color: CupertinoColors.transparent));
-      expect(box, paintsShadowRect(dx: 765, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 765, color: CupertinoColors.transparent));
-      expect(box, paintsShadowRect(dx: 755, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 755, color: CupertinoColors.transparent));
 
       await tester.pump(const Duration(milliseconds: 100));
 
@@ -1501,8 +1501,8 @@ void main() {
       expect(box, paintsShadowRect(dx: 296, color: const Color(0x03000000)));
       expect(box, paintsShadowRect(dx: 286, color: const Color(0x02000000)));
       expect(box, paintsShadowRect(dx: 276, color: const Color(0x01000000)));
-      expect(box, paintsShadowRect(dx: 266, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 266, color: CupertinoColors.transparent));
-      expect(box, paintsShadowRect(dx: 266, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 266, color: CupertinoColors.transparent));
 
       await tester.pumpAndSettle();
 
@@ -1512,7 +1512,7 @@ void main() {
       expect(box, paintsShadowRect(dx: -10, color: const Color(0x03000000)));
       expect(box, paintsShadowRect(dx: -20, color: const Color(0x02000000)));
       expect(box, paintsShadowRect(dx: -30, color: const Color(0x01000000)));
-      expect(box, paintsShadowRect(dx: -40, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: -40, color: CupertinoColors.transparent));
 
       // Start animation in reverse
       tester.state<NavigatorState>(find.byType(Navigator)).pop();
@@ -1523,16 +1523,16 @@ void main() {
       expect(box, paintsShadowRect(dx: 488, color: const Color(0x03000000)));
       expect(box, paintsShadowRect(dx: 478, color: const Color(0x02000000)));
       expect(box, paintsShadowRect(dx: 468, color: const Color(0x01000000)));
-      expect(box, paintsShadowRect(dx: 458, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 458, color: CupertinoColors.transparent));
 
       await tester.pump(const Duration(milliseconds: 150));
 
       // At the end of the animation, the shadow approaches full transparency
       expect(box, paintsShadowRect(dx: 794, color: const Color(0x01000000)));
-      expect(box, paintsShadowRect(dx: 784, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 784, color: CupertinoColors.transparent));
-      expect(box, paintsShadowRect(dx: 774, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 774, color: CupertinoColors.transparent));
-      expect(box, paintsShadowRect(dx: 764, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 764, color: CupertinoColors.transparent));
-      expect(box, paintsShadowRect(dx: 754, color: const Color(0x00000000)));
+      expect(box, paintsShadowRect(dx: 754, color: CupertinoColors.transparent));
     });
 
     testWidgets('when route is fullscreenDialog, it has no visible _CupertinoEdgeShadowDecoration', (WidgetTester tester) async {
@@ -2020,8 +2020,6 @@ void main() {
   });
 
   testWidgets('showCupertinoModalPopup transparent barrier color is transparent', (WidgetTester tester) async {
-    const Color kTransparentColor = Color(0x00000000);
-
     await tester.pumpWidget(CupertinoApp(
       home: CupertinoPageScaffold(
         child: Builder(builder: (BuildContext context) {
@@ -2030,7 +2028,7 @@ void main() {
               await showCupertinoModalPopup<void>(
                 context: context,
                 builder: (BuildContext context) => const SizedBox(),
-                barrierColor: kTransparentColor,
+                barrierColor: CupertinoColors.transparent,
               );
             },
             child: const Text('tap'),

packages/flutter/test/cupertino/text_selection_toolbar_button_test.dart

@@ -47,7 +47,7 @@ void main() {
       matching: find.byType(DecoratedBox),
     ));
     BoxDecoration boxDecoration = decoratedBox.decoration as BoxDecoration;
-    expect(boxDecoration.color, const Color(0x00000000));
+    expect(boxDecoration.color, CupertinoColors.transparent);
 
     // Make a "down" gesture on the button.
     final Offset center = tester.getCenter(find.byType(CupertinoTextSelectionToolbarButton));
@@ -72,7 +72,7 @@ void main() {
       matching: find.byType(DecoratedBox),
     ));
     boxDecoration = decoratedBox.decoration as BoxDecoration;
-    expect(boxDecoration.color, const Color(0x00000000));
+    expect(boxDecoration.color, CupertinoColors.transparent);
   });
 
   testWidgets('passing null to onPressed disables the button', (WidgetTester tester) async {

packages/flutter/test/cupertino/theme_test.dart

@@ -205,7 +205,7 @@ void main() {
     // Regression test for https://github.com/flutter/flutter/issues/47651.
     expect(
       const CupertinoTheme(
-        data: CupertinoThemeData(primaryColor: Color(0x00000000)),
+        data: CupertinoThemeData(primaryColor: CupertinoColors.transparent),
         child: SizedBox(),
       ).toStringDeep().trimRight(),
       isNot(contains('\n')),