Added opacity to cupertino switch when disabled (#29451)

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

@@ -77,7 +77,7 @@ class CupertinoSwitch extends StatefulWidget {
   /// change state until the parent widget rebuilds the switch with the new
   /// value.
   ///
-  /// If null, the switch will be displayed as disabled.
+  /// If null, the switch will be displayed as disabled, which has a reduced opacity.
   ///
   /// The callback provided to onChanged should update the state of the parent
   /// [StatefulWidget] using the [State.setState] method, so that the parent
@@ -135,12 +135,15 @@ class CupertinoSwitch extends StatefulWidget {
 class _CupertinoSwitchState extends State<CupertinoSwitch> with TickerProviderStateMixin {
   @override
   Widget build(BuildContext context) {
-    return _CupertinoSwitchRenderObjectWidget(
-      value: widget.value,
-      activeColor: widget.activeColor ?? CupertinoColors.activeGreen,
-      onChanged: widget.onChanged,
-      vsync: this,
-      dragStartBehavior: widget.dragStartBehavior,
+    return Opacity(
+      opacity: widget.onChanged == null ? _kCupertinoSwitchDisabledOpacity : 1.0,
+      child: _CupertinoSwitchRenderObjectWidget(
+        value: widget.value,
+        activeColor: widget.activeColor ?? CupertinoColors.activeGreen,
+        onChanged: widget.onChanged,
+        vsync: this,
+        dragStartBehavior: widget.dragStartBehavior,
+      ),
     );
   }
 }
@@ -193,6 +196,8 @@ const double _kTrackInnerEnd = _kTrackWidth - _kTrackInnerStart;
 const double _kTrackInnerLength = _kTrackInnerEnd - _kTrackInnerStart;
 const double _kSwitchWidth = 59.0;
 const double _kSwitchHeight = 39.0;
+// Opacity of a disabled switch, as eye-balled from iOS Simulator on Mac.
+const double _kCupertinoSwitchDisabledOpacity = 0.5;
 
 const Color _kTrackColor = CupertinoColors.lightBackgroundGray;
 const Duration _kReactionDuration = Duration(milliseconds: 300);

packages/flutter/test/cupertino/switch_test.dart

@@ -415,4 +415,102 @@ void main() {
     expect(value, isFalse);
   });
 
+  testWidgets('Switch is translucent when disabled', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: CupertinoSwitch(
+            value: false,
+            dragStartBehavior: DragStartBehavior.down,
+            onChanged: null,
+          ),
+        )
+      ),
+    );
+
+    expect(find.byType(Opacity), findsOneWidget);
+    expect(tester.widget<Opacity>(find.byType(Opacity).first).opacity, 0.5);
+  });
+
+  testWidgets('Switch is opaque when enabled', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: CupertinoSwitch(
+            value: false,
+            dragStartBehavior: DragStartBehavior.down,
+            onChanged: (bool newValue) {},
+          ),
+        )
+      ),
+    );
+
+    expect(find.byType(Opacity), findsOneWidget);
+    expect(tester.widget<Opacity>(find.byType(Opacity).first).opacity, 1.0);
+  });
+
+  testWidgets('Switch turns translucent after becoming disabled', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: CupertinoSwitch(
+            value: false,
+            dragStartBehavior: DragStartBehavior.down,
+            onChanged: (bool newValue) {},
+          ),
+        )
+      ),
+    );
+
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: CupertinoSwitch(
+            value: false,
+            dragStartBehavior: DragStartBehavior.down,
+            onChanged: null,
+          ),
+        )
+      ),
+    );
+
+    expect(find.byType(Opacity), findsOneWidget);
+    expect(tester.widget<Opacity>(find.byType(Opacity).first).opacity, 0.5);
+  });
+
+    testWidgets('Switch turns opaque after becoming enabled', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: CupertinoSwitch(
+            value: false,
+            dragStartBehavior: DragStartBehavior.down,
+            onChanged: null,
+          ),
+        )
+      ),
+    );
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: CupertinoSwitch(
+            value: false,
+            dragStartBehavior: DragStartBehavior.down,
+            onChanged: (bool newValue) {},
+          ),
+        )
+      ),
+    );
+
+    expect(find.byType(Opacity), findsOneWidget);
+    expect(tester.widget<Opacity>(find.byType(Opacity).first).opacity, 1.0);
+  });
+
 }