Material Switch optionally adapts per platform: Switch.adaptive() (#22688)

examples/flutter_gallery/lib/demo/material/selection_controls_demo.dart

@@ -187,7 +187,7 @@ class _SelectionControlsDemoState extends State<SelectionControlsDemo> {
       child: Row(
         mainAxisSize: MainAxisSize.min,
         children: <Widget>[
-          Switch(
+          Switch.adaptive(
             value: switchValue,
             onChanged: (bool value) {
               setState(() {
@@ -196,8 +196,8 @@ class _SelectionControlsDemoState extends State<SelectionControlsDemo> {
             }
           ),
           // Disabled switches
-          const Switch(value: true, onChanged: null),
-          const Switch(value: false, onChanged: null)
+          const Switch.adaptive(value: true, onChanged: null),
+          const Switch.adaptive(value: false, onChanged: null),
         ],
       ),
     );

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
@@ -23,6 +24,8 @@ const double _kSwitchWidth = _kTrackWidth - 2 * _kTrackRadius + 2 * kRadialReact
 const double _kSwitchHeight = 2 * kRadialReactionRadius + 8.0;
 const double _kSwitchHeightCollapsed = 2 * kRadialReactionRadius;
 
+enum _SwitchType { material, adaptive }
+
 /// A material design switch.
 ///
 /// Used to toggle the on/off state of a single setting.
@@ -65,7 +68,30 @@ class Switch extends StatefulWidget {
     this.activeThumbImage,
     this.inactiveThumbImage,
     this.materialTapTargetSize,
-  }) : super(key: key);
+  }) : _switchType = _SwitchType.material,
+       super(key: key);
+
+  /// Creates a [CupertinoSwitch] if the target platform is iOS, creates a
+  /// material design switch otherwise.
+  ///
+  /// If a [CupertinoSwitch] is created, the following parameters are
+  /// ignored: [activeTrackColor], [inactiveThumbColor], [inactiveTrackColor],
+  /// [activeThumbImage], [inactiveThumbImage], [materialTapTargetSize].
+  ///
+  /// The target platform is based on the current [Theme]: [ThemeData.platform].
+  const Switch.adaptive({
+    Key key,
+    @required this.value,
+    @required this.onChanged,
+    this.activeColor,
+    this.activeTrackColor,
+    this.inactiveThumbColor,
+    this.inactiveTrackColor,
+    this.activeThumbImage,
+    this.inactiveThumbImage,
+    this.materialTapTargetSize,
+  }) : _switchType = _SwitchType.adaptive,
+       super(key: key);
 
   /// Whether this switch is on or off.
   ///
@@ -104,22 +130,32 @@ class Switch extends StatefulWidget {
   /// The color to use on the track when this switch is on.
   ///
   /// Defaults to [ThemeData.toggleableActiveColor] with the opacity set at 50%.
+  ///
+  /// Ignored if this switch is created with [Switch.adaptive].
   final Color activeTrackColor;
 
   /// The color to use on the thumb when this switch is off.
   ///
   /// Defaults to the colors described in the Material design specification.
+  ///
+  /// Ignored if this switch is created with [Switch.adaptive].
   final Color inactiveThumbColor;
 
   /// The color to use on the track when this switch is off.
   ///
   /// Defaults to the colors described in the Material design specification.
+  ///
+  /// Ignored if this switch is created with [Switch.adaptive].
   final Color inactiveTrackColor;
 
   /// An image to use on the thumb of this switch when the switch is on.
+  ///
+  /// Ignored if this switch is created with [Switch.adaptive].
   final ImageProvider activeThumbImage;
 
   /// An image to use on the thumb of this switch when the switch is off.
+  ///
+  /// Ignored if this switch is created with [Switch.adaptive].
   final ImageProvider inactiveThumbImage;
 
   /// Configures the minimum size of the tap target.
@@ -131,6 +167,8 @@ class Switch extends StatefulWidget {
   ///   * [MaterialTapTargetSize], for a description of how this affects tap targets.
   final MaterialTapTargetSize materialTapTargetSize;
 
+  final _SwitchType _switchType;
+
   @override
   _SwitchState createState() => _SwitchState();
 
@@ -143,13 +181,25 @@ class Switch extends StatefulWidget {
 }
 
 class _SwitchState extends State<Switch> with TickerProviderStateMixin {
-  @override
-  Widget build(BuildContext context) {
-    assert(debugCheckHasMaterial(context));
-    final ThemeData themeData = Theme.of(context);
-    final bool isDark = themeData.brightness == Brightness.dark;
+  Size getSwitchSize(ThemeData theme) {
+    switch (widget.materialTapTargetSize ?? theme.materialTapTargetSize) {
+      case MaterialTapTargetSize.padded:
+        return const Size(_kSwitchWidth, _kSwitchHeight);
+        break;
+      case MaterialTapTargetSize.shrinkWrap:
+        return const Size(_kSwitchWidth, _kSwitchHeightCollapsed);
+        break;
+    }
+    assert(false);
+    return null;
+  }
 
-    final Color activeThumbColor = widget.activeColor ?? themeData.toggleableActiveColor;
+  Widget buildMaterialSwitch(BuildContext context) {
+    assert(debugCheckHasMaterial(context));
+    final ThemeData theme = Theme.of(context);
+    final bool isDark = theme.brightness == Brightness.dark;
+
+    final Color activeThumbColor = widget.activeColor ?? theme.toggleableActiveColor;
     final Color activeTrackColor = widget.activeTrackColor ?? activeThumbColor.withAlpha(0x80);
 
     Color inactiveThumbColor;
@@ -162,16 +212,6 @@ class _SwitchState extends State<Switch> with TickerProviderStateMixin {
       inactiveThumbColor = widget.inactiveThumbColor ?? (isDark ? Colors.grey.shade800 : Colors.grey.shade400);
       inactiveTrackColor = widget.inactiveTrackColor ?? (isDark ? Colors.white10 : Colors.black12);
     }
-    Size size;
-    switch (widget.materialTapTargetSize ?? themeData.materialTapTargetSize) {
-      case MaterialTapTargetSize.padded:
-        size = const Size(_kSwitchWidth, _kSwitchHeight);
-        break;
-      case MaterialTapTargetSize.shrinkWrap:
-        size = const Size(_kSwitchWidth, _kSwitchHeightCollapsed);
-        break;
-    }
-    final BoxConstraints additionalConstraints = BoxConstraints.tight(size);
 
     return _SwitchRenderObjectWidget(
       value: widget.value,
@@ -183,10 +223,46 @@ class _SwitchState extends State<Switch> with TickerProviderStateMixin {
       inactiveTrackColor: inactiveTrackColor,
       configuration: createLocalImageConfiguration(context),
       onChanged: widget.onChanged,
-      additionalConstraints: additionalConstraints,
+      additionalConstraints: BoxConstraints.tight(getSwitchSize(theme)),
       vsync: this,
     );
   }
+
+  Widget buildCupertinoSwitch(BuildContext context) {
+    final Size size = getSwitchSize(Theme.of(context));
+    return Container(
+      width: size.width,  // Same size as the Material switch.
+      height: size.height,
+      alignment: Alignment.center,
+      child: CupertinoSwitch(
+        value: widget.value,
+        onChanged: widget.onChanged,
+        activeColor: widget.activeColor,
+      ),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    switch (widget._switchType) {
+      case _SwitchType.material:
+        return buildMaterialSwitch(context);
+
+      case _SwitchType.adaptive: {
+        final ThemeData theme = Theme.of(context);
+        assert(theme.platform != null);
+        switch (theme.platform) {
+          case TargetPlatform.android:
+          case TargetPlatform.fuchsia:
+            return buildMaterialSwitch(context);
+          case TargetPlatform.iOS:
+            return buildCupertinoSwitch(context);
+        }
+      }
+    }
+    assert(false);
+    return null;
+  }
 }
 
 class _SwitchRenderObjectWidget extends LeafRenderObjectWidget {

packages/flutter/test/material/switch_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+
+import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
@@ -485,4 +487,46 @@ void main() {
     semanticsTester.dispose();
     SystemChannels.accessibility.setMockMessageHandler(null);
   });
+
+  testWidgets('Switch.adaptive', (WidgetTester tester) async {
+    bool value = false;
+
+    Widget buildFrame(TargetPlatform platform) {
+      return MaterialApp(
+        theme: ThemeData(platform: platform),
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return Material(
+              child: Center(
+                child: Switch.adaptive(
+                  value: value,
+                  onChanged: (bool newValue) {
+                    setState(() {
+                      value = newValue;
+                    });
+                  },
+                ),
+              ),
+            );
+          },
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.iOS));
+    expect(find.byType(CupertinoSwitch), findsOneWidget);
+
+    expect(value, isFalse);
+    await tester.tap(find.byType(Switch));
+    expect(value, isTrue);
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.android));
+    await tester.pumpAndSettle(); // Finish the theme change animation.
+    expect(find.byType(CupertinoSwitch), findsNothing);
+    expect(value, isTrue);
+    await tester.tap(find.byType(Switch));
+    expect(value, isFalse);
+
+  });
+
 }