[Material] Add support for high contrast theming to Material apps (#62337)

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

@@ -180,6 +180,8 @@ class MaterialApp extends StatefulWidget {
     this.color,
     this.theme,
     this.darkTheme,
+    this.highContrastTheme,
+    this.highContrastDarkTheme,
     this.themeMode = ThemeMode.system,
     this.locale,
     this.localizationsDelegates,
@@ -294,6 +296,35 @@ class MaterialApp extends StatefulWidget {
   ///    [MediaQueryData.platformBrightness].
   final ThemeData darkTheme;
 
+  /// The [ThemeData] to use when 'high contrast' is requested by the system.
+  ///
+  /// Some host platforms (for example, iOS) allow the users to increase
+  /// contrast through an accessibility setting.
+  ///
+  /// Uses [theme] instead when null.
+  ///
+  /// See also:
+  ///
+  ///  * [MediaQueryData.highContrast], which indicates the platform's
+  ///    desire to increase contrast.
+  final ThemeData highContrastTheme;
+
+  /// The [ThemeData] to use when a 'dark mode' and 'high contrast' is requested
+  /// by the system.
+  ///
+  /// Some host platforms (for example, iOS) allow the users to increase
+  /// contrast through an accessibility setting.
+  ///
+  /// This theme should have a [ThemeData.brightness] set to [Brightness.dark].
+  ///
+  /// Uses [darkTheme] instead when null.
+  ///
+  /// See also:
+  ///
+  ///  * [MediaQueryData.highContrast], which indicates the platform's
+  ///    desire to increase contrast.
+  final ThemeData highContrastDarkTheme;
+
   /// Determines which theme will be used by the application if both [theme]
   /// and [darkTheme] are provided.
   ///
@@ -616,17 +647,22 @@ class _MaterialAppState extends State<MaterialApp> {
         onGenerateInitialRoutes: widget.onGenerateInitialRoutes,
         onUnknownRoute: widget.onUnknownRoute,
         builder: (BuildContext context, Widget child) {
-          // Use a light theme, dark theme, or fallback theme.
+          // Resolve which theme to use based on brightness and high contrast.
           final ThemeMode mode = widget.themeMode ?? ThemeMode.system;
+          final Brightness platformBrightness = MediaQuery.platformBrightnessOf(context);
+          final bool useDarkTheme = mode == ThemeMode.dark
+              || (mode == ThemeMode.system && platformBrightness == ui.Brightness.dark);
+          final bool highContrast = MediaQuery.highContrastOf(context);
           ThemeData theme;
-          if (widget.darkTheme != null) {
-            final ui.Brightness platformBrightness = MediaQuery.platformBrightnessOf(context);
-            if (mode == ThemeMode.dark ||
-              (mode == ThemeMode.system && platformBrightness == ui.Brightness.dark)) {
+
+          if (useDarkTheme && highContrast) {
+            theme = widget.highContrastDarkTheme;
+          } else if (useDarkTheme) {
             theme = widget.darkTheme;
+          } else if (highContrast) {
+            theme = widget.highContrastTheme;
           }
-          }
-          theme ??= widget.theme ?? ThemeData.fallback();
+          theme ??= widget.theme ?? ThemeData.light();
 
           return AnimatedTheme(
             data: theme,

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

@@ -108,6 +108,67 @@ class ColorScheme with Diagnosticable {
        assert(onError != null),
        assert(brightness != null);
 
+
+  /// Create a high contrast ColorScheme based on a purple primary color that
+  /// matches the [baseline Material color scheme](https://material.io/design/color/the-color-system.html#color-theme-creation).
+  const ColorScheme.highContrastLight({
+    this.primary = const Color(0xff0000ba),
+    this.primaryVariant = const Color(0xff000088),
+    this.secondary = const Color(0xff66fff9),
+    this.secondaryVariant = const Color(0xff018786),
+    this.surface = Colors.white,
+    this.background = Colors.white,
+    this.error = const Color(0xff790000),
+    this.onPrimary = Colors.white,
+    this.onSecondary = Colors.black,
+    this.onSurface = Colors.black,
+    this.onBackground = Colors.black,
+    this.onError = Colors.white,
+    this.brightness = Brightness.light,
+  }) : assert(primary != null),
+        assert(primaryVariant != null),
+        assert(secondary != null),
+        assert(secondaryVariant != null),
+        assert(surface != null),
+        assert(background != null),
+        assert(error != null),
+        assert(onPrimary != null),
+        assert(onSecondary != null),
+        assert(onSurface != null),
+        assert(onBackground != null),
+        assert(onError != null),
+        assert(brightness != null);
+
+  /// Create a high contrast ColorScheme based on the dark
+  /// [baseline Material color scheme](https://material.io/design/color/dark-theme.html#ui-application).
+  const ColorScheme.highContrastDark({
+    this.primary = const Color(0xffefb7ff),
+    this.primaryVariant = const Color(0xffbe9eff),
+    this.secondary = const Color(0xff66fff9),
+    this.secondaryVariant = const Color(0xff66fff9),
+    this.surface = const Color(0xff121212),
+    this.background = const Color(0xff121212),
+    this.error = const Color(0xff9b374d),
+    this.onPrimary = Colors.black,
+    this.onSecondary = Colors.black,
+    this.onSurface = Colors.white,
+    this.onBackground = Colors.white,
+    this.onError = Colors.black,
+    this.brightness = Brightness.dark,
+  }) : assert(primary != null),
+        assert(primaryVariant != null),
+        assert(secondary != null),
+        assert(secondaryVariant != null),
+        assert(surface != null),
+        assert(background != null),
+        assert(error != null),
+        assert(onPrimary != null),
+        assert(onSecondary != null),
+        assert(onSurface != null),
+        assert(onBackground != null),
+        assert(onError != null),
+        assert(brightness != null);
+
   /// Create a color scheme from a [MaterialColor] swatch.
   ///
   /// This constructor is used by [ThemeData] to create its default

packages/flutter/lib/src/widgets/media_query.dart

@@ -843,6 +843,17 @@ class MediaQuery extends InheritedWidget {
     return MediaQuery.of(context, nullOk: true)?.platformBrightness ?? Brightness.light;
   }
 
+  /// Returns highContrast for the nearest MediaQuery ancestor or false, if no
+  /// such ancestor exists.
+  ///
+  /// See also:
+  ///
+  ///  * [MediaQueryData.highContrast], which indicates the platform's
+  ///    desire to increase contrast.
+  static bool highContrastOf(BuildContext context) {
+    return MediaQuery.of(context, nullOk: true)?.highContrast ?? false;
+  }
+
   /// Returns the boldText accessibility setting for the nearest MediaQuery
   /// ancestor, or false if no such ancestor exists.
   static bool boldTextOverride(BuildContext context) {

packages/flutter/test/material/app_test.dart

@@ -752,6 +752,66 @@ void main() {
     expect(appliedTheme.brightness, Brightness.dark);
   });
 
+  testWidgets('MaterialApp uses high contrast theme when appropriate', (WidgetTester tester) async {
+    tester.binding.window.platformBrightnessTestValue = Brightness.light;
+    tester.binding.window.accessibilityFeaturesTestValue = MockAccessibilityFeature();
+
+    ThemeData appliedTheme;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          primaryColor: Colors.lightBlue,
+        ),
+        highContrastTheme: ThemeData(
+          primaryColor: Colors.blue,
+        ),
+        home: Builder(
+          builder: (BuildContext context) {
+            appliedTheme = Theme.of(context);
+            return const SizedBox();
+          },
+        ),
+      ),
+    );
+
+    expect(appliedTheme.primaryColor, Colors.blue);
+    tester.binding.window.accessibilityFeaturesTestValue = null;
+  });
+
+  testWidgets('MaterialApp uses high contrast dark theme when appropriate', (WidgetTester tester) async {
+    tester.binding.window.platformBrightnessTestValue = Brightness.dark;
+    tester.binding.window.accessibilityFeaturesTestValue = MockAccessibilityFeature();
+
+    ThemeData appliedTheme;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          primaryColor: Colors.lightBlue,
+        ),
+        darkTheme: ThemeData(
+          primaryColor: Colors.lightGreen,
+        ),
+        highContrastTheme: ThemeData(
+          primaryColor: Colors.blue,
+        ),
+        highContrastDarkTheme: ThemeData(
+          primaryColor: Colors.green,
+        ),
+        home: Builder(
+          builder: (BuildContext context) {
+            appliedTheme = Theme.of(context);
+            return const SizedBox();
+          },
+        ),
+      ),
+    );
+
+    expect(appliedTheme.primaryColor, Colors.green);
+    tester.binding.window.accessibilityFeaturesTestValue = null;
+  });
+
   testWidgets('MaterialApp switches themes when the Window platformBrightness changes.', (WidgetTester tester) async {
     // Mock the Window to explicitly report a light platformBrightness.
     final TestWidgetsFlutterBinding binding = tester.binding;

packages/flutter/test/material/color_scheme_test.dart

@@ -45,4 +45,42 @@ void main() {
     expect(scheme.onError, const Color(0xff000000));
     expect(scheme.brightness, Brightness.dark);
   });
+
+  test('high contrast light scheme matches the spec', () {
+    // Colors are based off of the Material Design baseline default theme:
+    // https://material.io/design/color/dark-theme.html#ui-application
+    const ColorScheme scheme = ColorScheme.highContrastLight();
+    expect(scheme.primary, const Color(0xff0000ba));
+    expect(scheme.primaryVariant, const Color(0xff000088));
+    expect(scheme.secondary, const Color(0xff66fff9));
+    expect(scheme.secondaryVariant, const Color(0xff018786));
+    expect(scheme.background, const Color(0xffffffff));
+    expect(scheme.surface, const Color(0xffffffff));
+    expect(scheme.error, const Color(0xff790000));
+    expect(scheme.onPrimary, const Color(0xffffffff));
+    expect(scheme.onSecondary, const Color(0xff000000));
+    expect(scheme.onBackground, const Color(0xff000000));
+    expect(scheme.onSurface, const Color(0xff000000));
+    expect(scheme.onError, const Color(0xffffffff));
+    expect(scheme.brightness, Brightness.light);
+  });
+
+  test('high contrast dark scheme matches the spec', () {
+    // Colors are based off of the Material Design baseline dark theme:
+    // https://material.io/design/color/dark-theme.html#ui-application
+    const ColorScheme scheme = ColorScheme.highContrastDark();
+    expect(scheme.primary, const Color(0xffefb7ff));
+    expect(scheme.primaryVariant, const Color(0xffbe9eff));
+    expect(scheme.secondary, const Color(0xff66fff9));
+    expect(scheme.secondaryVariant, const Color(0xff66fff9));
+    expect(scheme.background, const Color(0xff121212));
+    expect(scheme.surface, const Color(0xff121212));
+    expect(scheme.error, const Color(0xff9b374d));
+    expect(scheme.onPrimary, const Color(0xff000000));
+    expect(scheme.onSecondary, const Color(0xff000000));
+    expect(scheme.onBackground, const Color(0xffffffff));
+    expect(scheme.onSurface, const Color(0xffffffff));
+    expect(scheme.onError, const Color(0xff000000));
+    expect(scheme.brightness, Brightness.dark);
+  });
 }

packages/flutter/test/widgets/media_query_test.dart

@@ -535,6 +535,33 @@ void main() {
     expect(insideBrightness, Brightness.dark);
   });
 
+  testWidgets('MediaQuery.highContrastOf', (WidgetTester tester) async {
+    bool outsideHighContrast;
+    bool insideHighContrast;
+
+    await tester.pumpWidget(
+      Builder(
+        builder: (BuildContext context) {
+          outsideHighContrast = MediaQuery.highContrastOf(context);
+          return MediaQuery(
+            data: const MediaQueryData(
+              highContrast: true,
+            ),
+            child: Builder(
+              builder: (BuildContext context) {
+                insideHighContrast = MediaQuery.highContrastOf(context);
+                return Container();
+              },
+            ),
+          );
+        },
+      ),
+    );
+
+    expect(outsideHighContrast, false);
+    expect(insideHighContrast, true);
+  });
+
   testWidgets('MediaQuery.boldTextOverride', (WidgetTester tester) async {
     bool outsideBoldTextOverride;
     bool insideBoldTextOverride;