Expose the duration and curve for theme animation in MaterialApp. (#107383)

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

@@ -220,6 +220,8 @@ class MaterialApp extends StatefulWidget {
     this.highContrastTheme,
     this.highContrastDarkTheme,
     this.themeMode = ThemeMode.system,
+    this.themeAnimationDuration = kThemeAnimationDuration,
+    this.themeAnimationCurve = Curves.linear,
     this.locale,
     this.localizationsDelegates,
     this.localeListResolutionCallback,
@@ -271,6 +273,8 @@ class MaterialApp extends StatefulWidget {
     this.highContrastTheme,
     this.highContrastDarkTheme,
     this.themeMode = ThemeMode.system,
+    this.themeAnimationDuration = kThemeAnimationDuration,
+    this.themeAnimationCurve = Curves.linear,
     this.locale,
     this.localizationsDelegates,
     this.localeListResolutionCallback,
@@ -472,6 +476,30 @@ class MaterialApp extends StatefulWidget {
   ///    system what kind of theme is being used.
   final ThemeMode? themeMode;
 
+  /// The duration of animated theme changes.
+  ///
+  /// When the theme changes (either by the [theme], [darkTheme] or [themeMode]
+  /// parameters changing) it is animated to the new theme over time.
+  /// The [themeAnimationDuration] determines how long this animation takes.
+  ///
+  /// To have the theme change immediately, you can set this to [Duration.zero].
+  ///
+  /// The default is [kThemeAnimationDuration].
+  ///
+  /// See also:
+  ///   [themeAnimationCurve], which defines the curve used for the animation.
+  final Duration themeAnimationDuration;
+
+  /// The curve to apply when animating theme changes.
+  ///
+  /// The default is [Curves.linear].
+  ///
+  /// This is ignored if [themeAnimationDuration] is [Duration.zero].
+  ///
+  /// See also:
+  ///   [themeAnimationDuration], which defines how long the animation is.
+  final Curve themeAnimationCurve;
+
   /// {@macro flutter.widgets.widgetsApp.color}
   final Color? color;
 
@@ -896,6 +924,8 @@ class _MaterialAppState extends State<MaterialApp> {
         cursorColor: effectiveCursorColor,
         child: AnimatedTheme(
           data: theme,
+          duration: widget.themeAnimationDuration,
+          curve: widget.themeAnimationCurve,
           child: widget.builder != null
             ? Builder(
                 builder: (BuildContext context) {

packages/flutter/test/material/app_test.dart

@@ -853,6 +853,88 @@ void main() {
     tester.binding.platformDispatcher.clearPlatformBrightnessTestValue();
   });
 
+  testWidgets('MaterialApp animates theme changes', (WidgetTester tester) async {
+    final ThemeData lightTheme = ThemeData.light();
+    final ThemeData darkTheme = ThemeData.dark();
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: lightTheme,
+        darkTheme: darkTheme,
+        themeMode: ThemeMode.light,
+        home: Builder(
+          builder: (BuildContext context) {
+            return const Scaffold();
+          },
+        ),
+      ),
+    );
+    expect(tester.widget<Material>(find.byType(Material)).color, lightTheme.scaffoldBackgroundColor);
+
+    // Change to dark theme
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData.light(),
+        darkTheme: ThemeData.dark(),
+        themeMode: ThemeMode.dark,
+        home: Builder(
+          builder: (BuildContext context) {
+            return const Scaffold();
+          },
+        ),
+      ),
+    );
+
+    // Wait half kThemeAnimationDuration = 200ms.
+    await tester.pump(const Duration(milliseconds: 100));
+
+    // Default curve is linear so background should be half way between
+    // the two colors.
+    final Color halfBGColor = Color.lerp(lightTheme.scaffoldBackgroundColor, darkTheme.scaffoldBackgroundColor, 0.5)!;
+    expect(tester.widget<Material>(find.byType(Material)).color, halfBGColor);
+  });
+
+  testWidgets('MaterialApp theme animation can be turned off', (WidgetTester tester) async {
+    final ThemeData lightTheme = ThemeData.light();
+    final ThemeData darkTheme = ThemeData.dark();
+    int scaffoldRebuilds = 0;
+
+    final Widget scaffold = Builder(
+      builder: (BuildContext context) {
+        scaffoldRebuilds++;
+        // Use Theme.of() to ensure we are building when the theme changes.
+        return Scaffold(backgroundColor: Theme.of(context).scaffoldBackgroundColor);
+      },
+    );
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: lightTheme,
+        darkTheme: darkTheme,
+        themeMode: ThemeMode.light,
+        themeAnimationDuration: Duration.zero,
+        home: scaffold,
+      ),
+    );
+    expect(tester.widget<Material>(find.byType(Material)).color, lightTheme.scaffoldBackgroundColor);
+    expect(scaffoldRebuilds, 1);
+
+    // Change to dark theme
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData.light(),
+        darkTheme: ThemeData.dark(),
+        themeMode: ThemeMode.dark,
+        themeAnimationDuration: Duration.zero,
+        home: scaffold,
+      ),
+    );
+
+    // Wait for any animation to finish.
+    await tester.pumpAndSettle();
+    expect(tester.widget<Material>(find.byType(Material)).color, darkTheme.scaffoldBackgroundColor);
+    expect(scaffoldRebuilds, 2);
+  });
+
   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;