Added backwardsCompatibility flag to AppBarTheme (#72472)

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

@@ -192,7 +192,7 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
     this.bottomOpacity = 1.0,
     this.toolbarHeight,
     this.leadingWidth,
-    this.backwardsCompatibility = true,
+    this.backwardsCompatibility,
     this.toolbarTextStyle,
     this.titleTextStyle,
     this.systemOverlayStyle,
@@ -201,7 +201,6 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
        assert(primary != null),
        assert(toolbarOpacity != null),
        assert(bottomOpacity != null),
-       assert(backwardsCompatibility != null),
        preferredSize = Size.fromHeight(toolbarHeight ?? kToolbarHeight + (bottom?.preferredSize.height ?? 0.0)),
        super(key: key);
 
@@ -609,16 +608,18 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   /// [iconTheme], [actionsIconTheme] properties, and the original use of
   /// the [textTheme] and [brightness] properties.
   ///
-  /// This property is true by default.
+  /// If this property is null, then [AppBarTheme.backwardsCompatibility] of
+  /// [ThemeData.appBarTheme] is used. If that is also null, the default
+  /// value is true.
   ///
   /// This is a temporary property. When setting it to false is no
-  /// longer considereed a breaking change, it will be depreacted and
+  /// longer considered a breaking change, it will be depreacted and
   /// its default value will be changed to false. App developers are
   /// encouraged to opt into the new features by setting it to false
   /// and using the [foregroundColor] and [systemOverlayStyle]
   /// properties as needed.
   /// {@endtemplate}
-  final bool backwardsCompatibility;
+  final bool? backwardsCompatibility;
 
   /// {@template flutter.material.appbar.toolbarTextStyle}
   /// The default text style for the AppBar's [leading], and
@@ -724,8 +725,9 @@ class _AppBarState extends State<AppBar> {
     final bool useCloseButton = parentRoute is PageRoute<dynamic> && parentRoute.fullscreenDialog;
 
     final double toolbarHeight = widget.toolbarHeight ?? kToolbarHeight;
+    final bool backwardsCompatibility = widget.backwardsCompatibility ?? appBarTheme.backwardsCompatibility ?? true;
 
-    final Color backgroundColor = widget.backwardsCompatibility
+    final Color backgroundColor = backwardsCompatibility
       ? widget.backgroundColor
         ?? appBarTheme.color
         ?? theme.primaryColor
@@ -737,7 +739,7 @@ class _AppBarState extends State<AppBar> {
       ?? appBarTheme.foregroundColor
       ?? (colorScheme.brightness == Brightness.dark ? colorScheme.onSurface : colorScheme.onPrimary);
 
-    IconThemeData overallIconTheme = widget.backwardsCompatibility
+    IconThemeData overallIconTheme = backwardsCompatibility
       ? widget.iconTheme
         ?? appBarTheme.iconTheme
         ?? theme.primaryIconTheme
@@ -749,7 +751,7 @@ class _AppBarState extends State<AppBar> {
       ?? appBarTheme.actionsIconTheme
       ?? overallIconTheme;
 
-    TextStyle? toolbarTextStyle = widget.backwardsCompatibility
+    TextStyle? toolbarTextStyle = backwardsCompatibility
       ? widget.textTheme?.bodyText2
         ?? appBarTheme.textTheme?.bodyText2
         ?? theme.primaryTextTheme.bodyText2
@@ -757,7 +759,7 @@ class _AppBarState extends State<AppBar> {
         ?? appBarTheme.toolbarTextStyle
         ?? theme.textTheme.bodyText2?.copyWith(color: foregroundColor);
 
-    TextStyle? titleTextStyle = widget.backwardsCompatibility
+    TextStyle? titleTextStyle = backwardsCompatibility
       ? widget.textTheme?.headline6
         ?? appBarTheme.textTheme?.headline6
         ?? theme.primaryTextTheme.headline6
@@ -951,7 +953,7 @@ class _AppBarState extends State<AppBar> {
     }
 
     final Brightness overlayStyleBrightness = widget.brightness ?? appBarTheme.brightness ?? colorScheme.brightness;
-    final SystemUiOverlayStyle overlayStyle = widget.backwardsCompatibility
+    final SystemUiOverlayStyle overlayStyle = backwardsCompatibility
       ? (overlayStyleBrightness == Brightness.dark ? SystemUiOverlayStyle.light : SystemUiOverlayStyle.dark)
       : widget.systemOverlayStyle
         ?? appBarTheme.systemOverlayStyle

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

@@ -42,6 +42,7 @@ class AppBarTheme with Diagnosticable {
     this.toolbarTextStyle,
     this.titleTextStyle,
     this.systemOverlayStyle,
+    this.backwardsCompatibility,
   });
 
   /// This property is obsolete, please use [systemOverlayStyle] instead.
@@ -161,6 +162,10 @@ class AppBarTheme with Diagnosticable {
   /// property in all descendant [AppBar] widgets.
   final SystemUiOverlayStyle? systemOverlayStyle;
 
+  /// Overrides the default value of [AppBar.backwardsCompatibility]
+  /// property in all descendant [AppBar] widgets.
+  final bool? backwardsCompatibility;
+
   /// Creates a copy of this object with the given fields replaced with the
   /// new values.
   AppBarTheme copyWith({
@@ -178,6 +183,7 @@ class AppBarTheme with Diagnosticable {
     TextStyle? toolbarTextStyle,
     TextStyle? titleTextStyle,
     SystemUiOverlayStyle? systemOverlayStyle,
+    bool? backwardsCompatibility,
   }) {
     return AppBarTheme(
       brightness: brightness ?? this.brightness,
@@ -194,6 +200,7 @@ class AppBarTheme with Diagnosticable {
       toolbarTextStyle: toolbarTextStyle ?? this.toolbarTextStyle,
       titleTextStyle: titleTextStyle ?? this.titleTextStyle,
       systemOverlayStyle: systemOverlayStyle ?? this.systemOverlayStyle,
+      backwardsCompatibility: backwardsCompatibility ?? this.backwardsCompatibility,
     );
   }
 
@@ -224,6 +231,7 @@ class AppBarTheme with Diagnosticable {
       toolbarTextStyle: TextStyle.lerp(a?.toolbarTextStyle, b?.toolbarTextStyle, t),
       titleTextStyle: TextStyle.lerp(a?.titleTextStyle, b?.titleTextStyle, t),
       systemOverlayStyle: t < 0.5 ? a?.systemOverlayStyle : b?.systemOverlayStyle,
+      backwardsCompatibility: t < 0.5 ? a?.backwardsCompatibility : b?.backwardsCompatibility,
     );
   }
 
@@ -244,6 +252,7 @@ class AppBarTheme with Diagnosticable {
       toolbarTextStyle,
       titleTextStyle,
       systemOverlayStyle,
+      backwardsCompatibility,
     );
   }
 
@@ -267,7 +276,8 @@ class AppBarTheme with Diagnosticable {
         && other.titleSpacing == titleSpacing
         && other.toolbarTextStyle == toolbarTextStyle
         && other.titleTextStyle == titleTextStyle
-        && other.systemOverlayStyle == systemOverlayStyle;
+        && other.systemOverlayStyle == systemOverlayStyle
+        && other.backwardsCompatibility == backwardsCompatibility;
   }
 
   @override
@@ -286,5 +296,6 @@ class AppBarTheme with Diagnosticable {
     properties.add(DiagnosticsProperty<double>('titleSpacing', titleSpacing, defaultValue: null));
     properties.add(DiagnosticsProperty<TextStyle>('toolbarTextStyle', toolbarTextStyle, defaultValue: null));
     properties.add(DiagnosticsProperty<TextStyle>('titleTextStyle', titleTextStyle, defaultValue: null));
+    properties.add(DiagnosticsProperty<bool>('backwardsCompatibility', backwardsCompatibility, defaultValue: null));
   }
 }

packages/flutter/test/material/app_bar_test.dart

@@ -2273,4 +2273,90 @@ void main() {
     final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
     expect(navToolBar.middleSpacing, NavigationToolbar.kMiddleSpacing);
   });
+
+  testWidgets('AppBar foregroundColor and backgroundColor', (WidgetTester tester) async {
+    const Color foregroundColor = Color(0xff00ff00);
+    const Color backgroundColor = Color(0xff00ffff);
+    final Key leadingIconKey = UniqueKey();
+    final Key actionIconKey = UniqueKey();
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          appBar: AppBar(
+            backwardsCompatibility: false,
+            foregroundColor: foregroundColor,
+            backgroundColor: backgroundColor,
+            leading: Icon(Icons.add_circle, key: leadingIconKey),
+            title: const Text('title'),
+            actions: <Widget>[Icon(Icons.add_circle, key: actionIconKey), const Text('action')],
+          ),
+        ),
+      ),
+    );
+
+    final Material appBarMaterial = tester.widget<Material>(
+      find.descendant(
+        of: find.byType(AppBar),
+        matching: find.byType(Material),
+      ),
+    );
+    expect(appBarMaterial.color, backgroundColor);
+
+    final TextStyle titleTextStyle = tester.widget<DefaultTextStyle>(
+      find.ancestor(of: find.text('title'), matching: find.byType(DefaultTextStyle)).first,
+    ).style;
+    expect(titleTextStyle.color, foregroundColor);
+
+    final IconThemeData leadingIconTheme = tester.widget<IconTheme>(
+      find.ancestor(of: find.byKey(leadingIconKey), matching: find.byType(IconTheme)).first,
+    ).data;
+    expect(leadingIconTheme.color, foregroundColor);
+
+    final IconThemeData actionIconTheme = tester.widget<IconTheme>(
+      find.ancestor(of: find.byKey(actionIconKey), matching: find.byType(IconTheme)).first,
+    ).data;
+    expect(actionIconTheme.color, foregroundColor);
+  });
+
+
+  testWidgets('AppBarTheme.backwardsCompatibility', (WidgetTester tester) async {
+    const Color foregroundColor = Color(0xff00ff00);
+    final Key leadingIconKey = UniqueKey();
+    final Key actionIconKey = UniqueKey();
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData.light().copyWith(
+          appBarTheme: const AppBarTheme(
+            backwardsCompatibility: false,
+          ),
+        ),
+        home: Scaffold(
+          appBar: AppBar(
+            foregroundColor: foregroundColor, // only applies if backwardsCompatibility is false
+            leading: Icon(Icons.add_circle, key: leadingIconKey),
+            title: const Text('title'),
+            actions: <Widget>[Icon(Icons.add_circle, key: actionIconKey), const Text('action')],
+          ),
+        ),
+      ),
+    );
+
+    final TextStyle titleTextStyle = tester.widget<DefaultTextStyle>(
+      find.ancestor(of: find.text('title'), matching: find.byType(DefaultTextStyle)).first,
+    ).style;
+    expect(titleTextStyle.color, foregroundColor);
+
+    final IconThemeData leadingIconTheme = tester.widget<IconTheme>(
+      find.ancestor(of: find.byKey(leadingIconKey), matching: find.byType(IconTheme)).first,
+    ).data;
+    expect(leadingIconTheme.color, foregroundColor);
+
+    final IconThemeData actionIconTheme = tester.widget<IconTheme>(
+      find.ancestor(of: find.byKey(actionIconKey), matching: find.byType(IconTheme)).first,
+    ).data;
+    expect(actionIconTheme.color, foregroundColor);
+  });
+
 }

packages/flutter/test/material/app_bar_theme_test.dart

@@ -440,6 +440,7 @@ void main() {
   testWidgets('AppBarTheme implements debugFillProperties', (WidgetTester tester) async {
     final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
     const AppBarTheme(
+      backwardsCompatibility: false,
       brightness: Brightness.dark,
       color: Color(0xff000001),
       elevation: 8.0,
@@ -460,6 +461,7 @@ void main() {
       'shadowColor: Color(0xff000002)',
       'centerTitle: true',
       'titleSpacing: 40.0',
+      'backwardsCompatibility: false',
     ]);
 
     // On the web, Dart doubles and ints are backed by the same kind of object because