[AppBarTheme] adds centerTitle property (#57736)

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

@@ -174,8 +174,8 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   /// [elevation] is specified, it must be non-negative.
   ///
   /// If [backgroundColor], [elevation], [brightness], [iconTheme],
-  /// [actionsIconTheme], or [textTheme] are null, then their [AppBarTheme]
-  /// values will be used. If the corresponding [AppBarTheme] property is null,
+  /// [actionsIconTheme], [textTheme] or [centerTitle] are null, then their
+  /// [AppBarTheme] values will be used. If the corresponding [AppBarTheme] property is null,
   /// then the default specified in the property's documentation will be used.
   ///
   /// Typically used in the [Scaffold.appBar] property.
@@ -388,7 +388,8 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
 
   /// Whether the title should be centered.
   ///
-  /// Defaults to being adapted to the current [TargetPlatform].
+  /// If this property is null, then [ThemeData.appBarTheme.centerTitle] is used,
+  /// if that is also null, then value is adapted to the current [TargetPlatform].
   final bool centerTitle;
 
   /// Whether the title should be wrapped with header [Semantics].
@@ -431,6 +432,8 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   bool _getEffectiveCenterTitle(ThemeData theme) {
     if (centerTitle != null)
       return centerTitle;
+    if (theme.appBarTheme.centerTitle != null)
+      return theme.appBarTheme.centerTitle;
     assert(theme.platform != null);
     switch (theme.platform) {
       case TargetPlatform.android:

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

@@ -37,6 +37,7 @@ class AppBarTheme with Diagnosticable {
     this.iconTheme,
     this.actionsIconTheme,
     this.textTheme,
+    this.centerTitle,
   });
 
   /// Default value for [AppBar.brightness].
@@ -69,6 +70,11 @@ class AppBarTheme with Diagnosticable {
   /// If null, [AppBar] uses [ThemeData.primaryTextTheme].
   final TextTheme textTheme;
 
+  /// Default value for [AppBar.centerTitle].
+  ///
+  /// If null, the value is adapted to current [TargetPlatform].
+  final bool centerTitle;
+
   /// Creates a copy of this object with the given fields replaced with the
   /// new values.
   AppBarTheme copyWith({
@@ -78,6 +84,7 @@ class AppBarTheme with Diagnosticable {
     double elevation,
     IconThemeData iconTheme,
     TextTheme textTheme,
+    bool centerTitle,
   }) {
     return AppBarTheme(
       brightness: brightness ?? this.brightness,
@@ -86,6 +93,7 @@ class AppBarTheme with Diagnosticable {
       iconTheme: iconTheme ?? this.iconTheme,
       actionsIconTheme: actionsIconTheme ?? this.actionsIconTheme,
       textTheme: textTheme ?? this.textTheme,
+      centerTitle: centerTitle ?? this.centerTitle,
     );
   }
 
@@ -108,6 +116,7 @@ class AppBarTheme with Diagnosticable {
       iconTheme: IconThemeData.lerp(a?.iconTheme, b?.iconTheme, t),
       actionsIconTheme: IconThemeData.lerp(a?.actionsIconTheme, b?.actionsIconTheme, t),
       textTheme: TextTheme.lerp(a?.textTheme, b?.textTheme, t),
+      centerTitle: t < 0.5 ? a?.centerTitle : b?.centerTitle,
     );
   }
 
@@ -120,6 +129,7 @@ class AppBarTheme with Diagnosticable {
       iconTheme,
       actionsIconTheme,
       textTheme,
+      centerTitle,
     );
   }
 
@@ -135,7 +145,8 @@ class AppBarTheme with Diagnosticable {
         && other.elevation == elevation
         && other.iconTheme == iconTheme
         && other.actionsIconTheme == actionsIconTheme
-        && other.textTheme == textTheme;
+        && other.textTheme == textTheme
+        && other.centerTitle == centerTitle;
   }
 
   @override
@@ -147,5 +158,6 @@ class AppBarTheme with Diagnosticable {
     properties.add(DiagnosticsProperty<IconThemeData>('iconTheme', iconTheme, defaultValue: null));
     properties.add(DiagnosticsProperty<IconThemeData>('actionsIconTheme', actionsIconTheme, defaultValue: null));
     properties.add(DiagnosticsProperty<TextTheme>('textTheme', textTheme, defaultValue: null));
+    properties.add(DiagnosticsProperty<bool>('centerTitle', centerTitle, defaultValue: null));
   }
 }

packages/flutter/test/material/app_bar_theme_test.dart

@@ -182,6 +182,43 @@ void main() {
     // Default value for ThemeData.typography is Typography.material2014()
     expect(text.style, Typography.material2014().englishLike.bodyText2.merge(Typography.material2014().white.bodyText2).merge(themeData.primaryTextTheme.bodyText2));
   });
+
+  testWidgets('AppBar uses AppBarTheme.centerTitle when centerTitle is null', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(appBarTheme: const AppBarTheme(centerTitle: true)),
+      home: Scaffold(appBar: AppBar(title: const Text('Title'))),
+    ));
+
+    final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
+    expect(navToolBar.centerMiddle, true);
+  });
+
+  testWidgets('AppBar.centerTitle takes priority over AppBarTheme.centerTitle', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(appBarTheme: const AppBarTheme(centerTitle: true)),
+      home: Scaffold(
+          appBar: AppBar(
+            title: const Text('Title'),
+            centerTitle: false,
+          )),
+    ));
+
+    final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
+    // The AppBar.centerTitle should be used instead of AppBarTheme.centerTitle.
+    expect(navToolBar.centerMiddle, false);
+  });
+
+  testWidgets('AppBar.centerTitle adapts to TargetPlatform when AppBarTheme.centerTitle is null', (WidgetTester tester) async{
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(platform: TargetPlatform.iOS),
+      home: Scaffold(appBar: AppBar(title: const Text('Title'))),
+    ));
+
+    final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
+    // When ThemeData.platform is TargetPlatform.iOS, and AppBarTheme is null,
+    // the value of NavigationToolBar.centerMiddle should be true.
+    expect(navToolBar.centerMiddle, true);
+  });
 }
 
 AppBarTheme _appBarTheme() {