[AppBarTheme] adds titleSpacing parameter (#67105)

* adds titleSpacing in AppBarTheme
Co-authored-by: Shi-Hao Hong <shihaohong@google.com>

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

@@ -203,7 +203,7 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
     this.primary = true,
     this.centerTitle,
     this.excludeHeaderSemantics = false,
-    this.titleSpacing = NavigationToolbar.kMiddleSpacing,
+    this.titleSpacing,
     this.toolbarOpacity = 1.0,
     this.bottomOpacity = 1.0,
     this.toolbarHeight,
@@ -211,7 +211,6 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   }) : assert(automaticallyImplyLeading != null),
        assert(elevation == null || elevation >= 0.0),
        assert(primary != null),
-       assert(titleSpacing != null),
        assert(toolbarOpacity != null),
        assert(bottomOpacity != null),
        preferredSize = Size.fromHeight(toolbarHeight ?? kToolbarHeight + (bottom?.preferredSize.height ?? 0.0)),
@@ -424,7 +423,7 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   /// [title] to take all the space available, set this value to 0.0.
   ///
   /// Defaults to [NavigationToolbar.kMiddleSpacing].
-  final double titleSpacing;
+  final double? titleSpacing;
 
   /// How opaque the toolbar part of the app bar is.
   ///
@@ -634,7 +633,7 @@ class _AppBarState extends State<AppBar> {
       middle: title,
       trailing: actions,
       centerMiddle: widget._getEffectiveCenterTitle(theme!),
-      middleSpacing: widget.titleSpacing,
+      middleSpacing: widget.titleSpacing ?? appBarTheme.titleSpacing ?? NavigationToolbar.kMiddleSpacing,
     );
 
     // If the toolbar is allocated less than toolbarHeight make it
@@ -848,7 +847,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
   final bool primary;
   final bool? centerTitle;
   final bool excludeHeaderSemantics;
-  final double titleSpacing;
+  final double? titleSpacing;
   final double? expandedHeight;
   final double collapsedHeight;
   final double topPadding;
@@ -1064,7 +1063,7 @@ class SliverAppBar extends StatefulWidget {
     this.primary = true,
     this.centerTitle,
     this.excludeHeaderSemantics = false,
-    this.titleSpacing = NavigationToolbar.kMiddleSpacing,
+    this.titleSpacing,
     this.collapsedHeight,
     this.expandedHeight,
     this.floating = false,
@@ -1079,7 +1078,6 @@ class SliverAppBar extends StatefulWidget {
   }) : assert(automaticallyImplyLeading != null),
        assert(forceElevated != null),
        assert(primary != null),
-       assert(titleSpacing != null),
        assert(floating != null),
        assert(pinned != null),
        assert(snap != null),
@@ -1259,7 +1257,7 @@ class SliverAppBar extends StatefulWidget {
   /// [title] to take all the space available, set this value to 0.0.
   ///
   /// Defaults to [NavigationToolbar.kMiddleSpacing].
-  final double titleSpacing;
+  final double? titleSpacing;
 
   /// Defines the height of the app bar when it is collapsed.
   ///

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

@@ -39,6 +39,7 @@ class AppBarTheme with Diagnosticable {
     this.actionsIconTheme,
     this.textTheme,
     this.centerTitle,
+    this.titleSpacing,
   });
 
   /// Default value for [AppBar.brightness].
@@ -81,6 +82,11 @@ class AppBarTheme with Diagnosticable {
   /// If null, the value is adapted to current [TargetPlatform].
   final bool? centerTitle;
 
+  /// Default value for [AppBar.titleSpacing].
+  ///
+  /// If null, [AppBar] uses default value of [NavigationToolbar.kMiddleSpacing].
+  final double? titleSpacing;
+
   /// Creates a copy of this object with the given fields replaced with the
   /// new values.
   AppBarTheme copyWith({
@@ -92,6 +98,7 @@ class AppBarTheme with Diagnosticable {
     IconThemeData? iconTheme,
     TextTheme? textTheme,
     bool? centerTitle,
+    double? titleSpacing,
   }) {
     return AppBarTheme(
       brightness: brightness ?? this.brightness,
@@ -102,6 +109,7 @@ class AppBarTheme with Diagnosticable {
       actionsIconTheme: actionsIconTheme ?? this.actionsIconTheme,
       textTheme: textTheme ?? this.textTheme,
       centerTitle: centerTitle ?? this.centerTitle,
+      titleSpacing: titleSpacing ?? this.titleSpacing,
     );
   }
 
@@ -126,6 +134,7 @@ class AppBarTheme with Diagnosticable {
       actionsIconTheme: IconThemeData.lerp(a?.actionsIconTheme, b?.actionsIconTheme, t),
       textTheme: TextTheme.lerp(a?.textTheme, b?.textTheme, t),
       centerTitle: t < 0.5 ? a?.centerTitle : b?.centerTitle,
+      titleSpacing: lerpDouble(a?.titleSpacing, b?.titleSpacing, t),
     );
   }
 
@@ -140,6 +149,7 @@ class AppBarTheme with Diagnosticable {
       actionsIconTheme,
       textTheme,
       centerTitle,
+      titleSpacing,
     );
   }
 
@@ -157,7 +167,8 @@ class AppBarTheme with Diagnosticable {
         && other.iconTheme == iconTheme
         && other.actionsIconTheme == actionsIconTheme
         && other.textTheme == textTheme
-        && other.centerTitle == centerTitle;
+        && other.centerTitle == centerTitle
+        && other.titleSpacing == titleSpacing;
   }
 
   @override
@@ -171,5 +182,6 @@ class AppBarTheme with Diagnosticable {
     properties.add(DiagnosticsProperty<IconThemeData>('actionsIconTheme', actionsIconTheme, defaultValue: null));
     properties.add(DiagnosticsProperty<TextTheme>('textTheme', textTheme, defaultValue: null));
     properties.add(DiagnosticsProperty<bool>('centerTitle', centerTitle, defaultValue: null));
+    properties.add(DiagnosticsProperty<double>('titleSpacing', titleSpacing, defaultValue: null));
   }
 }

packages/flutter/test/material/app_bar_test.dart

@@ -2211,4 +2211,27 @@ void main() {
     NavigationToolbar getAppBarWidget(Finder finder) => tester.widget<NavigationToolbar>(finder);
     expect(getAppBarWidget(appBarFinder).leading, null);
   });
+
+  testWidgets('AppBar.titleSpacing defaults to NavigationToolbar.kMiddleSpacing', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(
+          title: const Text('Title'),
+        ),
+      ),
+    ));
+
+    final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
+    expect(navToolBar.middleSpacing, NavigationToolbar.kMiddleSpacing);
+  });
+
+  testWidgets('SliverAppBar.titleSpacing defaults to NavigationToolbar.kMiddleSpacing', (WidgetTester tester) async {
+    await tester.pumpWidget(buildSliverAppBarApp(
+      floating: false,
+      pinned: false,
+    ));
+
+    final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
+    expect(navToolBar.middleSpacing, NavigationToolbar.kMiddleSpacing);
+  });
 }

packages/flutter/test/material/app_bar_theme_test.dart

@@ -7,6 +7,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -245,6 +246,116 @@ void main() {
     // The AppBar.shadowColor should be used instead of AppBarTheme.shadowColor.
     expect(appBar.shadowColor, Colors.yellow);
   });
+
+  testWidgets('AppBar uses AppBarTheme.titleSpacing', (WidgetTester tester) async {
+    const double kTitleSpacing = 10;
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(appBarTheme: const AppBarTheme(titleSpacing: kTitleSpacing)),
+      home: Scaffold(
+        appBar: AppBar(
+          title: const Text('Title'),
+        ),
+      ),
+    ));
+
+    final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
+    expect(navToolBar.middleSpacing, kTitleSpacing);
+  });
+
+  testWidgets('AppBar.titleSpacing takes priority over AppBarTheme.titleSpacing', (WidgetTester tester) async {
+    const double kTitleSpacing = 10;
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(appBarTheme: const AppBarTheme(titleSpacing: kTitleSpacing)),
+      home: Scaffold(
+        appBar: AppBar(
+          title: const Text('Title'),
+          titleSpacing: 40,
+        ),
+      ),
+    ));
+
+    final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
+    expect(navToolBar.middleSpacing, 40);
+  });
+
+  testWidgets('SliverAppBar uses AppBarTheme.titleSpacing', (WidgetTester tester) async {
+    const double kTitleSpacing = 10;
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(appBarTheme: const AppBarTheme(titleSpacing: kTitleSpacing)),
+      home: const CustomScrollView(
+        slivers: <Widget>[
+          SliverAppBar(
+            title: Text('Title'),
+          ),
+        ],
+      ),
+    ));
+
+    final NavigationToolbar navToolBar = tester.widget(find.byType(NavigationToolbar));
+    expect(navToolBar.middleSpacing, kTitleSpacing);
+  });
+
+  testWidgets('SliverAppBar.titleSpacing takes priority over AppBarTheme.titleSpacing ', (WidgetTester tester) async {
+    const double kTitleSpacing = 10;
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(appBarTheme: const AppBarTheme(titleSpacing: kTitleSpacing)),
+      home: const CustomScrollView(
+        slivers: <Widget>[
+          SliverAppBar(
+            title: Text('Title'),
+            titleSpacing: 40,
+          ),
+        ],
+      ),
+    ));
+
+    final NavigationToolbar navToolbar = tester.widget(find.byType(NavigationToolbar));
+    expect(navToolbar.middleSpacing, 40);
+  });
+
+  testWidgets('Default AppBarTheme debugFillProperties', (WidgetTester tester) async {
+    final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
+    const AppBarTheme().debugFillProperties(builder);
+
+    final List<String> description = builder.properties
+      .where((DiagnosticsNode node) => !node.isFiltered(DiagnosticLevel.info))
+      .map((DiagnosticsNode node) => node.toString())
+      .toList();
+
+    expect(description, <String>[]);
+  });
+
+  testWidgets('AppBarTheme implements debugFillProperties', (WidgetTester tester) async {
+    final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
+    const AppBarTheme(
+      brightness: Brightness.dark,
+      color: Color(0xff000001),
+      elevation: 8.0,
+      shadowColor: Color(0xff000002),
+      centerTitle: true,
+      titleSpacing: 40.0,
+    ).debugFillProperties(builder);
+
+    final List<String> description = builder.properties
+      .where((DiagnosticsNode node) => !node.isFiltered(DiagnosticLevel.info))
+      .map((DiagnosticsNode node) => node.toString())
+      .toList();
+
+    expect(description, <String>[
+      'brightness: Brightness.dark',
+      'color: Color(0xff000001)',
+      'elevation: 8.0',
+      'shadowColor: Color(0xff000002)',
+      'centerTitle: true',
+      'titleSpacing: 40.0',
+    ]);
+
+    // On the web, Dart doubles and ints are backed by the same kind of object because
+    // JavaScript does not support integers. So, the Dart double "4.0" is identical
+    // to "4", which results in the web evaluating to the value "4" regardless of which
+    // one is used. This results in a difference for doubles in debugFillProperties between
+    // the web and the rest of Flutter's target platforms.
+  }, skip: kIsWeb);
 }
 
 AppBarTheme _appBarTheme() {