Normalize BottomAppBarTheme (#168586)

This PR is to make `BottomAppBarTheme` conform to Flutter Material's
conventions for component themes:

- Added a `BottomAppBarThemeData` class which defines overrides for the
defaults for `BottomAppBar` properties.
- Added `BottomAppBarTheme` constructor parameters:
`BottomAppBarThemeData? data` and `Widget? child`. This is now the
preferred way to configure a `BottomAppBarTheme`:

```dart
BottomAppBarTheme(
	data: BottomAppBarThemeData(
	    color: xxx,
	    height: xxx,
	    elevation: xxx,
	    shape: xxx,
	    ...
	  ),
  	child: const BottomAppBar()
)
```
These two properties are made nullable to not break existing apps which
has customized `ThemeData.bottomAppBarTheme`.

- Update `BottomAppBarTheme` to be an `InheritedWidget` subclass.
- Changed the type of component theme defaults from `BottomAppBarTheme`
to `BottomAppBarThemeData`.
- Changed the `BottomAppBarTheme bottomAppBarTheme` property to
`BottomAppBarThemeData bottomAppBarTheme` in `ThemeData` and
`ThemeData.copyWith()`. This may cause breaking changes, a migration
guide will be created on website repo.
- Add new tests for `BottomAppBarThemeData` and update the existing
`BottomAppBarTheme` tests. And also turn to `true` for `useMaterial3`
(along with usages accordingly) at the common method `_withTheme()`
since `useMaterial3` is true by default.
- Addresses the "theme normalization" sub-project within #91772.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

---------

Signed-off-by: huycozy <huy@nevercode.io>

dev/integration_tests/new_gallery/lib/studies/reply/app.dart

@@ -152,7 +152,7 @@ class _RestorableEmailState extends RestorableListenable<EmailStore> {
 ThemeData _buildReplyLightTheme(BuildContext context) {
   final ThemeData base = ThemeData();
   return base.copyWith(
-    bottomAppBarTheme: const BottomAppBarTheme(color: ReplyColors.blue700),
+    bottomAppBarTheme: const BottomAppBarThemeData(color: ReplyColors.blue700),
     bottomSheetTheme: BottomSheetThemeData(
       backgroundColor: ReplyColors.blue700,
       modalBackgroundColor: Colors.white.withOpacity(0.7),
@@ -192,7 +192,7 @@ ThemeData _buildReplyLightTheme(BuildContext context) {
 ThemeData _buildReplyDarkTheme(BuildContext context) {
   final ThemeData base = ThemeData.dark();
   return base.copyWith(
-    bottomAppBarTheme: const BottomAppBarTheme(color: ReplyColors.darkBottomAppBarBackground),
+    bottomAppBarTheme: const BottomAppBarThemeData(color: ReplyColors.darkBottomAppBarBackground),
     bottomSheetTheme: BottomSheetThemeData(
       backgroundColor: ReplyColors.darkDrawerBackground,
       modalBackgroundColor: Colors.black.withOpacity(0.7),

dev/integration_tests/new_gallery/lib/themes/material_demo_theme_data.dart

@@ -20,7 +20,7 @@ class MaterialDemoThemeData {
       color: _colorScheme.primary,
       iconTheme: IconThemeData(color: _colorScheme.onPrimary),
     ),
-    bottomAppBarTheme: BottomAppBarTheme(color: _colorScheme.primary),
+    bottomAppBarTheme: BottomAppBarThemeData(color: _colorScheme.primary),
     checkboxTheme: CheckboxThemeData(
       fillColor: MaterialStateProperty.resolveWith<Color?>((Set<MaterialState> states) {
         if (states.contains(MaterialState.disabled)) {

dev/tools/gen_defaults/lib/bottom_app_bar_template.dart

@@ -14,7 +14,7 @@ class BottomAppBarTemplate extends TokenTemplate {
 
   @override
   String generate() => '''
-class _${blockName}DefaultsM3 extends BottomAppBarTheme {
+class _${blockName}DefaultsM3 extends BottomAppBarThemeData {
   _${blockName}DefaultsM3(this.context)
     : super(
       elevation: ${elevation('md.comp.bottom-app-bar.container')},

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

@@ -67,8 +67,8 @@ class BottomAppBar extends StatefulWidget {
   /// The [clipBehavior] argument defaults to [Clip.none].
   /// Additionally, [elevation] must be non-negative.
   ///
-  /// If [color], [elevation], or [shape] are null, their [BottomAppBarTheme] values will be used.
+  /// If [color], [elevation], or [shape] are null, their [BottomAppBarThemeData] values will be used.
-  /// If the corresponding [BottomAppBarTheme] property is null, then the default
+  /// If the corresponding [BottomAppBarThemeData] property is null, then the default
   /// specified in the property's documentation will be used.
   const BottomAppBar({
     super.key,
@@ -100,7 +100,7 @@ class BottomAppBar extends StatefulWidget {
 
   /// The bottom app bar's background color.
   ///
-  /// If this property is null then [BottomAppBarTheme.color] of
+  /// If this property is null then [BottomAppBarThemeData.color] of
   /// [ThemeData.bottomAppBarTheme] is used. If that's null and [ThemeData.useMaterial3]
   /// is true, the default value is [ColorScheme.surface]; if [ThemeData.useMaterial3]
   /// is false, then the default value is `Color(0xFF424242)` in dark theme and
@@ -113,14 +113,14 @@ class BottomAppBar extends StatefulWidget {
   /// This controls the size of the shadow below the bottom app bar. The
   /// value is non-negative.
   ///
-  /// If this property is null then [BottomAppBarTheme.elevation] of
+  /// If this property is null then [BottomAppBarThemeData.elevation] of
   /// [ThemeData.bottomAppBarTheme] is used. If that's null and
   /// [ThemeData.useMaterial3] is true, than the default value is 3 else is 8.
   final double? elevation;
 
   /// The notch that is made for the floating action button.
   ///
-  /// If this property is null then [BottomAppBarTheme.shape] of
+  /// If this property is null then [BottomAppBarThemeData.shape] of
   /// [ThemeData.bottomAppBarTheme] is used. If that's null then the shape will
   /// be rectangular with no notch.
   final NotchedShape? shape;
@@ -143,7 +143,7 @@ class BottomAppBar extends StatefulWidget {
   /// which provide more flexibility. The intention is to eventually remove surface tint color from
   /// the framework.
   ///
-  /// If this property is null, then [BottomAppBarTheme.surfaceTintColor]
+  /// If this property is null, then [BottomAppBarThemeData.surfaceTintColor]
   /// of [ThemeData.bottomAppBarTheme] is used. If that is also null, the default
   /// value is [Colors.transparent].
   ///
@@ -154,7 +154,7 @@ class BottomAppBar extends StatefulWidget {
 
   /// The color of the shadow below the app bar.
   ///
-  /// If this property is null, then [BottomAppBarTheme.shadowColor] of
+  /// If this property is null, then [BottomAppBarThemeData.shadowColor] of
   /// [ThemeData.bottomAppBarTheme] is used. If that is also null, the default value
   /// is fully opaque black for Material 2, and transparent for Material 3.
   ///
@@ -188,8 +188,8 @@ class _BottomAppBarState extends State<BottomAppBar> {
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
     final bool isMaterial3 = theme.useMaterial3;
-    final BottomAppBarTheme babTheme = BottomAppBarTheme.of(context);
+    final BottomAppBarThemeData babTheme = BottomAppBarTheme.of(context);
-    final BottomAppBarTheme defaults =
+    final BottomAppBarThemeData defaults =
         isMaterial3 ? _BottomAppBarDefaultsM3(context) : _BottomAppBarDefaultsM2(context);
 
     final bool hasFab = Scaffold.of(context).hasFloatingActionButton;
@@ -291,7 +291,7 @@ class _BottomAppBarClipper extends CustomClipper<Path> {
   }
 }
 
-class _BottomAppBarDefaultsM2 extends BottomAppBarTheme {
+class _BottomAppBarDefaultsM2 extends BottomAppBarThemeData {
   const _BottomAppBarDefaultsM2(this.context) : super(elevation: 8.0);
 
   final BuildContext context;
@@ -315,7 +315,7 @@ class _BottomAppBarDefaultsM2 extends BottomAppBarTheme {
 //   dev/tools/gen_defaults/bin/gen_defaults.dart.
 
 // dart format off
-class _BottomAppBarDefaultsM3 extends BottomAppBarTheme {
+class _BottomAppBarDefaultsM3 extends BottomAppBarThemeData {
   _BottomAppBarDefaultsM3(this.context)
     : super(
       elevation: 3.0,

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

@@ -15,11 +15,11 @@ import 'theme.dart';
 
 /// Defines default property values for descendant [BottomAppBar] widgets.
 ///
-/// Descendant widgets obtain the current [BottomAppBarTheme] object using
+/// Descendant widgets obtain the current [BottomAppBarThemeData] object using
-/// `BottomAppBarTheme.of(context)`. Instances of [BottomAppBarTheme] can be
+/// `[BottomAppBarTheme.of]`. Instances of [BottomAppBarThemeData] can be
-/// customized with [BottomAppBarTheme.copyWith].
+/// customized with [BottomAppBarThemeData.copyWith].
 ///
-/// Typically a [BottomAppBarTheme] is specified as part of the overall [Theme]
+/// Typically a [BottomAppBarThemeData] is specified as part of the overall [Theme]
 /// with [ThemeData.bottomAppBarTheme].
 ///
 /// All [BottomAppBarTheme] properties are `null` by default. When null, the
@@ -30,9 +30,192 @@ import 'theme.dart';
 ///  * [ThemeData], which describes the overall theme information for the
 ///    application.
 @immutable
-class BottomAppBarTheme with Diagnosticable {
+class BottomAppBarTheme extends InheritedTheme with Diagnosticable {
   /// Creates a theme that can be used for [ThemeData.bottomAppBarTheme].
   const BottomAppBarTheme({
+    super.key,
+    Color? color,
+    double? elevation,
+    NotchedShape? shape,
+    double? height,
+    Color? surfaceTintColor,
+    Color? shadowColor,
+    EdgeInsetsGeometry? padding,
+    BottomAppBarThemeData? data,
+    Widget? child,
+  }) : assert(
+         data == null ||
+             (color ??
+                     elevation ??
+                     shape ??
+                     height ??
+                     surfaceTintColor ??
+                     shadowColor ??
+                     padding) ==
+                 null,
+       ),
+       _color = color,
+       _elevation = elevation,
+       _shape = shape,
+       _height = height,
+       _surfaceTintColor = surfaceTintColor,
+       _shadowColor = shadowColor,
+       _padding = padding,
+       _data = data,
+       super(child: child ?? const SizedBox.shrink());
+
+  final BottomAppBarThemeData? _data;
+  final Color? _color;
+  final double? _elevation;
+  final NotchedShape? _shape;
+  final double? _height;
+  final Color? _surfaceTintColor;
+  final Color? _shadowColor;
+  final EdgeInsetsGeometry? _padding;
+
+  /// Overrides the default value for [BottomAppBar.color].
+  ///
+  /// This property is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.color] property in [data] instead.
+  Color? get color => _data != null ? _data.color : _color;
+
+  /// Overrides the default value for [BottomAppBar.elevation].
+  ///
+  /// This property is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.elevation] property in [data] instead.
+  double? get elevation => _data != null ? _data.elevation : _elevation;
+
+  /// Overrides the default value for [BottomAppBar.shape].
+  ///
+  /// This property is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.shape] property in [data] instead.
+  NotchedShape? get shape => _data != null ? _data.shape : _shape;
+
+  /// Overrides the default value for [BottomAppBar.height].
+  ///
+  /// This property is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.height] property in [data] instead.
+  double? get height => _data != null ? _data.height : _height;
+
+  /// Overrides the default value for [BottomAppBar.surfaceTintColor].
+  ///
+  /// This property is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.surfaceTintColor] property in [data] instead.
+  ///
+  /// If null, [BottomAppBar] will not display an overlay color.
+  ///
+  /// See [Material.surfaceTintColor] for more details.
+  Color? get surfaceTintColor => _data != null ? _data.surfaceTintColor : _surfaceTintColor;
+
+  /// Overrides the default value for [BottomAppBar.shadowColor].
+  ///
+  /// This property is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.shadowColor] property in [data] instead.
+  Color? get shadowColor => _data != null ? _data.shadowColor : _shadowColor;
+
+  /// Overrides the default value for [BottomAppBar.padding].
+  ///
+  /// This property is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.padding] property in [data] instead.
+  EdgeInsetsGeometry? get padding => _data != null ? _data.padding : _padding;
+
+  /// The properties used for all descendant [BottomAppBar] widgets.
+  BottomAppBarThemeData get data =>
+      _data ??
+      BottomAppBarThemeData(
+        color: _color,
+        elevation: _elevation,
+        shape: _shape,
+        height: _height,
+        surfaceTintColor: _surfaceTintColor,
+        shadowColor: _shadowColor,
+        padding: _padding,
+      );
+
+  /// Creates a copy of this object but with the given fields replaced with the
+  /// new values.
+  ///
+  /// This method is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.copyWith] method instead.
+  BottomAppBarTheme copyWith({
+    Color? color,
+    double? elevation,
+    NotchedShape? shape,
+    double? height,
+    Color? surfaceTintColor,
+    Color? shadowColor,
+    EdgeInsetsGeometry? padding,
+  }) {
+    return BottomAppBarTheme(
+      color: color ?? this.color,
+      elevation: elevation ?? this.elevation,
+      shape: shape ?? this.shape,
+      height: height ?? this.height,
+      surfaceTintColor: surfaceTintColor ?? this.surfaceTintColor,
+      shadowColor: shadowColor ?? this.shadowColor,
+      padding: padding ?? this.padding,
+    );
+  }
+
+  /// Returns the closest [BottomAppBarThemeData] instance given the build context.
+  static BottomAppBarThemeData of(BuildContext context) {
+    final BottomAppBarTheme? bottomAppBarTheme =
+        context.dependOnInheritedWidgetOfExactType<BottomAppBarTheme>();
+    return bottomAppBarTheme?.data ?? Theme.of(context).bottomAppBarTheme;
+  }
+
+  /// Linearly interpolate between two bottom app bar themes.
+  ///
+  /// {@macro dart.ui.shadow.lerp}
+  ///
+  /// This method is obsolete and will be deprecated in a future release:
+  /// please use the [BottomAppBarThemeData.lerp] instead.
+  static BottomAppBarTheme lerp(BottomAppBarTheme? a, BottomAppBarTheme? b, double t) {
+    if (identical(a, b) && a != null) {
+      return a;
+    }
+    return BottomAppBarTheme(
+      color: Color.lerp(a?.color, b?.color, t),
+      elevation: lerpDouble(a?.elevation, b?.elevation, t),
+      shape: t < 0.5 ? a?.shape : b?.shape,
+      height: lerpDouble(a?.height, b?.height, t),
+      surfaceTintColor: Color.lerp(a?.surfaceTintColor, b?.surfaceTintColor, t),
+      shadowColor: Color.lerp(a?.shadowColor, b?.shadowColor, t),
+      padding: EdgeInsetsGeometry.lerp(a?.padding, b?.padding, t),
+    );
+  }
+
+  @override
+  bool updateShouldNotify(BottomAppBarTheme oldWidget) => data != oldWidget.data;
+
+  @override
+  Widget wrap(BuildContext context, Widget child) {
+    return BottomAppBarTheme(data: data, child: child);
+  }
+}
+
+/// Defines default property values for descendant [BottomAppBar] widgets.
+///
+/// Descendant widgets obtain the current [BottomAppBarThemeData] object using
+/// [BottomAppBarTheme.of]. Instances of [BottomAppBarThemeData] can be
+/// customized with [BottomAppBarThemeData.copyWith].
+///
+/// Typically a [BottomAppBarThemeData] is specified as part of the overall [Theme]
+/// with [ThemeData.bottomAppBarTheme].
+///
+/// All [BottomAppBarThemeData] properties are `null` by default. When null, the [BottomAppBar]
+/// will use the values from [ThemeData] if they exist, otherwise it will
+/// provide its own defaults. See the individual [BottomAppBar] properties for details.
+///
+/// See also:
+///
+///  * [BottomAppBar], which is the widget that this theme configures.
+///  * [ThemeData], which describes the overall theme information for the
+///    application.
+@immutable
+class BottomAppBarThemeData with Diagnosticable {
+  /// Creates a bottom app bar theme that can be used with [ThemeData.bottomAppBarTheme].
+  const BottomAppBarThemeData({
     this.color,
     this.elevation,
     this.shape,
@@ -69,7 +252,7 @@ class BottomAppBarTheme with Diagnosticable {
 
   /// Creates a copy of this object but with the given fields replaced with the
   /// new values.
-  BottomAppBarTheme copyWith({
+  BottomAppBarThemeData copyWith({
     Color? color,
     double? elevation,
     NotchedShape? shape,
@@ -78,7 +261,7 @@ class BottomAppBarTheme with Diagnosticable {
     Color? shadowColor,
     EdgeInsetsGeometry? padding,
   }) {
-    return BottomAppBarTheme(
+    return BottomAppBarThemeData(
       color: color ?? this.color,
       elevation: elevation ?? this.elevation,
       shape: shape ?? this.shape,
@@ -89,19 +272,14 @@ class BottomAppBarTheme with Diagnosticable {
     );
   }
 
-  /// The [ThemeData.bottomAppBarTheme] property of the ambient [Theme].
+  /// Linearly interpolate between two bottom app bar themes.
-  static BottomAppBarTheme of(BuildContext context) {
-    return Theme.of(context).bottomAppBarTheme;
-  }
-
-  /// Linearly interpolate between two BAB themes.
   ///
   /// {@macro dart.ui.shadow.lerp}
-  static BottomAppBarTheme lerp(BottomAppBarTheme? a, BottomAppBarTheme? b, double t) {
+  static BottomAppBarThemeData lerp(BottomAppBarThemeData? a, BottomAppBarThemeData? b, double t) {
     if (identical(a, b) && a != null) {
       return a;
     }
-    return BottomAppBarTheme(
+    return BottomAppBarThemeData(
       color: Color.lerp(a?.color, b?.color, t),
       elevation: lerpDouble(a?.elevation, b?.elevation, t),
       shape: t < 0.5 ? a?.shape : b?.shape,
@@ -124,7 +302,7 @@ class BottomAppBarTheme with Diagnosticable {
     if (other.runtimeType != runtimeType) {
       return false;
     }
-    return other is BottomAppBarTheme &&
+    return other is BottomAppBarThemeData &&
         other.color == color &&
         other.elevation == elevation &&
         other.shape == shape &&
@@ -138,11 +316,13 @@ class BottomAppBarTheme with Diagnosticable {
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(ColorProperty('color', color, defaultValue: null));
-    properties.add(DiagnosticsProperty<double>('elevation', elevation, defaultValue: null));
+    properties.add(DoubleProperty('elevation', elevation, defaultValue: null));
-    properties.add(DiagnosticsProperty<NotchedShape>('shape', shape, defaultValue: null));
+    properties.add(DiagnosticsProperty<NotchedShape?>('shape', shape, defaultValue: null));
-    properties.add(DiagnosticsProperty<double>('height', height, defaultValue: null));
+    properties.add(DoubleProperty('height', height, defaultValue: null));
     properties.add(ColorProperty('surfaceTintColor', surfaceTintColor, defaultValue: null));
     properties.add(ColorProperty('shadowColor', shadowColor, defaultValue: null));
-    properties.add(DiagnosticsProperty<EdgeInsetsGeometry>('padding', padding, defaultValue: null));
+    properties.add(
+      DiagnosticsProperty<EdgeInsetsGeometry?>('padding', padding, defaultValue: null),
+    );
   }
 }

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

@@ -320,7 +320,8 @@ class ThemeData with Diagnosticable {
     AppBarTheme? appBarTheme,
     BadgeThemeData? badgeTheme,
     MaterialBannerThemeData? bannerTheme,
-    BottomAppBarTheme? bottomAppBarTheme,
+    // TODO(huycozy): Change the parameter type to BottomAppBarThemeData
+    Object? bottomAppBarTheme,
     BottomNavigationBarThemeData? bottomNavigationBarTheme,
     BottomSheetThemeData? bottomSheetTheme,
     ButtonThemeData? buttonTheme,
@@ -526,7 +527,16 @@ class ThemeData with Diagnosticable {
     appBarTheme ??= const AppBarTheme();
     badgeTheme ??= const BadgeThemeData();
     bannerTheme ??= const MaterialBannerThemeData();
-    bottomAppBarTheme ??= const BottomAppBarTheme();
+    // TODO(huycozy): Clean this up once the type of `bottomAppBarTheme` is changed to `BottomAppBarThemeData`
+    if (bottomAppBarTheme != null) {
+      if (bottomAppBarTheme is BottomAppBarTheme) {
+        bottomAppBarTheme = bottomAppBarTheme.data;
+      } else if (bottomAppBarTheme is! BottomAppBarThemeData) {
+        throw ArgumentError(
+          'bottomAppBarTheme must be either a BottomAppBarThemeData or a BottomAppBarTheme',
+        );
+      }
+    }
     bottomNavigationBarTheme ??= const BottomNavigationBarThemeData();
     bottomSheetTheme ??= const BottomSheetThemeData();
     cardTheme ??= const CardThemeData();
@@ -620,7 +630,9 @@ class ThemeData with Diagnosticable {
       appBarTheme: appBarTheme,
       badgeTheme: badgeTheme,
       bannerTheme: bannerTheme,
-      bottomAppBarTheme: bottomAppBarTheme,
+      // TODO(huycozy): Remove this type cast when bottomAppBarTheme is explicitly set to BottomAppBarThemeData
+      bottomAppBarTheme:
+          (bottomAppBarTheme as BottomAppBarThemeData?) ?? const BottomAppBarThemeData(),
       bottomNavigationBarTheme: bottomNavigationBarTheme,
       bottomSheetTheme: bottomSheetTheme,
       buttonTheme: buttonTheme,
@@ -1292,7 +1304,7 @@ class ThemeData with Diagnosticable {
   final MaterialBannerThemeData bannerTheme;
 
   /// A theme for customizing the shape, elevation, and color of a [BottomAppBar].
-  final BottomAppBarTheme bottomAppBarTheme;
+  final BottomAppBarThemeData bottomAppBarTheme;
 
   /// A theme for customizing the appearance and layout of [BottomNavigationBar]
   /// widgets.
@@ -1519,7 +1531,8 @@ class ThemeData with Diagnosticable {
     AppBarTheme? appBarTheme,
     BadgeThemeData? badgeTheme,
     MaterialBannerThemeData? bannerTheme,
-    BottomAppBarTheme? bottomAppBarTheme,
+    // TODO(huycozy): Change the parameter type to BottomAppBarThemeData
+    Object? bottomAppBarTheme,
     BottomNavigationBarThemeData? bottomNavigationBarTheme,
     BottomSheetThemeData? bottomSheetTheme,
     ButtonThemeData? buttonTheme,
@@ -1638,7 +1651,19 @@ class ThemeData with Diagnosticable {
       appBarTheme: appBarTheme ?? this.appBarTheme,
       badgeTheme: badgeTheme ?? this.badgeTheme,
       bannerTheme: bannerTheme ?? this.bannerTheme,
-      bottomAppBarTheme: bottomAppBarTheme ?? this.bottomAppBarTheme,
+      // TODO(huycozy): Remove these checks when bottomAppBarTheme is a BottomAppBarThemeData
+      bottomAppBarTheme: () {
+        if (bottomAppBarTheme != null) {
+          if (bottomAppBarTheme is BottomAppBarTheme) {
+            return bottomAppBarTheme.data;
+          } else if (bottomAppBarTheme is! BottomAppBarThemeData) {
+            throw ArgumentError(
+              'bottomAppBarTheme must be either a BottomAppBarThemeData or a BottomAppBarTheme',
+            );
+          }
+        }
+        return bottomAppBarTheme as BottomAppBarThemeData? ?? this.bottomAppBarTheme;
+      }(),
       bottomNavigationBarTheme: bottomNavigationBarTheme ?? this.bottomNavigationBarTheme,
       bottomSheetTheme: bottomSheetTheme ?? this.bottomSheetTheme,
       buttonTheme: buttonTheme ?? this.buttonTheme,
@@ -1955,7 +1980,7 @@ class ThemeData with Diagnosticable {
       appBarTheme: AppBarTheme.lerp(a.appBarTheme, b.appBarTheme, t),
       badgeTheme: BadgeThemeData.lerp(a.badgeTheme, b.badgeTheme, t),
       bannerTheme: MaterialBannerThemeData.lerp(a.bannerTheme, b.bannerTheme, t),
-      bottomAppBarTheme: BottomAppBarTheme.lerp(a.bottomAppBarTheme, b.bottomAppBarTheme, t),
+      bottomAppBarTheme: BottomAppBarThemeData.lerp(a.bottomAppBarTheme, b.bottomAppBarTheme, t),
       bottomNavigationBarTheme: BottomNavigationBarThemeData.lerp(
         a.bottomNavigationBarTheme,
         b.bottomNavigationBarTheme,
@@ -2036,7 +2061,6 @@ class ThemeData with Diagnosticable {
         // order in every place that they are separated by section comments (e.g.
         // GENERAL CONFIGURATION). Each section except for deprecations should be
         // alphabetical by symbol name.
-        // GENERAL CONFIGURATION
         mapEquals(other.adaptationMap, adaptationMap) &&
         other.applyElevationOverlayColor == applyElevationOverlayColor &&
         other.cupertinoOverrideTheme == cupertinoOverrideTheme &&
@@ -2529,7 +2553,7 @@ class ThemeData with Diagnosticable {
       ),
     );
     properties.add(
-      DiagnosticsProperty<BottomAppBarTheme>(
+      DiagnosticsProperty<BottomAppBarThemeData>(
         'bottomAppBarTheme',
         bottomAppBarTheme,
         defaultValue: defaultData.bottomAppBarTheme,

packages/flutter/test/material/bottom_app_bar_test.dart

@@ -205,7 +205,7 @@ void main() {
             return Theme(
               data: Theme.of(
                 context,
-              ).copyWith(bottomAppBarTheme: const BottomAppBarTheme(color: Color(0xffffff00))),
+              ).copyWith(bottomAppBarTheme: const BottomAppBarThemeData(color: Color(0xffffff00))),
               child: const Scaffold(
                 floatingActionButton: FloatingActionButton(onPressed: null),
                 bottomNavigationBar: BottomAppBar(),
@@ -230,7 +230,7 @@ void main() {
             return Theme(
               data: Theme.of(
                 context,
-              ).copyWith(bottomAppBarTheme: const BottomAppBarTheme(color: Color(0xffffff00))),
+              ).copyWith(bottomAppBarTheme: const BottomAppBarThemeData(color: Color(0xffffff00))),
               child: const Scaffold(
                 floatingActionButton: FloatingActionButton(onPressed: null),
                 bottomNavigationBar: BottomAppBar(color: Color(0xff0000ff)),
@@ -251,7 +251,7 @@ void main() {
   testWidgets('Material3 - Color overrides theme color', (WidgetTester tester) async {
     await tester.pumpWidget(
       MaterialApp(
-        theme: ThemeData(bottomAppBarTheme: const BottomAppBarTheme(color: Color(0xffffff00))),
+        theme: ThemeData(bottomAppBarTheme: const BottomAppBarThemeData(color: Color(0xffffff00))),
         home: Builder(
           builder: (BuildContext context) {
             return const Scaffold(

packages/flutter/test/material/bottom_app_bar_theme_test.dart

@@ -7,22 +7,133 @@
 @Tags(<String>['reduced-test-set'])
 library;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-  test('BottomAppBarTheme lerp special cases', () {
+  test('BottomAppBarThemeData copyWith, ==, hashCode, defaults', () {
-    expect(BottomAppBarTheme.lerp(null, null, 0), const BottomAppBarTheme());
+    expect(const BottomAppBarThemeData(), const BottomAppBarThemeData().copyWith());
-    const BottomAppBarTheme data = BottomAppBarTheme();
+    expect(
-    expect(identical(BottomAppBarTheme.lerp(data, data, 0.5), data), true);
+      const BottomAppBarThemeData().hashCode,
+      const BottomAppBarThemeData().copyWith().hashCode,
+    );
+    expect(const BottomAppBarThemeData().color, null);
+    expect(const BottomAppBarThemeData().elevation, null);
+    expect(const BottomAppBarThemeData().shadowColor, null);
+    expect(const BottomAppBarThemeData().shape, null);
+    expect(const BottomAppBarThemeData().height, null);
+    expect(const BottomAppBarThemeData().surfaceTintColor, null);
+    expect(const BottomAppBarThemeData().padding, null);
+  });
+
+  test('BottomAppBarThemeData lerp special cases', () {
+    const BottomAppBarThemeData theme = BottomAppBarThemeData();
+    expect(identical(BottomAppBarThemeData.lerp(theme, theme, 0.5), theme), true);
+  });
+
+  testWidgets('Default BottomAppBarThemeData debugFillProperties', (WidgetTester tester) async {
+    final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
+    const BottomAppBarThemeData().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('BottomAppBarThemeData implements debugFillProperties', (WidgetTester tester) async {
+    final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
+    const BottomAppBarThemeData(
+      color: Color(0xffff0000),
+      elevation: 1.0,
+      shape: CircularNotchedRectangle(),
+      height: 1.0,
+      shadowColor: Color(0xff0000ff),
+      surfaceTintColor: Color(0xff00ff00),
+      padding: EdgeInsets.all(8),
+    ).debugFillProperties(builder);
+
+    final List<String> description =
+        builder.properties
+            .where((DiagnosticsNode node) => !node.isFiltered(DiagnosticLevel.info))
+            .map((DiagnosticsNode node) => node.toString())
+            .toList();
+
+    expect(description, <String>[
+      'color: ${const Color(0xffff0000)}',
+      'elevation: 1.0',
+      "shape: Instance of 'CircularNotchedRectangle'",
+      'height: 1.0',
+      'surfaceTintColor: ${const Color(0xff00ff00)}',
+      'shadowColor: ${const Color(0xff0000ff)}',
+      'padding: EdgeInsets.all(8.0)',
+    ]);
+  });
+
+  testWidgets('Local BottomAppBarTheme overrides defaults', (WidgetTester tester) async {
+    const Color color = Colors.blueAccent;
+    const double elevation = 1.0;
+    const Color shadowColor = Colors.black87;
+    const double height = 100.0;
+    const Color surfaceTintColor = Colors.transparent;
+    const NotchedShape shape = CircularNotchedRectangle();
+    const EdgeInsetsGeometry padding = EdgeInsets.all(8);
+    const BottomAppBarThemeData themeData = BottomAppBarThemeData(
+      color: color,
+      elevation: elevation,
+      shadowColor: shadowColor,
+      shape: shape,
+      height: height,
+      surfaceTintColor: surfaceTintColor,
+      padding: padding,
+    );
+
+    await tester.pumpWidget(_withTheme(localBABTheme: themeData));
+
+    final PhysicalShape widget = _getBabRenderObject(tester);
+    expect(widget.color, themeData.color);
+    expect(widget.elevation, themeData.elevation);
+    expect(widget.shadowColor, themeData.shadowColor);
+
+    final RenderBox renderBox = tester.renderObject<RenderBox>(find.byType(BottomAppBar));
+    expect(renderBox.size.height, themeData.height);
+
+    final bool hasFab =
+        Scaffold.of(tester.element(find.byType(BottomAppBar))).hasFloatingActionButton;
+    if (hasFab) {
+      expect(widget.clipper.toString(), '_BottomAppBarClipper');
+    } else {
+      expect(widget.clipper, isA<ShapeBorderClipper>());
+      final ShapeBorderClipper clipper = widget.clipper as ShapeBorderClipper;
+      expect(clipper.shape, isA<RoundedRectangleBorder>());
+    }
+
+    final Color effectiveColor = ElevationOverlay.applySurfaceTint(
+      themeData.color!,
+      themeData.surfaceTintColor,
+      themeData.elevation!,
+    );
+    expect(widget.color, effectiveColor);
+
+    // The BottomAppBar has two Padding widgets in its hierarchy:
+    // 1. The first Padding is from the SafeArea widget.
+    // 2. The second Padding is the one that applies the theme's padding.
+    final Padding paddingWidget = tester.widget<Padding>(
+      find.descendant(of: find.byType(BottomAppBar), matching: find.byType(Padding).at(1)),
+    );
+    expect(paddingWidget.padding, padding);
   });
 
   group('Material 2 tests', () {
     testWidgets('Material2 - BAB theme overrides color', (WidgetTester tester) async {
       const Color themedColor = Colors.black87;
-      const BottomAppBarTheme theme = BottomAppBarTheme(color: themedColor);
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(color: themedColor);
 
-      await tester.pumpWidget(_withTheme(theme, useMaterial3: false));
+      await tester.pumpWidget(_withTheme(babTheme: theme, useMaterial3: false));
 
       final PhysicalShape widget = _getBabRenderObject(tester);
       expect(widget.color, themedColor);
@@ -31,7 +142,7 @@ void main() {
     testWidgets('Material2 - BAB color - Widget', (WidgetTester tester) async {
       const Color babThemeColor = Colors.black87;
       const Color babColor = Colors.pink;
-      const BottomAppBarTheme theme = BottomAppBarTheme(color: babThemeColor);
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(color: babThemeColor);
 
       await tester.pumpWidget(
         MaterialApp(
@@ -46,7 +157,7 @@ void main() {
 
     testWidgets('Material2 - BAB color - BabTheme', (WidgetTester tester) async {
       const Color babThemeColor = Colors.black87;
-      const BottomAppBarTheme theme = BottomAppBarTheme(color: babThemeColor);
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(color: babThemeColor);
 
       await tester.pumpWidget(
         MaterialApp(
@@ -66,7 +177,7 @@ void main() {
         MaterialApp(
           theme: ThemeData(
             useMaterial3: false,
-            bottomAppBarTheme: const BottomAppBarTheme(color: themeColor),
+            bottomAppBarTheme: const BottomAppBarThemeData(color: themeColor),
           ),
           home: const Scaffold(body: BottomAppBar()),
         ),
@@ -90,13 +201,13 @@ void main() {
     });
 
     testWidgets('Material2 - BAB theme customizes shape', (WidgetTester tester) async {
-      const BottomAppBarTheme theme = BottomAppBarTheme(
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(
         color: Colors.white30,
         shape: CircularNotchedRectangle(),
         elevation: 1.0,
       );
 
-      await tester.pumpWidget(_withTheme(theme, useMaterial3: false));
+      await tester.pumpWidget(_withTheme(babTheme: theme, useMaterial3: false));
 
       await expectLater(
         find.byKey(_painterKey),
@@ -122,8 +233,8 @@ void main() {
   group('Material 3 tests', () {
     testWidgets('Material3 - BAB theme overrides color', (WidgetTester tester) async {
       const Color themedColor = Colors.black87;
-      const BottomAppBarTheme theme = BottomAppBarTheme(color: themedColor, elevation: 0);
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(color: themedColor, elevation: 0);
-      await tester.pumpWidget(_withTheme(theme, useMaterial3: true));
+      await tester.pumpWidget(_withTheme(babTheme: theme));
 
       final PhysicalShape widget = _getBabRenderObject(tester);
       expect(widget.color, themedColor);
@@ -132,7 +243,7 @@ void main() {
     testWidgets('Material3 - BAB color - Widget', (WidgetTester tester) async {
       const Color babThemeColor = Colors.black87;
       const Color babColor = Colors.pink;
-      const BottomAppBarTheme theme = BottomAppBarTheme(color: babThemeColor);
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(color: babThemeColor);
 
       await tester.pumpWidget(
         MaterialApp(
@@ -149,7 +260,7 @@ void main() {
 
     testWidgets('Material3 - BAB color - BabTheme', (WidgetTester tester) async {
       const Color babThemeColor = Colors.black87;
-      const BottomAppBarTheme theme = BottomAppBarTheme(color: babThemeColor);
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(color: babThemeColor);
 
       await tester.pumpWidget(
         MaterialApp(
@@ -180,12 +291,12 @@ void main() {
     testWidgets('Material3 - BAB theme overrides surfaceTintColor', (WidgetTester tester) async {
       const Color color = Colors.blue; // base color that the surface tint will be applied to
       const Color babThemeSurfaceTintColor = Colors.black87;
-      const BottomAppBarTheme theme = BottomAppBarTheme(
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(
         color: color,
         surfaceTintColor: babThemeSurfaceTintColor,
         elevation: 0,
       );
-      await tester.pumpWidget(_withTheme(theme, useMaterial3: true));
+      await tester.pumpWidget(_withTheme(babTheme: theme));
 
       final PhysicalShape widget = _getBabRenderObject(tester);
       expect(widget.color, ElevationOverlay.applySurfaceTint(color, babThemeSurfaceTintColor, 0));
@@ -193,11 +304,11 @@ void main() {
 
     testWidgets('Material3 - BAB theme overrides shadowColor', (WidgetTester tester) async {
       const Color babThemeShadowColor = Colors.yellow;
-      const BottomAppBarTheme theme = BottomAppBarTheme(
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(
         shadowColor: babThemeShadowColor,
         elevation: 0,
       );
-      await tester.pumpWidget(_withTheme(theme, useMaterial3: true));
+      await tester.pumpWidget(_withTheme(babTheme: theme));
 
       final PhysicalShape widget = _getBabRenderObject(tester);
       expect(widget.shadowColor, babThemeShadowColor);
@@ -207,7 +318,9 @@ void main() {
       const Color color = Colors.white10; // base color that the surface tint will be applied to
       const Color babThemeSurfaceTintColor = Colors.black87;
       const Color babSurfaceTintColor = Colors.pink;
-      const BottomAppBarTheme theme = BottomAppBarTheme(surfaceTintColor: babThemeSurfaceTintColor);
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(
+        surfaceTintColor: babThemeSurfaceTintColor,
+      );
       await tester.pumpWidget(
         MaterialApp(
           theme: ThemeData(bottomAppBarTheme: theme),
@@ -224,7 +337,7 @@ void main() {
     testWidgets('Material3 - BAB surfaceTintColor - BabTheme', (WidgetTester tester) async {
       const Color color = Colors.blue; // base color that the surface tint will be applied to
       const Color babThemeColor = Colors.black87;
-      const BottomAppBarTheme theme = BottomAppBarTheme(surfaceTintColor: babThemeColor);
+      const BottomAppBarThemeData theme = BottomAppBarThemeData(surfaceTintColor: babThemeColor);
 
       await tester.pumpWidget(
         MaterialApp(
@@ -247,20 +360,23 @@ PhysicalShape _getBabRenderObject(WidgetTester tester) {
 
 final Key _painterKey = UniqueKey();
 
-Widget _withTheme(BottomAppBarTheme theme, {required bool useMaterial3}) {
+Widget _withTheme({
+  BottomAppBarThemeData? babTheme,
+  BottomAppBarThemeData? localBABTheme,
+  bool useMaterial3 = true,
+}) {
+  Widget babWidget = const BottomAppBar(
+    child: Row(children: <Widget>[Icon(Icons.add), Expanded(child: SizedBox()), Icon(Icons.add)]),
+  );
+  if (localBABTheme != null) {
+    babWidget = BottomAppBarTheme(data: localBABTheme, child: babWidget);
+  }
   return MaterialApp(
-    theme: ThemeData(useMaterial3: useMaterial3, bottomAppBarTheme: theme),
+    theme: ThemeData(useMaterial3: useMaterial3, bottomAppBarTheme: babTheme),
     home: Scaffold(
       floatingActionButton: const FloatingActionButton(onPressed: null),
       floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
-      bottomNavigationBar: RepaintBoundary(
+      bottomNavigationBar: RepaintBoundary(key: _painterKey, child: babWidget),
-        key: _painterKey,
-        child: const BottomAppBar(
-          child: Row(
-            children: <Widget>[Icon(Icons.add), Expanded(child: SizedBox()), Icon(Icons.add)],
-          ),
-        ),
-      ),
     ),
   );
 }

packages/flutter/test/material/theme_data_test.dart

@@ -1332,7 +1332,7 @@ void main() {
       appBarTheme: const AppBarTheme(backgroundColor: Colors.black),
       badgeTheme: const BadgeThemeData(backgroundColor: Colors.black),
       bannerTheme: const MaterialBannerThemeData(backgroundColor: Colors.black),
-      bottomAppBarTheme: const BottomAppBarTheme(color: Colors.black),
+      bottomAppBarTheme: const BottomAppBarThemeData(color: Colors.black),
       bottomNavigationBarTheme: const BottomNavigationBarThemeData(
         type: BottomNavigationBarType.fixed,
       ),
@@ -1461,7 +1461,7 @@ void main() {
       appBarTheme: const AppBarTheme(backgroundColor: Colors.white),
       badgeTheme: const BadgeThemeData(backgroundColor: Colors.black),
       bannerTheme: const MaterialBannerThemeData(backgroundColor: Colors.white),
-      bottomAppBarTheme: const BottomAppBarTheme(color: Colors.white),
+      bottomAppBarTheme: const BottomAppBarThemeData(color: Colors.white),
       bottomNavigationBarTheme: const BottomNavigationBarThemeData(
         type: BottomNavigationBarType.shifting,
       ),