[Material] Implement App Bar Theme (#26597)

This change creates an `AppBarTheme` to be used with `AppBar` widgets. This allows for users to theme their AppBars separately from their overall Theme if they choose.

packages/flutter/lib/material.dart

@@ -18,6 +18,7 @@ export 'src/material/about.dart';
 export 'src/material/animated_icons.dart';
 export 'src/material/app.dart';
 export 'src/material/app_bar.dart';
+export 'src/material/app_bar_theme.dart';
 export 'src/material/arc.dart';
 export 'src/material/back_button.dart';
 export 'src/material/bottom_app_bar.dart';

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

@@ -9,6 +9,7 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
+import 'app_bar_theme.dart';
 import 'back_button.dart';
 import 'constants.dart';
 import 'debug.dart';
@@ -130,9 +131,14 @@ class _ToolbarContainerLayout extends SingleChildLayoutDelegate {
 class AppBar extends StatefulWidget implements PreferredSizeWidget {
   /// Creates a material design app bar.
   ///
-  /// The arguments [elevation], [primary], [toolbarOpacity], [bottomOpacity]
-  /// and [automaticallyImplyLeading] must not be null. Additionally,
-  /// [elevation] must be non-negative.
+  /// The arguments [primary], [toolbarOpacity], [bottomOpacity]
+  /// and [automaticallyImplyLeading] must not be null. Additionally, if
+  /// [elevation] is specified, it must be non-negative.
+  ///
+  /// If [backgroundColor], [elevation], [brightness], [iconTheme], or
+  /// [textTheme] are null, 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.
   AppBar({
@@ -143,7 +149,7 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
     this.actions,
     this.flexibleSpace,
     this.bottom,
-    this.elevation = 4.0,
+    this.elevation,
     this.backgroundColor,
     this.brightness,
     this.iconTheme,
@@ -154,7 +160,7 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
     this.toolbarOpacity = 1.0,
     this.bottomOpacity = 1.0,
   }) : assert(automaticallyImplyLeading != null),
-       assert(elevation != null && elevation >= 0.0),
+       assert(elevation == null || elevation >= 0.0),
        assert(primary != null),
        assert(titleSpacing != null),
        assert(toolbarOpacity != null),
@@ -269,33 +275,39 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   ///
   /// This controls the size of the shadow below the app bar.
   ///
-  /// Defaults to 4, the appropriate elevation for app bars.
-  ///
   /// The value is non-negative.
+  ///
+  /// If this property is null then [ThemeData.appBarTheme.elevation] is used,
+  /// if that is also null, the default value is 4, the appropriate elevation
+  /// for app bars.
   final double elevation;
 
   /// The color to use for the app bar's material. Typically this should be set
   /// along with [brightness], [iconTheme], [textTheme].
   ///
-  /// Defaults to [ThemeData.primaryColor].
+  /// If this property is null then [ThemeData.appBarTheme.color] is used,
+  /// if that is also null then [ThemeData.primaryColor] is used.
   final Color backgroundColor;
 
   /// The brightness of the app bar's material. Typically this is set along
   /// with [backgroundColor], [iconTheme], [textTheme].
   ///
-  /// Defaults to [ThemeData.primaryColorBrightness].
+  /// If this property is null then [ThemeData.appBarTheme.brightness] is used,
+  /// if that is also null then [ThemeData.primaryColorBrightness] is used.
   final Brightness brightness;
 
   /// The color, opacity, and size to use for app bar icons. Typically this
   /// is set along with [backgroundColor], [brightness], [textTheme].
   ///
-  /// Defaults to [ThemeData.primaryIconTheme].
+  /// If this property is null then [ThemeData.appBarTheme.iconTheme] is used,
+  /// if that is also null then [ThemeData.primaryIconTheme] is used.
   final IconThemeData iconTheme;
 
   /// The typographic styles to use for text in the app bar. Typically this is
   /// set along with [brightness] [backgroundColor], [iconTheme].
   ///
-  /// Defaults to [ThemeData.primaryTextTheme].
+  /// If this property is null then [ThemeData.appBarTheme.textTheme] is used,
+  /// if that is also null then [ThemeData.primaryTextTheme] is used.
   final TextTheme textTheme;
 
   /// Whether this app bar is being displayed at the top of the screen.
@@ -361,6 +373,8 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
 }
 
 class _AppBarState extends State<AppBar> {
+  static const double _defaultElevation = 4.0;
+
   void _handleDrawerButton() {
     Scaffold.of(context).openDrawer();
   }
@@ -374,6 +388,7 @@ class _AppBarState extends State<AppBar> {
     assert(!widget.primary || debugCheckHasMediaQuery(context));
     assert(debugCheckHasMaterialLocalizations(context));
     final ThemeData themeData = Theme.of(context);
+    final AppBarTheme appBarTheme = AppBarTheme.of(context);
     final ScaffoldState scaffold = Scaffold.of(context, nullOk: true);
     final ModalRoute<dynamic> parentRoute = ModalRoute.of(context);
 
@@ -382,9 +397,15 @@ class _AppBarState extends State<AppBar> {
     final bool canPop = parentRoute?.canPop ?? false;
     final bool useCloseButton = parentRoute is PageRoute<dynamic> && parentRoute.fullscreenDialog;
 
-    IconThemeData appBarIconTheme = widget.iconTheme ?? themeData.primaryIconTheme;
-    TextStyle centerStyle = widget.textTheme?.title ?? themeData.primaryTextTheme.title;
-    TextStyle sideStyle = widget.textTheme?.body1 ?? themeData.primaryTextTheme.body1;
+    IconThemeData appBarIconTheme = widget.iconTheme
+      ?? appBarTheme.iconTheme
+      ?? themeData.primaryIconTheme;
+    TextStyle centerStyle = widget.textTheme?.title
+      ?? appBarTheme.textTheme?.title
+      ?? themeData.primaryTextTheme.title;
+    TextStyle sideStyle = widget.textTheme?.body1
+      ?? appBarTheme.textTheme?.body1
+      ?? themeData.primaryTextTheme.body1;
 
     if (widget.toolbarOpacity != 1.0) {
       final double opacity = const Interval(0.25, 1.0, curve: Curves.fastOutSlowIn).transform(widget.toolbarOpacity);
@@ -517,18 +538,24 @@ class _AppBarState extends State<AppBar> {
         ],
       );
     }
-    final Brightness brightness = widget.brightness ?? themeData.primaryColorBrightness;
+    final Brightness brightness = widget.brightness
+      ?? appBarTheme.brightness
+      ?? themeData.primaryColorBrightness;
     final SystemUiOverlayStyle overlayStyle = brightness == Brightness.dark
-        ? SystemUiOverlayStyle.light
-        : SystemUiOverlayStyle.dark;
+      ? SystemUiOverlayStyle.light
+      : SystemUiOverlayStyle.dark;
 
     return Semantics(
       container: true,
       child: AnnotatedRegion<SystemUiOverlayStyle>(
         value: overlayStyle,
         child: Material(
-          color: widget.backgroundColor ?? themeData.primaryColor,
-          elevation: widget.elevation,
+          color: widget.backgroundColor
+            ?? appBarTheme.color
+            ?? themeData.primaryColor,
+          elevation: widget.elevation
+            ?? appBarTheme.elevation
+            ?? _defaultElevation,
           child: Semantics(
             explicitChildNodes: true,
             child: appBar,

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

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui' show lerpDouble;
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+import 'text_theme.dart';
+import 'theme.dart';
+
+/// Defines default property values for descendant [AppBar] widgets.
+///
+/// Descendant widgets obtain the current [AppBarTheme] object using
+/// `AppBarTheme.of(context)`. Instances of [AppBarTheme] can be customized
+/// with [AppBarTheme.copyWith].
+///
+/// Typically an [AppBarTheme] is specified as part of the overall [Theme] with
+/// [ThemeData.appBarTheme].
+///
+/// All [AppBarTheme] properties are `null` by default. When null, the [AppBar]
+/// will use the values from [ThemeData] if they exist, otherwise it will
+/// provide its own defaults.
+///
+/// See also:
+///
+///  * [ThemeData], which describes the overall theme information for the
+///    application.
+class AppBarTheme extends Diagnosticable {
+  /// Creates a theme that can be used for [ThemeData.AppBarTheme].
+  const AppBarTheme({
+    this.brightness,
+    this.color,
+    this.elevation,
+    this.iconTheme,
+    this.textTheme,
+  });
+
+  /// Default value for [AppBar.brightness].
+  ///
+  /// If null, [AppBar] uses [ThemeData.primaryColorBrightness].
+  final Brightness brightness;
+
+  /// Default value for [AppBar.color].
+  ///
+  /// If null, [AppBar] uses [ThemeData.primaryColor].
+  final Color color;
+
+  /// Default value for [AppBar.elevation].
+  ///
+  /// If null, [AppBar] uses a default value of 4.0.
+  final double elevation;
+
+  /// Default value for [AppBar.iconTheme].
+  ///
+  /// If null, [AppBar] uses [ThemeData.primaryIconTheme].
+  final IconThemeData iconTheme;
+
+  /// Default value for [AppBar.textTheme].
+  ///
+  /// If null, [AppBar] uses [ThemeData.primaryTextTheme].
+  final TextTheme textTheme;
+
+  /// Creates a copy of this object with the given fields replaced with the
+  /// new values.
+  AppBarTheme copyWith({
+    Brightness brightness,
+    Color color,
+    double elevation,
+    IconThemeData iconTheme,
+    TextTheme textTheme,
+  }) {
+    return AppBarTheme(
+      brightness: brightness ?? this.brightness,
+      color: color ?? this.color,
+      elevation: elevation ?? this.elevation,
+      iconTheme: iconTheme ?? this.iconTheme,
+      textTheme: textTheme ?? this.textTheme,
+    );
+  }
+
+  /// The [ThemeData.appBarTheme] property of the ambient [Theme].
+  static AppBarTheme of(BuildContext context) {
+    return Theme.of(context).appBarTheme;
+  }
+
+  /// Linearly interpolate between two AppBar themes.
+  ///
+  /// The argument `t` must not be null.
+  ///
+  /// {@macro dart.ui.shadow.lerp}
+  static AppBarTheme lerp(AppBarTheme a, AppBarTheme b, double t) {
+    assert(t != null);
+    return AppBarTheme(
+      brightness: t < 0.5 ? a?.brightness : b?.brightness,
+      color: Color.lerp(a?.color, b?.color, t),
+      elevation: lerpDouble(a?.elevation, b?.elevation, t),
+      iconTheme: IconThemeData.lerp(a?.iconTheme, b?.iconTheme, t),
+      textTheme: TextTheme.lerp(a?.textTheme, b?.textTheme, t),
+    );
+  }
+
+  @override
+  int get hashCode {
+    return hashValues(
+      brightness,
+      color,
+      elevation,
+      iconTheme,
+      textTheme,
+    );
+  }
+
+  @override
+  bool operator ==(dynamic other) {
+    if (identical(this, other))
+      return true;
+    if (other.runtimeType != runtimeType)
+      return false;
+    final AppBarTheme typedOther = other;
+    return typedOther.brightness == brightness
+        && typedOther.color == color
+        && typedOther.elevation == elevation
+        && typedOther.iconTheme == iconTheme
+        && typedOther.textTheme == textTheme;
+  }
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(DiagnosticsProperty<Brightness>('brightness', brightness, defaultValue: null));
+    properties.add(DiagnosticsProperty<Color>('color', color, defaultValue: null));
+    properties.add(DiagnosticsProperty<double>('elevation', elevation, defaultValue: null));
+    properties.add(DiagnosticsProperty<IconThemeData>('iconTheme', iconTheme, defaultValue: null));
+    properties.add(DiagnosticsProperty<TextTheme>('textTheme', textTheme, defaultValue: null));
+  }
+}

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

@@ -9,6 +9,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
+import 'app_bar_theme.dart';
 import 'bottom_app_bar_theme.dart';
 import 'button_theme.dart';
 import 'chip_theme.dart';
@@ -152,6 +153,7 @@ class ThemeData extends Diagnosticable {
     TargetPlatform platform,
     MaterialTapTargetSize materialTapTargetSize,
     PageTransitionsTheme pageTransitionsTheme,
+    AppBarTheme appBarTheme,
     BottomAppBarTheme bottomAppBarTheme,
     ColorScheme colorScheme,
     DialogTheme dialogTheme,
@@ -244,6 +246,7 @@ class ThemeData extends Diagnosticable {
       valueIndicatorTextStyle: accentTextTheme.body2,
     );
     tabBarTheme ??= const TabBarTheme();
+    appBarTheme ??= const AppBarTheme();
     bottomAppBarTheme ??= const BottomAppBarTheme();
     chipTheme ??= ChipThemeData.fromDefaults(
       secondaryColor: primaryColor,
@@ -297,6 +300,7 @@ class ThemeData extends Diagnosticable {
       platform: platform,
       materialTapTargetSize: materialTapTargetSize,
       pageTransitionsTheme: pageTransitionsTheme,
+      appBarTheme: appBarTheme,
       bottomAppBarTheme: bottomAppBarTheme,
       colorScheme: colorScheme,
       dialogTheme: dialogTheme,
@@ -359,6 +363,7 @@ class ThemeData extends Diagnosticable {
     @required this.platform,
     @required this.materialTapTargetSize,
     @required this.pageTransitionsTheme,
+    @required this.appBarTheme,
     @required this.bottomAppBarTheme,
     @required this.colorScheme,
     @required this.dialogTheme,
@@ -406,6 +411,7 @@ class ThemeData extends Diagnosticable {
        assert(platform != null),
        assert(materialTapTargetSize != null),
        assert(pageTransitionsTheme != null),
+       assert(appBarTheme != null),
        assert(bottomAppBarTheme != null),
        assert(colorScheme != null),
        assert(dialogTheme != null),
@@ -625,6 +631,10 @@ class ThemeData extends Diagnosticable {
   /// builder is not found, a builder whose platform is null is used.
   final PageTransitionsTheme pageTransitionsTheme;
 
+  /// A theme for customizing the color, elevation, brightness, iconTheme and
+  /// textTheme of [AppBar]s.
+  final AppBarTheme appBarTheme;
+
   /// A theme for customizing the shape, elevation, and color of a [BottomAppBar].
   final BottomAppBarTheme bottomAppBarTheme;
 
@@ -702,6 +712,7 @@ class ThemeData extends Diagnosticable {
     TargetPlatform platform,
     MaterialTapTargetSize materialTapTargetSize,
     PageTransitionsTheme pageTransitionsTheme,
+    AppBarTheme appBarTheme,
     BottomAppBarTheme bottomAppBarTheme,
     ColorScheme colorScheme,
     DialogTheme dialogTheme,
@@ -753,6 +764,7 @@ class ThemeData extends Diagnosticable {
       platform: platform ?? this.platform,
       materialTapTargetSize: materialTapTargetSize ?? this.materialTapTargetSize,
       pageTransitionsTheme: pageTransitionsTheme ?? this.pageTransitionsTheme,
+      appBarTheme: appBarTheme ?? this.appBarTheme,
       bottomAppBarTheme: bottomAppBarTheme ?? this.bottomAppBarTheme,
       colorScheme: colorScheme ?? this.colorScheme,
       dialogTheme: dialogTheme ?? this.dialogTheme,
@@ -882,6 +894,7 @@ class ThemeData extends Diagnosticable {
       platform: t < 0.5 ? a.platform : b.platform,
       materialTapTargetSize: t < 0.5 ? a.materialTapTargetSize : b.materialTapTargetSize,
       pageTransitionsTheme: t < 0.5 ? a.pageTransitionsTheme : b.pageTransitionsTheme,
+      appBarTheme: AppBarTheme.lerp(a.appBarTheme, b.appBarTheme, t),
       bottomAppBarTheme: BottomAppBarTheme.lerp(a.bottomAppBarTheme, b.bottomAppBarTheme, t),
       colorScheme: ColorScheme.lerp(a.colorScheme, b.colorScheme, t),
       dialogTheme: DialogTheme.lerp(a.dialogTheme, b.dialogTheme, t),
@@ -941,6 +954,7 @@ class ThemeData extends Diagnosticable {
            (otherData.platform == platform) &&
            (otherData.materialTapTargetSize == materialTapTargetSize) &&
            (otherData.pageTransitionsTheme == pageTransitionsTheme) &&
+           (otherData.appBarTheme == appBarTheme) &&
            (otherData.bottomAppBarTheme == bottomAppBarTheme) &&
            (otherData.colorScheme == colorScheme) &&
            (otherData.dialogTheme == dialogTheme) &&
@@ -1000,6 +1014,7 @@ class ThemeData extends Diagnosticable {
           platform,
           materialTapTargetSize,
           pageTransitionsTheme,
+          appBarTheme,
           bottomAppBarTheme,
           colorScheme,
           dialogTheme,
@@ -1054,6 +1069,7 @@ class ThemeData extends Diagnosticable {
     properties.add(DiagnosticsProperty<ChipThemeData>('chipTheme', chipTheme));
     properties.add(DiagnosticsProperty<MaterialTapTargetSize>('materialTapTargetSize', materialTapTargetSize));
     properties.add(DiagnosticsProperty<PageTransitionsTheme>('pageTransitionsTheme', pageTransitionsTheme));
+    properties.add(DiagnosticsProperty<AppBarTheme>('appBarTheme', appBarTheme, defaultValue: defaultData.appBarTheme));
     properties.add(DiagnosticsProperty<BottomAppBarTheme>('bottomAppBarTheme', bottomAppBarTheme, defaultValue: defaultData.bottomAppBarTheme));
     properties.add(DiagnosticsProperty<ColorScheme>('colorScheme', colorScheme, defaultValue: defaultData.colorScheme));
     properties.add(DiagnosticsProperty<DialogTheme>('dialogTheme', dialogTheme, defaultValue: defaultData.dialogTheme));

packages/flutter/test/material/app_bar_theme_test.dart

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('AppBarTheme copyWith, ==, hashCode basics', () {
+    expect(const AppBarTheme(), const AppBarTheme().copyWith());
+    expect(const AppBarTheme().hashCode, const AppBarTheme().copyWith().hashCode);
+  });
+
+  testWidgets('Passing no AppBarTheme returns defaults', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(appBar: AppBar()),
+    ));
+
+    final Material widget = _getAppBarMaterial(tester);
+    final IconTheme iconTheme = _getAppBarIconTheme(tester);
+    final DefaultTextStyle text = _getAppBarText(tester);
+
+    expect(SystemChrome.latestStyle.statusBarBrightness, Brightness.dark);
+    expect(widget.color, Colors.blue);
+    expect(widget.elevation, 4.0);
+    expect(iconTheme.data, const IconThemeData(color: Colors.white));
+    expect(text.style, Typography().englishLike.body1.merge(Typography().white.body1));
+  });
+
+  testWidgets('AppBar uses values from AppBarTheme', (WidgetTester tester) async {
+    final AppBarTheme appBarTheme = _appBarTheme();
+
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(appBarTheme: appBarTheme),
+      home: Scaffold(appBar: AppBar(title: const Text('App Bar Title'),)),
+    ));
+
+    final Material widget = _getAppBarMaterial(tester);
+    final IconTheme iconTheme = _getAppBarIconTheme(tester);
+    final DefaultTextStyle text = _getAppBarText(tester);
+
+    expect(SystemChrome.latestStyle.statusBarBrightness, appBarTheme.brightness);
+    expect(widget.color, appBarTheme.color);
+    expect(widget.elevation, appBarTheme.elevation);
+    expect(iconTheme.data, appBarTheme.iconTheme);
+    expect(text.style, appBarTheme.textTheme.body1);
+  });
+
+  testWidgets('AppBar widget properties take priority over theme', (WidgetTester tester) async {
+    const Brightness brightness = Brightness.dark;
+    const Color color = Colors.orange;
+    const double elevation = 3.0;
+    const IconThemeData iconThemeData = IconThemeData(color: Colors.green);
+    const TextTheme textTheme = TextTheme(title: TextStyle(color: Colors.orange), body1: TextStyle(color: Colors.pink));
+
+    final ThemeData themeData = _themeData().copyWith(appBarTheme: _appBarTheme());
+
+    await tester.pumpWidget(MaterialApp(
+      theme: themeData,
+      home: Scaffold(appBar: AppBar(
+        backgroundColor: color,
+        brightness: brightness,
+        elevation: elevation,
+        iconTheme: iconThemeData,
+        textTheme: textTheme,)
+      ),
+    ));
+
+    final Material widget = _getAppBarMaterial(tester);
+    final IconTheme iconTheme = _getAppBarIconTheme(tester);
+    final DefaultTextStyle text = _getAppBarText(tester);
+
+    expect(SystemChrome.latestStyle.statusBarBrightness, brightness);
+    expect(widget.color, color);
+    expect(widget.elevation, elevation);
+    expect(iconTheme.data, iconThemeData);
+    expect(text.style, textTheme.body1);
+  });
+
+  testWidgets('AppBarTheme properties take priority over ThemeData properties', (WidgetTester tester) async {
+    final AppBarTheme appBarTheme = _appBarTheme();
+    final ThemeData themeData = _themeData().copyWith(appBarTheme: _appBarTheme());
+
+    await tester.pumpWidget(MaterialApp(
+      theme: themeData,
+      home: Scaffold(appBar: AppBar()),
+    ));
+
+    final Material widget = _getAppBarMaterial(tester);
+    final IconTheme iconTheme = _getAppBarIconTheme(tester);
+    final DefaultTextStyle text = _getAppBarText(tester);
+
+    expect(SystemChrome.latestStyle.statusBarBrightness, appBarTheme.brightness);
+    expect(widget.color, appBarTheme.color);
+    expect(widget.elevation, appBarTheme.elevation);
+    expect(iconTheme.data, appBarTheme.iconTheme);
+    expect(text.style, appBarTheme.textTheme.body1);
+  });
+
+  testWidgets('ThemeData properties are used when no AppBarTheme is set', (WidgetTester tester) async {
+    final ThemeData themeData = _themeData();
+
+    await tester.pumpWidget(MaterialApp(
+      theme: themeData,
+      home: Scaffold(appBar: AppBar()),
+    ));
+
+    final Material widget = _getAppBarMaterial(tester);
+    final IconTheme iconTheme = _getAppBarIconTheme(tester);
+    final DefaultTextStyle text = _getAppBarText(tester);
+
+    expect(SystemChrome.latestStyle.statusBarBrightness, themeData.brightness);
+    expect(widget.color, themeData.primaryColor);
+    expect(widget.elevation, 4.0);
+    expect(iconTheme.data, themeData.primaryIconTheme);
+    expect(text.style, Typography().englishLike.body1.merge(Typography().white.body1).merge(themeData.primaryTextTheme.body1));
+  });
+}
+
+AppBarTheme _appBarTheme() {
+  const Brightness brightness = Brightness.light;
+  const Color color = Colors.lightBlue;
+  const double elevation = 6.0;
+  const IconThemeData iconThemeData = IconThemeData(color: Colors.black);
+  const TextTheme textTheme = TextTheme(body1: TextStyle(color: Colors.yellow));
+  return const AppBarTheme(
+    brightness: brightness,
+    color: color,
+    elevation: elevation,
+    iconTheme: iconThemeData,
+    textTheme: textTheme
+  );
+}
+
+ThemeData _themeData() {
+  return ThemeData(
+    primaryColor: Colors.purple,
+    brightness: Brightness.dark,
+    primaryIconTheme: const IconThemeData(color: Colors.green),
+    primaryTextTheme: const TextTheme(title: TextStyle(color: Colors.orange), body1: TextStyle(color: Colors.pink))
+  );
+}
+
+Material _getAppBarMaterial(WidgetTester tester) {
+  return tester.widget<Material>(
+    find.descendant(
+      of: find.byType(AppBar),
+      matching: find.byType(Material),
+    ),
+  );
+}
+
+IconTheme _getAppBarIconTheme(WidgetTester tester) {
+  return tester.widget<IconTheme>(
+    find.descendant(
+      of: find.byType(AppBar),
+      matching: find.byType(IconTheme),
+    ),
+  );
+}
+
+DefaultTextStyle _getAppBarText(WidgetTester tester) {
+  return tester.widget<DefaultTextStyle>(
+    find.descendant(
+      of: find.byType(CustomSingleChildLayout),
+      matching: find.byType(DefaultTextStyle),
+    ).first,
+  );
+}