[Material] Bottom app bar theme (#24156)

* [WIP] BAB theme

* [WIP] BAB theme

* Update goldens

* Extract helper function in tests

* Update Goldens version

* Add tests

* [WIP] Hans first round comments

* Added test

* Added docs

* Hans second round comments

* Fixed analyzer error

* Hans third round comments

* ambient

* Change [BottomAppBarTheme.of] to static

* Final doc change

packages/flutter/lib/material.dart

@@ -21,6 +21,7 @@ export 'src/material/app_bar.dart';
 export 'src/material/arc.dart';
 export 'src/material/back_button.dart';
 export 'src/material/bottom_app_bar.dart';
+export 'src/material/bottom_app_bar_theme.dart';
 export 'src/material/bottom_navigation_bar.dart';
 export 'src/material/bottom_sheet.dart';
 export 'src/material/button.dart';

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

@@ -6,6 +6,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
+import 'bottom_app_bar_theme.dart';
 import 'material.dart';
 import 'scaffold.dart';
 import 'theme.dart';
@@ -41,19 +42,23 @@ import 'theme.dart';
 class BottomAppBar extends StatefulWidget {
   /// Creates a bottom application bar.
   ///
-  /// The [color], [elevation], and [clipBehavior] arguments must not be null.
+  /// The [clipBehavior] argument must not be null.
   /// Additionally, [elevation] must be non-negative.
+  ///
+  /// If [color], [elevation], or [shape] are null, their [BottomAppBarTheme] values will be used.
+  /// If the corresponding [BottomAppBarTheme] property is null, then the default
+  /// specified in the property's documentation will be used.
   const BottomAppBar({
     Key key,
     this.color,
-    this.elevation = 8.0,
+    this.elevation,
     this.shape,
     this.clipBehavior = Clip.none,
     this.notchMargin = 4.0,
     this.child,
-  }) : assert(elevation != null),
-       assert(elevation >= 0.0),
-       assert(clipBehavior != null),
+  }) : assert(clipBehavior != null),
+       assert(elevation == null || elevation >= 0.0),
+       assert(notchMargin != null),
        super(key: key);
 
   /// The widget below this widget in the tree.
@@ -66,7 +71,8 @@ class BottomAppBar extends StatefulWidget {
 
   /// The bottom app bar's background color.
   ///
-  /// When null defaults to [ThemeData.bottomAppBarColor].
+  /// If this property is null then [ThemeData.bottomAppBarTheme.color] is used,
+  /// if that's null then [ThemeData.bottomAppBarColor] is used.
   final Color color;
 
   /// The z-coordinate at which to place this bottom app bar relative to its
@@ -75,12 +81,14 @@ class BottomAppBar extends StatefulWidget {
   /// This controls the size of the shadow below the bottom app bar. The
   /// value is non-negative.
   ///
-  /// Defaults to 8, the appropriate elevation for bottom app bars.
+  /// If this property is null then [ThemeData.bottomAppBarTheme.elevation] is used,
+  /// if that's null, the default value is 8.
   final double elevation;
 
   /// The notch that is made for the floating action button.
   ///
-  /// If null the bottom app bar will be rectangular with no notch.
+  /// If this property is null then [ThemeData.bottomAppBarTheme.shape] is used,
+  /// if that's null then the shape will be rectangular with no notch.
   final NotchedShape shape;
 
   /// {@macro flutter.widgets.Clip}
@@ -98,6 +106,7 @@ class BottomAppBar extends StatefulWidget {
 
 class _BottomAppBarState extends State<BottomAppBar> {
   ValueListenable<ScaffoldGeometry> geometryListenable;
+  static const double _defaultElevation = 8.0;
 
   @override
   void didChangeDependencies() {
@@ -107,17 +116,20 @@ class _BottomAppBarState extends State<BottomAppBar> {
 
   @override
   Widget build(BuildContext context) {
-    final CustomClipper<Path> clipper = widget.shape != null
+    final ThemeData theme = Theme.of(context);
+    final BottomAppBarTheme babTheme = BottomAppBarTheme.of(context);
+    final NotchedShape notchedShape = widget.shape ?? babTheme.shape;
+    final CustomClipper<Path> clipper = notchedShape != null
       ? _BottomAppBarClipper(
         geometry: geometryListenable,
-        shape: widget.shape,
+        shape: notchedShape,
         notchMargin: widget.notchMargin,
       )
       : const ShapeBorderClipper(shape: RoundedRectangleBorder());
     return PhysicalShape(
       clipper: clipper,
-      elevation: widget.elevation,
-      color: widget.color ?? Theme.of(context).bottomAppBarColor,
+      elevation: widget.elevation ?? babTheme.elevation ?? _defaultElevation,
+      color: widget.color ?? babTheme.color ?? theme.bottomAppBarColor,
       clipBehavior: widget.clipBehavior,
       child: Material(
         type: MaterialType.transparency,

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

+// Copyright 2018 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 'theme.dart';
+
+/// Defines default property values for descendant [BottomAppBar] widgets.
+///
+/// Descendant widgets obtain the current [BottomAppBarTheme] object using
+/// `BottomAppBarTheme.of(context)`. Instances of [BottomAppBarTheme] can be
+/// customized with [BottomAppBarTheme.copyWith].
+///
+/// Typically a [BottomAppBarTheme] is specified as part of the overall [Theme]
+/// with [ThemeData.bottomAppBarTheme].
+///
+/// All [BottomAppBarTheme] properties are `null` by default. When null, the
+/// [BottomAppBar] constructor provides defaults.
+///
+/// See also:
+///
+///  * [ThemeData], which describes the overall theme information for the
+///    application.
+class BottomAppBarTheme extends Diagnosticable {
+  /// Creates a theme that can be used for [ThemeData.BottomAppBarTheme].
+  const BottomAppBarTheme({
+    this.color,
+    this.elevation,
+    this.shape,
+  });
+
+  /// Default value for [BottomAppBar.color].
+  ///
+  /// If null, [BottomAppBar] uses [ThemeData.bottomAppBarColor].
+  final Color color;
+
+  /// Default value for [BottomAppBar.elevation].
+  final double elevation;
+
+  /// Default value for [BottomAppBar.shape].
+  final NotchedShape shape;
+
+  /// Creates a copy of this object but with the given fields replaced with the
+  /// new values.
+  BottomAppBarTheme copyWith({
+    Color color,
+    double elevation,
+    NotchedShape shape,
+  }) {
+    return BottomAppBarTheme(
+      color: color ?? this.color,
+      elevation: elevation ?? this.elevation,
+      shape: shape ?? this.shape,
+    );
+  }
+
+  /// The [ThemeData.bottomAppBarTheme] property of the ambient [Theme].
+  static BottomAppBarTheme of(BuildContext context) {
+    return Theme.of(context).bottomAppBarTheme;
+  }
+
+  /// Linearly interpolate between two BAB themes.
+  ///
+  /// The argument `t` must not be null.
+  ///
+  /// {@macro dart.ui.shadow.lerp}
+  static BottomAppBarTheme lerp(BottomAppBarTheme a, BottomAppBarTheme b, double t) {
+    assert(t != null);
+    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,
+    );
+  }
+
+  @override
+  int get hashCode {
+    return hashValues(
+      color,
+      elevation,
+      shape,
+    );
+  }
+
+  @override
+  bool operator ==(dynamic other) {
+    if (identical(this, other))
+      return true;
+    if (other.runtimeType != runtimeType)
+      return false;
+    final BottomAppBarTheme typedOther = other;
+    return typedOther.color == color
+        && typedOther.elevation == elevation
+        && typedOther.shape == shape;
+  }
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(DiagnosticsProperty<Color>('color', color, defaultValue: null));
+    properties.add(DiagnosticsProperty<double>('elevation', elevation, defaultValue: null));
+    properties.add(DiagnosticsProperty<NotchedShape>('shape', shape, 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 'bottom_app_bar_theme.dart';
 import 'button_theme.dart';
 import 'chip_theme.dart';
 import 'color_scheme.dart';
@@ -151,6 +152,7 @@ class ThemeData extends Diagnosticable {
     TargetPlatform platform,
     MaterialTapTargetSize materialTapTargetSize,
     PageTransitionsTheme pageTransitionsTheme,
+    BottomAppBarTheme bottomAppBarTheme,
     ColorScheme colorScheme,
     DialogTheme dialogTheme,
     Typography typography,
@@ -242,6 +244,7 @@ class ThemeData extends Diagnosticable {
       valueIndicatorTextStyle: accentTextTheme.body2,
     );
     tabBarTheme ??= const TabBarTheme();
+    bottomAppBarTheme ??= const BottomAppBarTheme();
     chipTheme ??= ChipThemeData.fromDefaults(
       secondaryColor: primaryColor,
       brightness: brightness,
@@ -294,6 +297,7 @@ class ThemeData extends Diagnosticable {
       platform: platform,
       materialTapTargetSize: materialTapTargetSize,
       pageTransitionsTheme: pageTransitionsTheme,
+      bottomAppBarTheme: bottomAppBarTheme,
       colorScheme: colorScheme,
       dialogTheme: dialogTheme,
       typography: typography,
@@ -355,6 +359,7 @@ class ThemeData extends Diagnosticable {
     @required this.platform,
     @required this.materialTapTargetSize,
     @required this.pageTransitionsTheme,
+    @required this.bottomAppBarTheme,
     @required this.colorScheme,
     @required this.dialogTheme,
     @required this.typography,
@@ -401,6 +406,7 @@ class ThemeData extends Diagnosticable {
        assert(platform != null),
        assert(materialTapTargetSize != null),
        assert(pageTransitionsTheme != null),
+       assert(bottomAppBarTheme != null),
        assert(colorScheme != null),
        assert(dialogTheme != null),
        assert(typography != null);
@@ -619,6 +625,9 @@ class ThemeData extends Diagnosticable {
   /// builder is not found, a builder whose platform is null is used.
   final PageTransitionsTheme pageTransitionsTheme;
 
+  /// A theme for customizing the shape, elevation, and color of a [BottomAppBar].
+  final BottomAppBarTheme bottomAppBarTheme;
+
   /// A set of thirteen colors that can be used to configure the
   /// color properties of most components.
   ///
@@ -693,6 +702,7 @@ class ThemeData extends Diagnosticable {
     TargetPlatform platform,
     MaterialTapTargetSize materialTapTargetSize,
     PageTransitionsTheme pageTransitionsTheme,
+    BottomAppBarTheme bottomAppBarTheme,
     ColorScheme colorScheme,
     DialogTheme dialogTheme,
     Typography typography,
@@ -743,6 +753,7 @@ class ThemeData extends Diagnosticable {
       platform: platform ?? this.platform,
       materialTapTargetSize: materialTapTargetSize ?? this.materialTapTargetSize,
       pageTransitionsTheme: pageTransitionsTheme ?? this.pageTransitionsTheme,
+      bottomAppBarTheme: bottomAppBarTheme ?? this.bottomAppBarTheme,
       colorScheme: colorScheme ?? this.colorScheme,
       dialogTheme: dialogTheme ?? this.dialogTheme,
       typography: typography ?? this.typography,
@@ -871,6 +882,7 @@ 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,
+      bottomAppBarTheme: BottomAppBarTheme.lerp(a.bottomAppBarTheme, b.bottomAppBarTheme, t),
       colorScheme: ColorScheme.lerp(a.colorScheme, b.colorScheme, t),
       dialogTheme: DialogTheme.lerp(a.dialogTheme, b.dialogTheme, t),
       typography: Typography.lerp(a.typography, b.typography, t),
@@ -929,6 +941,7 @@ class ThemeData extends Diagnosticable {
            (otherData.platform == platform) &&
            (otherData.materialTapTargetSize == materialTapTargetSize) &&
            (otherData.pageTransitionsTheme == pageTransitionsTheme) &&
+           (otherData.bottomAppBarTheme == bottomAppBarTheme) &&
            (otherData.colorScheme == colorScheme) &&
            (otherData.dialogTheme == dialogTheme) &&
            (otherData.typography == typography) &&
@@ -987,6 +1000,7 @@ class ThemeData extends Diagnosticable {
           platform,
           materialTapTargetSize,
           pageTransitionsTheme,
+          bottomAppBarTheme,
           colorScheme,
           dialogTheme,
           typography,
@@ -1040,6 +1054,7 @@ 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<BottomAppBarTheme>('bottomAppBarTheme', bottomAppBarTheme, defaultValue: defaultData.bottomAppBarTheme));
     properties.add(DiagnosticsProperty<ColorScheme>('colorScheme', colorScheme, defaultValue: defaultData.colorScheme));
     properties.add(DiagnosticsProperty<DialogTheme>('dialogTheme', dialogTheme, defaultValue: defaultData.dialogTheme));
     properties.add(DiagnosticsProperty<Typography>('typography', typography, defaultValue: defaultData.typography));

packages/flutter/test/material/bottom_app_bar_theme_test.dart

+// Copyright 2018 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:io' show Platform;
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('BAB theme overrides color', (WidgetTester tester) async {
+    const Color themedColor = Colors.black87;
+    const BottomAppBarTheme theme = BottomAppBarTheme(color: themedColor);
+
+    await tester.pumpWidget(_withTheme(theme));
+
+    final PhysicalShape widget = _getBabRenderObject(tester);
+    expect(widget.color, themedColor);
+  });
+
+  testWidgets('BAB color - Widget', (WidgetTester tester) async {
+    const Color themeColor = Colors.white10;
+    const Color babThemeColor = Colors.black87;
+    const Color babColor = Colors.pink;
+    const BottomAppBarTheme theme = BottomAppBarTheme(color: babThemeColor);
+
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(bottomAppBarTheme: theme, bottomAppBarColor: themeColor),
+      home: const Scaffold(body: BottomAppBar(color: babColor)),
+    ));
+
+    final PhysicalShape widget = _getBabRenderObject(tester);
+    expect(widget.color, babColor);
+  });
+
+  testWidgets('BAB color - BabTheme', (WidgetTester tester) async {
+    const Color themeColor = Colors.white10;
+    const Color babThemeColor = Colors.black87;
+    const BottomAppBarTheme theme = BottomAppBarTheme(color: babThemeColor);
+
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(bottomAppBarTheme: theme, bottomAppBarColor: themeColor),
+      home: const Scaffold(body: BottomAppBar()),
+    ));
+
+    final PhysicalShape widget = _getBabRenderObject(tester);
+    expect(widget.color, babThemeColor);
+  });
+
+  testWidgets('BAB color - Theme', (WidgetTester tester) async {
+    const Color themeColor = Colors.white10;
+
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(bottomAppBarColor: themeColor),
+      home: const Scaffold(body: BottomAppBar()),
+    ));
+
+    final PhysicalShape widget = _getBabRenderObject(tester);
+    expect(widget.color, themeColor);
+  });
+
+  testWidgets('BAB color - Default', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(),
+      home: const Scaffold(body: BottomAppBar()),
+    ));
+
+    final PhysicalShape widget = _getBabRenderObject(tester);
+
+    expect(widget.color, Colors.white);
+  });
+
+  testWidgets('BAB theme customizes shape', (WidgetTester tester) async {
+    const BottomAppBarTheme theme = BottomAppBarTheme(
+        color: Colors.white30,
+        shape: CircularNotchedRectangle(),
+        elevation: 1.0,
+    );
+
+    await tester.pumpWidget(_withTheme(theme));
+
+    await expectLater(
+      find.byKey(_painterKey),
+      matchesGoldenFile('bottom_app_bar_theme.custom_shape.png'),
+      skip: !Platform.isLinux,
+    );
+  });
+
+  testWidgets('BAB theme does not affect defaults', (WidgetTester tester) async {
+    await tester.pumpWidget(const MaterialApp(
+      home: Scaffold(body: BottomAppBar()),
+    ));
+
+    final PhysicalShape widget = _getBabRenderObject(tester);
+
+    expect(widget.color, Colors.white);
+    expect(widget.elevation, equals(8.0));
+  });
+}
+
+PhysicalShape _getBabRenderObject(WidgetTester tester) {
+  return tester.widget<PhysicalShape>(
+      find.descendant(
+          of: find.byType(BottomAppBar),
+          matching: find.byType(PhysicalShape),
+      ),
+  );
+}
+
+final Key _painterKey = UniqueKey();
+
+Widget _withTheme(BottomAppBarTheme theme) {
+  return MaterialApp(
+    theme: ThemeData(bottomAppBarTheme: theme),
+    home: Scaffold(
+        floatingActionButton: const FloatingActionButton(onPressed: null),
+        floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
+        bottomNavigationBar: RepaintBoundary(
+          key: _painterKey,
+          child: BottomAppBar(
+            child: Row(
+              children: const <Widget>[
+                Icon(Icons.add),
+                Expanded(child: SizedBox()),
+                Icon(Icons.add),
+              ],
+            ),
+          ),
+        )
+    ),
+  );
+}