[Material] Added BottomNavigationBarTheme (#54714)

packages/flutter/lib/material.dart

@@ -28,6 +28,7 @@ export 'src/material/banner_theme.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_navigation_bar_theme.dart';
 export 'src/material/bottom_sheet.dart';
 export 'src/material/bottom_sheet_theme.dart';
 export 'src/material/button.dart';

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

@@ -8,7 +8,7 @@ import 'dart:math' as math;
 import 'package:flutter/widgets.dart';
 import 'package:vector_math/vector_math_64.dart' show Vector3;
 
-import 'colors.dart';
+import 'bottom_navigation_bar_theme.dart';
 import 'constants.dart';
 import 'debug.dart';
 import 'ink_well.dart';
@@ -172,21 +172,21 @@ class BottomNavigationBar extends StatefulWidget {
     @required this.items,
     this.onTap,
     this.currentIndex = 0,
-    this.elevation = 8.0,
-    BottomNavigationBarType type,
+    this.elevation,
+    this.type,
     Color fixedColor,
     this.backgroundColor,
     this.iconSize = 24.0,
     Color selectedItemColor,
     this.unselectedItemColor,
-    this.selectedIconTheme = const IconThemeData(),
-    this.unselectedIconTheme = const IconThemeData(),
+    this.selectedIconTheme,
+    this.unselectedIconTheme,
     this.selectedFontSize = 14.0,
     this.unselectedFontSize = 12.0,
     this.selectedLabelStyle,
     this.unselectedLabelStyle,
     this.showSelectedLabels = true,
-    bool showUnselectedLabels,
+    this.showUnselectedLabels,
   }) : assert(items != null),
        assert(items.length >= 2),
        assert(
@@ -194,7 +194,7 @@ class BottomNavigationBar extends StatefulWidget {
         'Every item must have a non-null title',
        ),
        assert(0 <= currentIndex && currentIndex < items.length),
-       assert(elevation != null && elevation >= 0.0),
+       assert(elevation == null || elevation >= 0.0),
        assert(iconSize != null && iconSize >= 0.0),
        assert(
          selectedItemColor == null || fixedColor == null,
@@ -203,9 +203,7 @@ class BottomNavigationBar extends StatefulWidget {
        assert(selectedFontSize != null && selectedFontSize >= 0.0),
        assert(unselectedFontSize != null && unselectedFontSize >= 0.0),
        assert(showSelectedLabels != null),
-       type = _type(type, items),
        selectedItemColor = selectedItemColor ?? fixedColor,
-       showUnselectedLabels = showUnselectedLabels ?? _defaultShowUnselected(_type(type, items)),
        super(key: key);
 
   /// Defines the appearance of the button items that are arrayed within the
@@ -316,37 +314,6 @@ class BottomNavigationBar extends StatefulWidget {
   /// Whether the labels are shown for the unselected [BottomNavigationBarItem]s.
   final bool showSelectedLabels;
 
-  // Used by the [BottomNavigationBar] constructor to set the [type] parameter.
-  //
-  // If type is provided, it is returned. Otherwise,
-  // [BottomNavigationBarType.fixed] is used for 3 or fewer items, and
-  // [BottomNavigationBarType.shifting] is used for 4+ items.
-  static BottomNavigationBarType _type(
-    BottomNavigationBarType type,
-    List<BottomNavigationBarItem> items,
-  ) {
-    if (type != null) {
-      return type;
-    }
-    return items.length <= 3 ? BottomNavigationBarType.fixed : BottomNavigationBarType.shifting;
-  }
-
-  // Used by the [BottomNavigationBar] constructor to set the [showUnselected]
-  // parameter.
-  //
-  // Unselected labels are shown by default for [BottomNavigationBarType.fixed],
-  // and hidden by default for [BottomNavigationBarType.shifting].
-  static bool _defaultShowUnselected(BottomNavigationBarType type) {
-    switch (type) {
-      case BottomNavigationBarType.shifting:
-        return false;
-      case BottomNavigationBarType.fixed:
-        return true;
-    }
-    assert(false);
-    return false;
-  }
-
   @override
   _BottomNavigationBarState createState() => _BottomNavigationBarState();
 }
@@ -401,10 +368,17 @@ class _BottomNavigationTile extends StatelessWidget {
     // (which is an integer) by a large number.
     int size;
 
+    final BottomNavigationBarThemeData bottomTheme = BottomNavigationBarTheme.of(context);
+
     final double selectedFontSize = selectedLabelStyle.fontSize;
 
-    final double selectedIconSize = selectedIconTheme?.size ?? iconSize;
-    final double unselectedIconSize = unselectedIconTheme?.size ?? iconSize;
+    final double selectedIconSize = selectedIconTheme?.size
+      ?? bottomTheme?.selectedIconTheme?.size
+      ?? iconSize;
+    final double unselectedIconSize = unselectedIconTheme?.size
+      ?? bottomTheme?.unselectedIconTheme?.size
+      ?? iconSize;
+
     // The amount that the selected icon is bigger than the unselected icons,
     // (or zero if the selected icon is not bigger than the unselected icons).
     final double selectedIconDiff = math.max(selectedIconSize - unselectedIconSize, 0);
@@ -491,17 +465,17 @@ class _BottomNavigationTile extends StatelessWidget {
                       iconSize: iconSize,
                       selected: selected,
                       item: item,
-                      selectedIconTheme: selectedIconTheme,
-                      unselectedIconTheme: unselectedIconTheme,
+                      selectedIconTheme: selectedIconTheme ?? bottomTheme.selectedIconTheme,
+                      unselectedIconTheme: unselectedIconTheme ?? bottomTheme.unselectedIconTheme,
                     ),
                     _Label(
                       colorTween: colorTween,
                       animation: animation,
                       item: item,
-                      selectedLabelStyle: selectedLabelStyle,
-                      unselectedLabelStyle: unselectedLabelStyle,
-                      showSelectedLabels: showSelectedLabels,
-                      showUnselectedLabels: showUnselectedLabels,
+                      selectedLabelStyle: selectedLabelStyle ?? bottomTheme.selectedLabelStyle,
+                      unselectedLabelStyle: unselectedLabelStyle ?? bottomTheme.unselectedLabelStyle,
+                      showSelectedLabels: showSelectedLabels ?? bottomTheme.showUnselectedLabels,
+                      showUnselectedLabels: showUnselectedLabels ?? bottomTheme.showUnselectedLabels,
                     ),
                   ],
                 ),
@@ -693,6 +667,33 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
     _backgroundColor = widget.items[widget.currentIndex].backgroundColor;
   }
 
+  // Computes the default value for the [type] parameter.
+  //
+  // If type is provided, it is returned. Next, if the bottom navigation bar
+  // theme provides a type, it is used. Finally, the default behavior will be
+  // [BottomNavigationBarType.fixed] for 3 or fewer items, and
+  // [BottomNavigationBarType.shifting] is used for 4+ items.
+  BottomNavigationBarType get _effectiveType {
+    return widget.type
+      ?? BottomNavigationBarTheme.of(context).type
+      ?? (widget.items.length <= 3 ? BottomNavigationBarType.fixed : BottomNavigationBarType.shifting);
+  }
+
+  // Computes the default value for the [showUnselected] parameter.
+  //
+  // Unselected labels are shown by default for [BottomNavigationBarType.fixed],
+  // and hidden by default for [BottomNavigationBarType.shifting].
+  bool get _defaultShowUnselected {
+    switch (_effectiveType) {
+      case BottomNavigationBarType.shifting:
+        return false;
+      case BottomNavigationBarType.fixed:
+        return true;
+    }
+    assert(false);
+    return false;
+  }
+
   @override
   void initState() {
     super.initState();
@@ -757,7 +758,7 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
     }
 
     if (widget.currentIndex != oldWidget.currentIndex) {
-      switch (widget.type) {
+      switch (_effectiveType) {
         case BottomNavigationBarType.fixed:
           break;
         case BottomNavigationBarType.shifting:
@@ -785,11 +786,18 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
     assert(localizations != null);
 
     final ThemeData themeData = Theme.of(context);
+    final BottomNavigationBarThemeData bottomTheme = BottomNavigationBarTheme.of(context);
 
     final TextStyle effectiveSelectedLabelStyle =
-      _effectiveTextStyle(widget.selectedLabelStyle, widget.selectedFontSize);
+      _effectiveTextStyle(
+        widget.selectedLabelStyle ?? bottomTheme.selectedLabelStyle,
+        widget.selectedFontSize,
+      );
     final TextStyle effectiveUnselectedLabelStyle =
-      _effectiveTextStyle(widget.unselectedLabelStyle, widget.unselectedFontSize);
+      _effectiveTextStyle(
+        widget.unselectedLabelStyle ?? bottomTheme.unselectedLabelStyle,
+        widget.unselectedFontSize,
+      );
 
     Color themeColor;
     switch (themeData.brightness) {
@@ -802,17 +810,26 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
     }
 
     ColorTween colorTween;
-    switch (widget.type) {
+    switch (_effectiveType) {
       case BottomNavigationBarType.fixed:
         colorTween = ColorTween(
-          begin: widget.unselectedItemColor ?? themeData.textTheme.caption.color,
-          end: widget.selectedItemColor ?? widget.fixedColor ?? themeColor,
+          begin: widget.unselectedItemColor
+            ?? bottomTheme.unselectedItemColor
+            ?? themeData.textTheme.caption.color,
+          end: widget.selectedItemColor
+            ?? bottomTheme.selectedItemColor
+            ?? widget.fixedColor
+            ?? themeColor,
         );
         break;
       case BottomNavigationBarType.shifting:
         colorTween = ColorTween(
-          begin: widget.unselectedItemColor ?? Colors.white,
-          end: widget.selectedItemColor ?? Colors.white,
+          begin: widget.unselectedItemColor
+            ?? bottomTheme.unselectedItemColor
+            ?? themeData.colorScheme.surface,
+          end: widget.selectedItemColor
+            ?? bottomTheme.selectedItemColor
+            ?? themeData.colorScheme.surface,
         );
         break;
     }
@@ -820,12 +837,12 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
     final List<Widget> tiles = <Widget>[];
     for (int i = 0; i < widget.items.length; i++) {
       tiles.add(_BottomNavigationTile(
-        widget.type,
+        _effectiveType,
         widget.items[i],
         _animations[i],
         widget.iconSize,
-        selectedIconTheme: widget.selectedIconTheme,
-        unselectedIconTheme: widget.unselectedIconTheme,
+        selectedIconTheme: widget.selectedIconTheme ?? bottomTheme.selectedIconTheme,
+        unselectedIconTheme: widget.unselectedIconTheme ?? bottomTheme.unselectedIconTheme,
         selectedLabelStyle: effectiveSelectedLabelStyle,
         unselectedLabelStyle: effectiveUnselectedLabelStyle,
         onTap: () {
@@ -835,8 +852,8 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
         colorTween: colorTween,
         flex: _evaluateFlex(_animations[i]),
         selected: i == widget.currentIndex,
-        showSelectedLabels: widget.showSelectedLabels,
-        showUnselectedLabels: widget.showUnselectedLabels,
+        showSelectedLabels: widget.showSelectedLabels ?? bottomTheme.showSelectedLabels,
+        showUnselectedLabels: widget.showUnselectedLabels ?? bottomTheme.showUnselectedLabels ?? _defaultShowUnselected,
         indexLabel: localizations.tabLabel(tabIndex: i + 1, tabCount: widget.items.length),
       ));
     }
@@ -859,12 +876,14 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
     assert(debugCheckHasMaterialLocalizations(context));
     assert(debugCheckHasMediaQuery(context));
 
+    final BottomNavigationBarThemeData bottomTheme = BottomNavigationBarTheme.of(context);
+
     // Labels apply up to _bottomMargin padding. Remainder is media padding.
     final double additionalBottomPadding = math.max(MediaQuery.of(context).padding.bottom - widget.selectedFontSize / 2.0, 0.0);
     Color backgroundColor;
-    switch (widget.type) {
+    switch (_effectiveType) {
       case BottomNavigationBarType.fixed:
-        backgroundColor = widget.backgroundColor;
+        backgroundColor = widget.backgroundColor ?? bottomTheme.backgroundColor;
         break;
       case BottomNavigationBarType.shifting:
         backgroundColor = _backgroundColor;
@@ -873,7 +892,7 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
     return Semantics(
       explicitChildNodes: true,
       child: Material(
-        elevation: widget.elevation,
+        elevation: widget.elevation ?? bottomTheme.elevation ?? 8.0,
         color: backgroundColor,
         child: ConstrainedBox(
           constraints: BoxConstraints(minHeight: kBottomNavigationBarHeight + additionalBottomPadding),

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

+// Copyright 2014 The Flutter 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 'bottom_navigation_bar.dart';
+import 'theme.dart';
+
+/// Defines default property values for descendant [BottomNavigationBar]
+/// widgets.
+///
+/// Descendant widgets obtain the current [BottomNavigationBarThemeData] object
+/// using `BottomNavigationBarTheme.of(context)`. Instances of
+/// [BottomNavigationBarThemeData] can be customized with
+/// [BottomNavigationBarThemeData.copyWith].
+///
+/// Typically a [BottomNavigationBarThemeData] is specified as part of the
+/// overall [Theme] with [ThemeData.bottomNavigationBarTheme].
+///
+/// All [BottomNavigationBarThemeData] properties are `null` by default. When
+/// null, the [BottomNavigationBar]'s build method provides defaults.
+///
+/// See also:
+///
+///  * [ThemeData], which describes the overall theme information for the
+///    application.
+@immutable
+class BottomNavigationBarThemeData with Diagnosticable {
+  /// Creates a theme that can be used for [ThemeData.BottomNavigationBarTheme].
+  const BottomNavigationBarThemeData({
+    this.backgroundColor,
+    this.elevation,
+    this.selectedIconTheme,
+    this.unselectedIconTheme,
+    this.selectedItemColor,
+    this.unselectedItemColor,
+    this.selectedLabelStyle,
+    this.unselectedLabelStyle,
+    this.showSelectedLabels,
+    this.showUnselectedLabels,
+    this.type,
+  });
+
+  /// The color of the [BottomNavigationBar] itself.
+  ///
+  /// See [BottomNavigationBar.backgroundColor].
+  final Color backgroundColor;
+
+  /// The z-coordinate of the [BottomNavigationBar].
+  ///
+  /// See [BottomNavigationBar.elevation].
+  final double elevation;
+
+  /// The size, opacity, and color of the icon in the currently selected
+  /// [BottomNavigationBarItem.icon].
+  ///
+  /// See [BottomNavigationBar.selectedIconTheme].
+  final IconThemeData selectedIconTheme;
+
+  /// The size, opacity, and color of the icon in the currently unselected
+  /// [BottomNavigationBarItem.icon]s.
+  ///
+  /// See [BottomNavigationBar.unselectedIconTheme].
+  final IconThemeData unselectedIconTheme;
+
+  /// The color of the selected [BottomNavigationBarItem.icon] and
+  /// [BottomNavigationBarItem.label].
+  ///
+  /// See [BottomNavigationBar.selectedItemColor].
+  final Color selectedItemColor;
+
+  /// The color of the unselected [BottomNavigationBarItem.icon] and
+  /// [BottomNavigationBarItem.label]s.
+  ///
+  /// See [BottomNavigationBar.unselectedItemColor].
+  final Color unselectedItemColor;
+
+  /// The [TextStyle] of the [BottomNavigationBarItem] labels when they are
+  /// selected.
+  ///
+  /// See [BottomNavigationBar.selectedLabelStyle].
+  final TextStyle selectedLabelStyle;
+
+  /// The [TextStyle] of the [BottomNavigationBarItem] labels when they are not
+  /// selected.
+  ///
+  /// See [BottomNavigationBar.unselectedLabelStyle].
+  final TextStyle unselectedLabelStyle;
+
+  /// Whether the labels are shown for the unselected [BottomNavigationBarItem]s.
+  ///
+  /// See [BottomNavigationBar.showSelectedLabels].
+  final bool showSelectedLabels;
+
+  /// Whether the labels are shown for the selected [BottomNavigationBarItem].
+  ///
+  /// See [BottomNavigationBar.showUnselectedLabels].
+  final bool showUnselectedLabels;
+
+  /// Defines the layout and behavior of a [BottomNavigationBar].
+  ///
+  /// See [BottomNavigationBar.type].
+  final BottomNavigationBarType type;
+
+  /// Creates a copy of this object but with the given fields replaced with the
+  /// new values.
+  BottomNavigationBarThemeData copyWith({
+    Color backgroundColor,
+    double elevation,
+    IconThemeData selectedIconTheme,
+    IconThemeData unselectedIconTheme,
+    Color selectedItemColor,
+    Color unselectedItemColor,
+    TextStyle selectedLabelStyle,
+    TextStyle unselectedLabelStyle,
+    bool showSelectedLabels,
+    bool showUnselectedLabels,
+    BottomNavigationBarType type,
+  }) {
+    return BottomNavigationBarThemeData(
+      backgroundColor: backgroundColor ?? this.backgroundColor,
+      elevation: elevation ?? this.elevation,
+      selectedIconTheme: selectedIconTheme ?? this.selectedIconTheme,
+      unselectedIconTheme: unselectedIconTheme ?? this.unselectedIconTheme,
+      selectedItemColor: selectedItemColor ?? this.selectedItemColor,
+      unselectedItemColor: unselectedItemColor ?? this.unselectedItemColor,
+      selectedLabelStyle: selectedLabelStyle ?? this.selectedLabelStyle,
+      unselectedLabelStyle: unselectedLabelStyle ?? this.unselectedLabelStyle,
+      showSelectedLabels: showSelectedLabels ?? this.showSelectedLabels,
+      showUnselectedLabels: showUnselectedLabels ?? this.showUnselectedLabels,
+      type: type ?? this.type,
+    );
+  }
+
+  /// Linearly interpolate between two [BottomNavigationBarThemeData].
+  ///
+  /// The argument `t` must not be null.
+  ///
+  /// {@macro dart.ui.shadow.lerp}
+  static BottomNavigationBarThemeData lerp(BottomNavigationBarThemeData a, BottomNavigationBarThemeData b, double t) {
+    assert(t != null);
+    return BottomNavigationBarThemeData(
+      backgroundColor: Color.lerp(a?.backgroundColor, b?.backgroundColor, t),
+      elevation: lerpDouble(a?.elevation, b?.elevation, t),
+      selectedIconTheme: IconThemeData.lerp(a?.selectedIconTheme, b?.selectedIconTheme, t),
+      unselectedIconTheme: IconThemeData.lerp(a?.unselectedIconTheme, b?.unselectedIconTheme, t),
+      selectedItemColor: Color.lerp(a?.selectedItemColor, b?.selectedItemColor, t),
+      unselectedItemColor: Color.lerp(a?.unselectedItemColor, b?.unselectedItemColor, t),
+      selectedLabelStyle: TextStyle.lerp(a?.selectedLabelStyle, b?.selectedLabelStyle, t),
+      unselectedLabelStyle: TextStyle.lerp(a?.unselectedLabelStyle, b?.unselectedLabelStyle, t),
+      showSelectedLabels: t < 0.5 ? a?.showSelectedLabels : b?.showSelectedLabels,
+      showUnselectedLabels: t < 0.5 ? a?.showUnselectedLabels : b?.showUnselectedLabels,
+      type: t < 0.5 ? a?.type : b?.type,
+    );
+  }
+
+  @override
+  int get hashCode {
+    return hashValues(
+      backgroundColor,
+      elevation,
+      selectedIconTheme,
+      unselectedIconTheme,
+      selectedItemColor,
+      unselectedItemColor,
+      selectedLabelStyle,
+      unselectedLabelStyle,
+      showSelectedLabels,
+      showUnselectedLabels,
+      type,
+    );
+  }
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other))
+      return true;
+    if (other.runtimeType != runtimeType)
+      return false;
+    return other is BottomNavigationBarThemeData
+        && other.backgroundColor == backgroundColor
+        && other.elevation == elevation
+        && other.selectedIconTheme == selectedIconTheme
+        && other.unselectedIconTheme == unselectedIconTheme
+        && other.selectedItemColor == selectedItemColor
+        && other.unselectedItemColor == unselectedItemColor
+        && other.selectedLabelStyle == selectedLabelStyle
+        && other.unselectedLabelStyle == unselectedLabelStyle
+        && other.showSelectedLabels == showSelectedLabels
+        && other.showUnselectedLabels == showUnselectedLabels
+        && other.type == type;
+  }
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(ColorProperty('backgroundColor', backgroundColor, defaultValue: null));
+    properties.add(DoubleProperty('elevation', elevation, defaultValue: null));
+    properties.add(DiagnosticsProperty<IconThemeData>('selectedIconTheme', selectedIconTheme, defaultValue: null));
+    properties.add(DiagnosticsProperty<IconThemeData>('unselectedIconTheme', unselectedIconTheme, defaultValue: null));
+    properties.add(ColorProperty('selectedItemColor', selectedItemColor, defaultValue: null));
+    properties.add(ColorProperty('unselectedItemColor', unselectedItemColor, defaultValue: null));
+    properties.add(DiagnosticsProperty<TextStyle>('selectedLabelStyle', selectedLabelStyle, defaultValue: null));
+    properties.add(DiagnosticsProperty<TextStyle>('unselectedLabelStyle', unselectedLabelStyle, defaultValue: null));
+    properties.add(DiagnosticsProperty<bool>('showSelectedLabels', showSelectedLabels, defaultValue: null));
+    properties.add(DiagnosticsProperty<bool>('showUnselectedLabels', showUnselectedLabels, defaultValue: null));
+    properties.add(DiagnosticsProperty<BottomNavigationBarType>('type', type, defaultValue: null));
+  }
+}
+
+/// Applies a bottom navigation bar theme to descendant [BottomNavigationBar]
+/// widgets.
+///
+/// Descendant widgets obtain the current theme's [BottomNavigationBarTheme]
+/// object using [BottomNavigationBarTheme.of]. When a widget uses
+/// [BottomNavigationBarTheme.of], it is automatically rebuilt if the theme
+/// later changes.
+///
+/// A bottom navigation theme can be specified as part of the overall Material
+/// theme using [ThemeData.bottomNavigationBarTheme].
+///
+/// See also:
+///
+///  * [BottomNavigationBarThemeData], which describes the actual configuration
+///    of a bottom navigation bar theme.
+class BottomNavigationBarTheme extends InheritedWidget {
+  /// Constructs a bottom navigation bar theme that configures all descendant
+  /// [BottomNavigationBar] widgets.
+  ///
+  /// The [data] must not be null.
+  const BottomNavigationBarTheme({
+    Key key,
+    @required this.data,
+    Widget child,
+  }) : assert(data != null), super(key: key, child: child);
+
+  /// The properties used for all descendant [BottomNavigationBar] widgets.
+  final BottomNavigationBarThemeData data;
+
+  /// Returns the configuration [data] from the closest
+  /// [BottomNavigationBarTheme] ancestor. If there is no ancestor, it returns
+  /// [ThemeData.bottomNavigationBarTheme]. Applications can assume that the
+  /// returned value will not be null.
+  ///
+  /// Typical usage is as follows:
+  ///
+  /// ```dart
+  /// BottomNavigationBarThemeData theme = BottomNavigationBarTheme.of(context);
+  /// ```
+  static BottomNavigationBarThemeData of(BuildContext context) {
+    final BottomNavigationBarTheme bottomNavTheme = context.dependOnInheritedWidgetOfExactType<BottomNavigationBarTheme>();
+    return bottomNavTheme?.data ?? Theme.of(context).bottomNavigationBarTheme;
+  }
+
+  @override
+  bool updateShouldNotify(BottomNavigationBarTheme oldWidget) => data != oldWidget.data;
+}

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

@@ -12,6 +12,7 @@ import 'package:flutter/widgets.dart';
 import 'app_bar_theme.dart';
 import 'banner_theme.dart';
 import 'bottom_app_bar_theme.dart';
+import 'bottom_navigation_bar_theme.dart';
 import 'bottom_sheet_theme.dart';
 import 'button_bar_theme.dart';
 import 'button_theme.dart';
@@ -266,6 +267,7 @@ class ThemeData with Diagnosticable {
     MaterialBannerThemeData bannerTheme,
     DividerThemeData dividerTheme,
     ButtonBarThemeData buttonBarTheme,
+    BottomNavigationBarThemeData bottomNavigationBarTheme,
   }) {
     brightness ??= Brightness.light;
     final bool isDark = brightness == Brightness.dark;
@@ -374,6 +376,7 @@ class ThemeData with Diagnosticable {
     bannerTheme ??= const MaterialBannerThemeData();
     dividerTheme ??= const DividerThemeData();
     buttonBarTheme ??= const ButtonBarThemeData();
+    bottomNavigationBarTheme ??= const BottomNavigationBarThemeData();
 
     return ThemeData.raw(
       brightness: brightness,
@@ -440,6 +443,7 @@ class ThemeData with Diagnosticable {
       bannerTheme: bannerTheme,
       dividerTheme: dividerTheme,
       buttonBarTheme: buttonBarTheme,
+      bottomNavigationBarTheme: bottomNavigationBarTheme,
     );
   }
 
@@ -518,6 +522,7 @@ class ThemeData with Diagnosticable {
     @required this.bannerTheme,
     @required this.dividerTheme,
     @required this.buttonBarTheme,
+    @required this.bottomNavigationBarTheme,
   }) : assert(brightness != null),
        assert(visualDensity != null),
        assert(primaryColor != null),
@@ -578,7 +583,8 @@ class ThemeData with Diagnosticable {
        assert(popupMenuTheme != null),
        assert(bannerTheme != null),
        assert(dividerTheme != null),
-       assert(buttonBarTheme != null);
+       assert(buttonBarTheme != null),
+       assert(bottomNavigationBarTheme != null);
 
   /// Create a [ThemeData] based on the colors in the given [colorScheme] and
   /// text styles of the optional [textTheme].
@@ -1025,6 +1031,10 @@ class ThemeData with Diagnosticable {
   /// A theme for customizing the appearance and layout of [ButtonBar] widgets.
   final ButtonBarThemeData buttonBarTheme;
 
+  /// A theme for customizing the appearance and layout of [BottomNavigationBar]
+  /// widgets.
+  final BottomNavigationBarThemeData bottomNavigationBarTheme;
+
   /// Creates a copy of this theme but with the given fields replaced with the new values.
   ThemeData copyWith({
     Brightness brightness,
@@ -1091,6 +1101,7 @@ class ThemeData with Diagnosticable {
     MaterialBannerThemeData bannerTheme,
     DividerThemeData dividerTheme,
     ButtonBarThemeData buttonBarTheme,
+    BottomNavigationBarThemeData bottomNavigationBarTheme,
   }) {
     cupertinoOverrideTheme = cupertinoOverrideTheme?.noDefault();
     return ThemeData.raw(
@@ -1158,6 +1169,7 @@ class ThemeData with Diagnosticable {
       bannerTheme: bannerTheme ?? this.bannerTheme,
       dividerTheme: dividerTheme ?? this.dividerTheme,
       buttonBarTheme: buttonBarTheme ?? this.buttonBarTheme,
+      bottomNavigationBarTheme: bottomNavigationBarTheme ?? this.bottomNavigationBarTheme,
     );
   }
 
@@ -1303,6 +1315,7 @@ class ThemeData with Diagnosticable {
       bannerTheme: MaterialBannerThemeData.lerp(a.bannerTheme, b.bannerTheme, t),
       dividerTheme: DividerThemeData.lerp(a.dividerTheme, b.dividerTheme, t),
       buttonBarTheme: ButtonBarThemeData.lerp(a.buttonBarTheme, b.buttonBarTheme, t),
+      bottomNavigationBarTheme: BottomNavigationBarThemeData.lerp(a.bottomNavigationBarTheme, b.bottomNavigationBarTheme, t),
     );
   }
 
@@ -1375,7 +1388,8 @@ class ThemeData with Diagnosticable {
         && other.popupMenuTheme == popupMenuTheme
         && other.bannerTheme == bannerTheme
         && other.dividerTheme == dividerTheme
-        && other.buttonBarTheme == buttonBarTheme;
+        && other.buttonBarTheme == buttonBarTheme
+        && other.bottomNavigationBarTheme == bottomNavigationBarTheme;
   }
 
   @override
@@ -1448,6 +1462,7 @@ class ThemeData with Diagnosticable {
       bannerTheme,
       dividerTheme,
       buttonBarTheme,
+      bottomNavigationBarTheme,
     ];
     return hashList(values);
   }
@@ -1516,6 +1531,7 @@ class ThemeData with Diagnosticable {
     properties.add(DiagnosticsProperty<MaterialBannerThemeData>('bannerTheme', bannerTheme, defaultValue: defaultData.bannerTheme, level: DiagnosticLevel.debug));
     properties.add(DiagnosticsProperty<DividerThemeData>('dividerTheme', dividerTheme, defaultValue: defaultData.dividerTheme, level: DiagnosticLevel.debug));
     properties.add(DiagnosticsProperty<ButtonBarThemeData>('buttonBarTheme', buttonBarTheme, defaultValue: defaultData.buttonBarTheme, level: DiagnosticLevel.debug));
+    properties.add(DiagnosticsProperty<BottomNavigationBarThemeData>('bottomNavigationBarTheme', bottomNavigationBarTheme, defaultValue: defaultData.bottomNavigationBarTheme, level: DiagnosticLevel.debug));
   }
 }
 

packages/flutter/test/material/bottom_navigation_bar_theme_test.dart

+// Copyright 2014 The Flutter 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_test/flutter_test.dart';
+import 'package:vector_math/vector_math_64.dart' show Vector3;
+
+
+void main() {
+  test('BottomNavigationBarThemeData copyWith, ==, hashCode basics', () {
+    expect(const BottomNavigationBarThemeData(), const BottomNavigationBarThemeData().copyWith());
+    expect(const BottomNavigationBarThemeData().hashCode, const BottomNavigationBarThemeData().copyWith().hashCode);
+  });
+
+  test('BottomNavigationBarThemeData defaults', () {
+    const BottomNavigationBarThemeData themeData = BottomNavigationBarThemeData();
+    expect(themeData.backgroundColor, null);
+    expect(themeData.elevation, null);
+    expect(themeData.selectedIconTheme, null);
+    expect(themeData.unselectedIconTheme, null);
+    expect(themeData.selectedItemColor, null);
+    expect(themeData.unselectedItemColor, null);
+    expect(themeData.selectedLabelStyle, null);
+    expect(themeData.unselectedLabelStyle, null);
+    expect(themeData.showSelectedLabels, null);
+    expect(themeData.showUnselectedLabels, null);
+    expect(themeData.type, null);
+
+    const BottomNavigationBarTheme theme = BottomNavigationBarTheme(data: BottomNavigationBarThemeData());
+    expect(theme.data.backgroundColor, null);
+    expect(theme.data.elevation, null);
+    expect(theme.data.selectedIconTheme, null);
+    expect(theme.data.unselectedIconTheme, null);
+    expect(theme.data.selectedItemColor, null);
+    expect(theme.data.unselectedItemColor, null);
+    expect(theme.data.selectedLabelStyle, null);
+    expect(theme.data.unselectedLabelStyle, null);
+    expect(theme.data.showSelectedLabels, null);
+    expect(theme.data.showUnselectedLabels, null);
+    expect(theme.data.type, null);
+  });
+
+  testWidgets('Default BottomNavigationBarThemeData debugFillProperties', (WidgetTester tester) async {
+    final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
+    const BottomNavigationBarThemeData().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('BottomNavigationBarThemeData implements debugFillProperties', (WidgetTester tester) async {
+    final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
+    const BottomNavigationBarThemeData(
+      backgroundColor: Color(0xfffffff0),
+      elevation: 10.0,
+      selectedIconTheme: IconThemeData(size: 1.0),
+      unselectedIconTheme: IconThemeData(size: 2.0),
+      selectedItemColor: Color(0xfffffff1),
+      unselectedItemColor: Color(0xfffffff2),
+      selectedLabelStyle: TextStyle(fontSize: 3.0),
+      unselectedLabelStyle: TextStyle(fontSize: 4.0),
+      showSelectedLabels: true,
+      showUnselectedLabels: true,
+      type: BottomNavigationBarType.fixed,
+    ).debugFillProperties(builder);
+
+    final List<String> description = builder.properties
+        .where((DiagnosticsNode node) => !node.isFiltered(DiagnosticLevel.info))
+        .map((DiagnosticsNode node) => node.toString())
+        .toList();
+
+    expect(description[0], 'backgroundColor: Color(0xfffffff0)');
+    expect(description[1], 'elevation: 10.0');
+
+    // Ignore instance address for IconThemeData.
+    expect(description[2].contains('selectedIconTheme: IconThemeData'), isTrue);
+    expect(description[2].contains('(size: 1.0)'), isTrue);
+    expect(description[3].contains('unselectedIconTheme: IconThemeData'), isTrue);
+    expect(description[3].contains('(size: 2.0)'), isTrue);
+
+    expect(description[4], 'selectedItemColor: Color(0xfffffff1)');
+    expect(description[5], 'unselectedItemColor: Color(0xfffffff2)');
+    expect(description[6], 'selectedLabelStyle: TextStyle(inherit: true, size: 3.0)');
+    expect(description[7], 'unselectedLabelStyle: TextStyle(inherit: true, size: 4.0)');
+    expect(description[8], 'showSelectedLabels: true');
+    expect(description[9], 'showUnselectedLabels: true');
+    expect(description[10], 'type: BottomNavigationBarType.fixed');
+  });
+
+  testWidgets('BottomNavigationBar is themable', (WidgetTester tester) async {
+    const Color backgroundColor = Color(0xFF000001);
+    const Color selectedItemColor = Color(0xFF000002);
+    const Color unselectedItemColor = Color(0xFF000003);
+    const IconThemeData selectedIconTheme = IconThemeData(size: 10);
+    const IconThemeData unselectedIconTheme = IconThemeData(size: 11);
+    const TextStyle selectedTextStyle = TextStyle(fontSize: 22);
+    const TextStyle unselectedTextStyle = TextStyle(fontSize: 21);
+    const double elevation = 9.0;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          bottomNavigationBarTheme: const BottomNavigationBarThemeData(
+            backgroundColor: backgroundColor,
+            selectedItemColor: selectedItemColor,
+            unselectedItemColor: unselectedItemColor,
+            selectedIconTheme: selectedIconTheme,
+            unselectedIconTheme: unselectedIconTheme,
+            elevation: elevation,
+            showUnselectedLabels: true,
+            showSelectedLabels: true,
+            type: BottomNavigationBarType.fixed,
+            selectedLabelStyle: selectedTextStyle,
+            unselectedLabelStyle: unselectedTextStyle,
+          ),
+        ),
+        home: Scaffold(
+          bottomNavigationBar: BottomNavigationBar(
+            items: const <BottomNavigationBarItem>[
+              BottomNavigationBarItem(
+                icon: Icon(Icons.ac_unit),
+                title: Text('AC'),
+              ),
+              BottomNavigationBarItem(
+                icon: Icon(Icons.access_alarm),
+                title: Text('Alarm'),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final TextStyle selectedFontStyle = tester.renderObject<RenderParagraph>(find.text('AC')).text.style;
+    final TextStyle selectedIcon = _iconStyle(tester, Icons.ac_unit);
+    final TextStyle unselectedIcon = _iconStyle(tester, Icons.access_alarm);
+    expect(selectedFontStyle.fontSize, selectedFontStyle.fontSize);
+    // Unselected label has a font size of 22 but is scaled down to be font size 21.
+    expect(
+      tester.firstWidget<Transform>(find.ancestor(of: find.text('Alarm'), matching: find.byType(Transform))).transform,
+      equals(Matrix4.diagonal3(Vector3.all(unselectedTextStyle.fontSize / selectedTextStyle.fontSize))),
+    );
+    expect(selectedIcon.color, equals(selectedItemColor));
+    expect(selectedIcon.fontSize, equals(selectedIconTheme.size));
+    expect(unselectedIcon.color, equals(unselectedItemColor));
+    expect(unselectedIcon.fontSize, equals(unselectedIconTheme.size));
+    // There should not be any [Opacity] or [FadeTransition] widgets
+    // since showUnselectedLabels and showSelectedLabels are true.
+    final Finder findOpacity = find.descendant(
+      of: find.byType(BottomNavigationBar),
+      matching: find.byType(Opacity),
+    );
+    final Finder findFadeTransition = find.descendant(
+      of: find.byType(BottomNavigationBar),
+      matching: find.byType(FadeTransition),
+    );
+    expect(findOpacity, findsNothing);
+    expect(findFadeTransition, findsNothing);
+    expect(_material(tester).elevation, equals(elevation));
+    expect(_material(tester).color, equals(backgroundColor));
+  });
+
+  testWidgets('BottomNavigationBar properties are taken over the theme values', (WidgetTester tester) async {
+    const Color themeBackgroundColor = Color(0xFF000001);
+    const Color themeSelectedItemColor = Color(0xFF000002);
+    const Color themeUnselectedItemColor = Color(0xFF000003);
+    const IconThemeData themeSelectedIconTheme = IconThemeData(size: 10);
+    const IconThemeData themeUnselectedIconTheme = IconThemeData(size: 11);
+    const TextStyle themeSelectedTextStyle = TextStyle(fontSize: 22);
+    const TextStyle themeUnselectedTextStyle = TextStyle(fontSize: 21);
+    const double themeElevation = 9.0;
+
+    const Color backgroundColor = Color(0xFF000004);
+    const Color selectedItemColor = Color(0xFF000005);
+    const Color unselectedItemColor = Color(0xFF000006);
+    const IconThemeData selectedIconTheme = IconThemeData(size: 15);
+    const IconThemeData unselectedIconTheme = IconThemeData(size: 16);
+    const TextStyle selectedTextStyle = TextStyle(fontSize: 25);
+    const TextStyle unselectedTextStyle = TextStyle(fontSize: 26);
+    const double elevation = 7.0;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          bottomNavigationBarTheme: const BottomNavigationBarThemeData(
+            backgroundColor: themeBackgroundColor,
+            selectedItemColor: themeSelectedItemColor,
+            unselectedItemColor: themeUnselectedItemColor,
+            selectedIconTheme: themeSelectedIconTheme,
+            unselectedIconTheme: themeUnselectedIconTheme,
+            elevation: themeElevation,
+            showUnselectedLabels: false,
+            showSelectedLabels: false,
+            type: BottomNavigationBarType.shifting,
+            selectedLabelStyle: themeSelectedTextStyle,
+            unselectedLabelStyle: themeUnselectedTextStyle,
+          ),
+        ),
+        home: Scaffold(
+          bottomNavigationBar: BottomNavigationBar(
+            backgroundColor: backgroundColor,
+            selectedItemColor: selectedItemColor,
+            unselectedItemColor: unselectedItemColor,
+            selectedIconTheme: selectedIconTheme,
+            unselectedIconTheme: unselectedIconTheme,
+            elevation: elevation,
+            showUnselectedLabels: true,
+            showSelectedLabels: true,
+            type: BottomNavigationBarType.fixed,
+            selectedLabelStyle: selectedTextStyle,
+            unselectedLabelStyle: unselectedTextStyle,
+            items: const <BottomNavigationBarItem>[
+              BottomNavigationBarItem(
+                icon: Icon(Icons.ac_unit),
+                title: Text('AC'),
+              ),
+              BottomNavigationBarItem(
+                icon: Icon(Icons.access_alarm),
+                title: Text('Alarm'),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final TextStyle selectedFontStyle = tester.renderObject<RenderParagraph>(find.text('AC')).text.style;
+    final TextStyle selectedIcon = _iconStyle(tester, Icons.ac_unit);
+    final TextStyle unselectedIcon = _iconStyle(tester, Icons.access_alarm);
+    expect(selectedFontStyle.fontSize, selectedFontStyle.fontSize);
+    // Unselected label has a font size of 22 but is scaled down to be font size 21.
+    expect(
+      tester.firstWidget<Transform>(find.ancestor(of: find.text('Alarm'), matching: find.byType(Transform))).transform,
+      equals(Matrix4.diagonal3(Vector3.all(unselectedTextStyle.fontSize / selectedTextStyle.fontSize))),
+    );
+    expect(selectedIcon.color, equals(selectedItemColor));
+    expect(selectedIcon.fontSize, equals(selectedIconTheme.size));
+    expect(unselectedIcon.color, equals(unselectedItemColor));
+    expect(unselectedIcon.fontSize, equals(unselectedIconTheme.size));
+    // There should not be any [Opacity] or [FadeTransition] widgets
+    // since showUnselectedLabels and showSelectedLabels are true.
+    final Finder findOpacity = find.descendant(
+      of: find.byType(BottomNavigationBar),
+      matching: find.byType(Opacity),
+    );
+    final Finder findFadeTransition = find.descendant(
+      of: find.byType(BottomNavigationBar),
+      matching: find.byType(FadeTransition),
+    );
+    expect(findOpacity, findsNothing);
+    expect(findFadeTransition, findsNothing);
+    expect(_material(tester).elevation, equals(elevation));
+    expect(_material(tester).color, equals(backgroundColor));
+  });
+}
+
+TextStyle _iconStyle(WidgetTester tester, IconData icon) {
+  final RichText iconRichText = tester.widget<RichText>(
+    find.descendant(of: find.byIcon(icon), matching: find.byType(RichText)),
+  );
+  return iconRichText.text.style;
+}
+
+Material _material(WidgetTester tester) {
+  return tester.firstWidget<Material>(
+    find.descendant(of: find.byType(BottomNavigationBar), matching: find.byType(Material)),
+  );
+}

packages/flutter/test/material/theme_data_test.dart

@@ -280,6 +280,7 @@ void main() {
       bannerTheme: const MaterialBannerThemeData(backgroundColor: Colors.black),
       dividerTheme: const DividerThemeData(color: Colors.black),
       buttonBarTheme: const ButtonBarThemeData(alignment: MainAxisAlignment.start),
+      bottomNavigationBarTheme: const BottomNavigationBarThemeData(type: BottomNavigationBarType.fixed),
     );
 
     final SliderThemeData otherSliderTheme = SliderThemeData.fromPrimaryColors(
@@ -360,6 +361,7 @@ void main() {
       bannerTheme: const MaterialBannerThemeData(backgroundColor: Colors.white),
       dividerTheme: const DividerThemeData(color: Colors.white),
       buttonBarTheme: const ButtonBarThemeData(alignment: MainAxisAlignment.end),
+      bottomNavigationBarTheme: const BottomNavigationBarThemeData(type: BottomNavigationBarType.shifting),
     );
 
     final ThemeData themeDataCopy = theme.copyWith(
@@ -426,6 +428,7 @@ void main() {
       bannerTheme: otherTheme.bannerTheme,
       dividerTheme: otherTheme.dividerTheme,
       buttonBarTheme: otherTheme.buttonBarTheme,
+      bottomNavigationBarTheme: otherTheme.bottomNavigationBarTheme,
     );
 
     expect(themeDataCopy.brightness, equals(otherTheme.brightness));
@@ -493,6 +496,7 @@ void main() {
     expect(themeDataCopy.bannerTheme, equals(otherTheme.bannerTheme));
     expect(themeDataCopy.dividerTheme, equals(otherTheme.dividerTheme));
     expect(themeDataCopy.buttonBarTheme, equals(otherTheme.buttonBarTheme));
+    expect(themeDataCopy.bottomNavigationBarTheme, equals(otherTheme.bottomNavigationBarTheme));
   });
 
   testWidgets('ThemeData.toString has less than 200 characters output', (WidgetTester tester) async {