[Material] TabBarTheme text style parameters (#26533)

* Add labelStyle + unselectedLabelStyle to TabBarTheme

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

@@ -29,7 +29,9 @@ class TabBarTheme extends Diagnosticable {
     this.indicator,
     this.indicatorSize,
     this.labelColor,
+    this.labelStyle,
     this.unselectedLabelColor,
+    this.unselectedLabelStyle,
   });
 
   /// Default value for [TabBar.indicator].
@@ -41,22 +43,32 @@ class TabBarTheme extends Diagnosticable {
   /// Default value for [TabBar.labelColor].
   final Color labelColor;
 
+  /// Default value for [TabBar.labelStyle].
+  final TextStyle labelStyle;
+
   /// Default value for [TabBar.unselectedLabelColor].
   final Color unselectedLabelColor;
 
+  /// Default value for [TabBar.unselectedLabelStyle].
+  final TextStyle unselectedLabelStyle;
+
   /// Creates a copy of this object but with the given fields replaced with the
   /// new values.
   TabBarTheme copyWith({
     Decoration indicator,
     TabBarIndicatorSize indicatorSize,
     Color labelColor,
+    TextStyle labelStyle,
     Color unselectedLabelColor,
+    TextStyle unselectedLabelStyle,
   }) {
     return TabBarTheme(
-        indicator: indicator ?? this.indicator,
-        indicatorSize: indicatorSize ?? this.indicatorSize,
-        labelColor: labelColor ?? this.labelColor,
-        unselectedLabelColor: unselectedLabelColor ?? this.unselectedLabelColor
+      indicator: indicator ?? this.indicator,
+      indicatorSize: indicatorSize ?? this.indicatorSize,
+      labelColor: labelColor ?? this.labelColor,
+      labelStyle: labelStyle ?? this.labelStyle,
+      unselectedLabelColor: unselectedLabelColor ?? this.unselectedLabelColor,
+      unselectedLabelStyle: unselectedLabelStyle ?? this.unselectedLabelStyle,
     );
   }
 
@@ -78,13 +90,22 @@ class TabBarTheme extends Diagnosticable {
       indicator: Decoration.lerp(a.indicator, b.indicator, t),
       indicatorSize: t < 0.5 ? a.indicatorSize : b.indicatorSize,
       labelColor: Color.lerp(a.labelColor, b.labelColor, t),
-      unselectedLabelColor: Color.lerp(a.unselectedLabelColor, b.unselectedLabelColor, t)
+      labelStyle: TextStyle.lerp(a.labelStyle, b.labelStyle, t),
+      unselectedLabelColor: Color.lerp(a.unselectedLabelColor, b.unselectedLabelColor, t),
+      unselectedLabelStyle: TextStyle.lerp(a.unselectedLabelStyle, b.unselectedLabelStyle, t),
     );
   }
 
   @override
   int get hashCode {
-    return hashValues(indicator, indicatorSize, labelColor, unselectedLabelColor);
+    return hashValues(
+      indicator,
+      indicatorSize,
+      labelColor,
+      labelStyle,
+      unselectedLabelColor,
+      unselectedLabelStyle,
+    );
   }
 
   @override
@@ -97,6 +118,8 @@ class TabBarTheme extends Diagnosticable {
     return typedOther.indicator == indicator
         && typedOther.indicatorSize == indicatorSize
         && typedOther.labelColor == labelColor
-        && typedOther.unselectedLabelColor == unselectedLabelColor;
+        && typedOther.labelStyle == labelStyle
+        && typedOther.unselectedLabelColor == unselectedLabelColor
+        && typedOther.unselectedLabelStyle == unselectedLabelStyle;
   }
 }

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

@@ -154,20 +154,21 @@ class _TabStyle extends AnimatedWidget {
     final ThemeData themeData = Theme.of(context);
     final TabBarTheme tabBarTheme = TabBarTheme.of(context);
 
-    final TextStyle defaultStyle = labelStyle ?? themeData.primaryTextTheme.body2;
-    final TextStyle defaultUnselectedStyle = unselectedLabelStyle ?? labelStyle ?? themeData.primaryTextTheme.body2;
+    final TextStyle defaultStyle = labelStyle ?? tabBarTheme.labelStyle ?? themeData.primaryTextTheme.body2;
+    final TextStyle defaultUnselectedStyle = unselectedLabelStyle
+      ?? tabBarTheme.unselectedLabelStyle
+      ?? labelStyle
+      ?? themeData.primaryTextTheme.body2;
     final Animation<double> animation = listenable;
     final TextStyle textStyle = selected
       ? TextStyle.lerp(defaultStyle, defaultUnselectedStyle, animation.value)
       : TextStyle.lerp(defaultUnselectedStyle, defaultStyle, animation.value);
-    final Color selectedColor =
-        labelColor
-         ?? tabBarTheme.labelColor
-         ?? themeData.primaryTextTheme.body2.color;
-    final Color unselectedColor =
-        unselectedLabelColor
-        ?? tabBarTheme.unselectedLabelColor
-        ?? selectedColor.withAlpha(0xB2); // 70% alpha
+    final Color selectedColor = labelColor
+       ?? tabBarTheme.labelColor
+       ?? themeData.primaryTextTheme.body2.color;
+    final Color unselectedColor = unselectedLabelColor
+      ?? tabBarTheme.unselectedLabelColor
+      ?? selectedColor.withAlpha(0xB2); // 70% alpha
     final Color color = selected
       ? Color.lerp(selectedColor, unselectedColor, animation.value)
       : Color.lerp(unselectedColor, selectedColor, animation.value);

packages/flutter/test/material/tab_bar_theme_test.dart

@@ -20,19 +20,18 @@ const List<Tab> _tabs = <Tab>[
   Tab(text: _tab3Text, icon: Icon(Icons.looks_3)),
 ];
 
-Widget _buildTabBar({ List<Tab> tabs = _tabs }) {
-  final TabController _tabController = TabController(length: 3, vsync: const TestVSync());
-
-  return RepaintBoundary(
-    key: _painterKey,
-    child: TabBar(tabs: tabs, controller: _tabController),
-  );
-}
-
 Widget _withTheme(TabBarTheme theme) {
   return MaterialApp(
     theme: ThemeData(tabBarTheme: theme),
-    home: Scaffold(body: _buildTabBar()),
+    home: Scaffold(
+      body: RepaintBoundary(
+        key: _painterKey,
+        child: TabBar(
+          tabs: _tabs,
+          controller: TabController(length: _tabs.length, vsync: const TestVSync()),
+        ),
+      ),
+    ),
   );
 }
 
@@ -42,6 +41,19 @@ RenderParagraph _iconRenderObject(WidgetTester tester, IconData icon) {
 }
 
 void main() {
+  testWidgets('Tab bar defaults', (WidgetTester tester) async {
+    await tester.pumpWidget(_withTheme(null));
+
+    final RenderParagraph selectedRenderObject = tester.renderObject<RenderParagraph>(find.text(_tab1Text));
+    expect(selectedRenderObject.text.style.fontFamily, equals('Roboto'));
+    expect(selectedRenderObject.text.style.fontSize, equals(14.0));
+    expect(selectedRenderObject.text.style.color, equals(Colors.white));
+    final RenderParagraph unselectedRenderObject = tester.renderObject<RenderParagraph>(find.text(_tab2Text));
+    expect(unselectedRenderObject.text.style.fontFamily, equals('Roboto'));
+    expect(unselectedRenderObject.text.style.fontSize, equals(14.0));
+    expect(unselectedRenderObject.text.style.color, equals(Colors.white.withAlpha(0xB2)));
+  });
+
   testWidgets('Tab bar theme overrides label color (selected)', (WidgetTester tester) async {
     const Color labelColor = Colors.black;
     const TabBarTheme tabBarTheme = TabBarTheme(labelColor: labelColor);
@@ -54,6 +66,51 @@ void main() {
     expect(iconRenderObject.text.style.color, equals(labelColor));
   });
 
+  testWidgets('Tab bar theme overrides label styles', (WidgetTester tester) async {
+    const TextStyle labelStyle = TextStyle(fontFamily: 'foobar');
+    const TextStyle unselectedLabelStyle = TextStyle(fontFamily: 'baz');
+    const TabBarTheme tabBarTheme = TabBarTheme(
+      labelStyle: labelStyle,
+      unselectedLabelStyle: unselectedLabelStyle,
+    );
+
+    await tester.pumpWidget(_withTheme(tabBarTheme));
+
+    final RenderParagraph selectedRenderObject = tester.renderObject<RenderParagraph>(find.text(_tab1Text));
+    expect(selectedRenderObject.text.style.fontFamily, equals(labelStyle.fontFamily));
+    final RenderParagraph unselectedRenderObject = tester.renderObject<RenderParagraph>(find.text(_tab2Text));
+    expect(unselectedRenderObject.text.style.fontFamily, equals(unselectedLabelStyle.fontFamily));
+  });
+
+  testWidgets('Tab bar label styles override theme label styles', (WidgetTester tester) async {
+    const TextStyle labelStyle = TextStyle(fontFamily: '1');
+    const TextStyle unselectedLabelStyle = TextStyle(fontFamily: '2');
+    const TextStyle themeLabelStyle = TextStyle(fontFamily: '3');
+    const TextStyle themeUnselectedLabelStyle = TextStyle(fontFamily: '4');
+    const TabBarTheme tabBarTheme = TabBarTheme(
+      labelStyle: themeLabelStyle,
+      unselectedLabelStyle: themeUnselectedLabelStyle,
+    );
+
+    await tester.pumpWidget(
+      MaterialApp(
+          theme: ThemeData(tabBarTheme: tabBarTheme),
+          home: Scaffold(body: TabBar(
+            tabs: _tabs,
+            controller: TabController(length: _tabs.length, vsync: const TestVSync()),
+            labelStyle: labelStyle,
+            unselectedLabelStyle: unselectedLabelStyle,
+          ),
+        )
+      ),
+    );
+
+    final RenderParagraph selectedRenderObject = tester.renderObject<RenderParagraph>(find.text(_tab1Text));
+    expect(selectedRenderObject.text.style.fontFamily, equals(labelStyle.fontFamily));
+    final RenderParagraph unselectedRenderObject = tester.renderObject<RenderParagraph>(find.text(_tab2Text));
+    expect(unselectedRenderObject.text.style.fontFamily, equals(unselectedLabelStyle.fontFamily));
+  });
+
   testWidgets('Tab bar theme overrides label color (unselected)', (WidgetTester tester) async {
     const Color unselectedLabelColor = Colors.black;
     const TabBarTheme tabBarTheme = TabBarTheme(unselectedLabelColor: unselectedLabelColor);