make sure bottom nav bar always contributes labels (#19934)

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

@@ -242,6 +242,7 @@ class _BottomNavigationTile extends StatelessWidget {
           ).evaluate(animation),
         ),
         child: new FadeTransition(
+          alwaysIncludeSemantics: true,
           opacity: animation,
           child: DefaultTextStyle.merge(
             style: const TextStyle(

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -792,7 +792,14 @@ class RenderAnimatedOpacity extends RenderProxyBox {
   /// Creates a partially transparent render object.
   ///
   /// The [opacity] argument must not be null.
-  RenderAnimatedOpacity({ @required Animation<double> opacity, RenderBox child }) : super(child) {
+  RenderAnimatedOpacity({
+    @required Animation<double> opacity,
+    bool alwaysIncludeSemantics = false,
+    RenderBox child,
+  }) : assert(opacity != null),
+       assert(alwaysIncludeSemantics != null),
+       _alwaysIncludeSemantics = alwaysIncludeSemantics,
+       super(child) {
     this.opacity = opacity;
   }
 
@@ -823,6 +830,18 @@ class RenderAnimatedOpacity extends RenderProxyBox {
     _updateOpacity();
   }
 
+  /// Whether child semantics are included regardless of the opacity.
+  ///
+  /// Defaults to false.
+  bool get alwaysIncludeSemantics => _alwaysIncludeSemantics;
+  bool _alwaysIncludeSemantics;
+  set alwaysIncludeSemantics(bool value) {
+    if (value == _alwaysIncludeSemantics)
+      return;
+    _alwaysIncludeSemantics = value;
+    markNeedsSemanticsUpdate();
+  }
+
   @override
   void attach(PipelineOwner owner) {
     super.attach(owner);
@@ -866,7 +885,7 @@ class RenderAnimatedOpacity extends RenderProxyBox {
 
   @override
   void visitChildrenForSemantics(RenderObjectVisitor visitor) {
-    if (child != null && _alpha != 0)
+    if (child != null && (_alpha != 0 || alwaysIncludeSemantics))
       visitor(child);
   }
 

packages/flutter/lib/src/widgets/transitions.dart

@@ -342,6 +342,7 @@ class FadeTransition extends SingleChildRenderObjectWidget {
     Key key,
     @required this.opacity,
     Widget child,
+    this.alwaysIncludeSemantics = false,
   }) : super(key: key, child: child);
 
   /// The animation that controls the opacity of the child.
@@ -352,17 +353,29 @@ class FadeTransition extends SingleChildRenderObjectWidget {
   /// completely transparent.
   final Animation<double> opacity;
 
+  /// Whether the semantic information of the children is always included.
+  ///
+  /// Defaults to false.
+  ///
+  /// When true, regardless of the opacity settings the child semantic
+  /// information is exposed as if the widget were fully visible. This is
+  /// useful in cases where labels may be hidden during animations that
+  /// would otherwise contribute relevant semantics.
+  final bool alwaysIncludeSemantics;
+
   @override
   RenderAnimatedOpacity createRenderObject(BuildContext context) {
     return new RenderAnimatedOpacity(
       opacity: opacity,
+      alwaysIncludeSemantics: alwaysIncludeSemantics,
     );
   }
 
   @override
   void updateRenderObject(BuildContext context, RenderAnimatedOpacity renderObject) {
     renderObject
-      ..opacity = opacity;
+      ..opacity = opacity
+      ..alwaysIncludeSemantics = alwaysIncludeSemantics;
   }
 
   @override

packages/flutter/test/material/bottom_navigation_bar_test.dart

@@ -539,7 +539,7 @@ void main() {
     expect(find.byKey(stroked), findsOneWidget);
   });
 
-  testWidgets('BottomNavigationBar semantics', (WidgetTester tester) async {
+  testWidgets('BottomNavigationBar.fixed semantics', (WidgetTester tester) async {
     final SemanticsTester semantics = new SemanticsTester(tester);
 
     await tester.pumpWidget(
@@ -567,13 +567,10 @@ void main() {
     final TestSemantics expected = new TestSemantics.root(
       children: <TestSemantics>[
         new TestSemantics(
-          id: 1,
           children: <TestSemantics>[
             new TestSemantics(
-              id: 2,
               children: <TestSemantics>[
                 new TestSemantics(
-                  id: 3,
                   flags: <SemanticsFlag>[
                     SemanticsFlag.isSelected,
                     SemanticsFlag.isHeader,
@@ -583,7 +580,6 @@ void main() {
                   textDirection: TextDirection.ltr,
                 ),
                 new TestSemantics(
-                  id: 4,
                   flags: <SemanticsFlag>[
                     SemanticsFlag.isHeader,
                   ],
@@ -592,7 +588,6 @@ void main() {
                   textDirection: TextDirection.ltr,
                 ),
                 new TestSemantics(
-                  id: 5,
                   flags: <SemanticsFlag>[
                     SemanticsFlag.isHeader,
                   ],
@@ -606,7 +601,75 @@ void main() {
         ),
       ],
     );
-    expect(semantics, hasSemantics(expected, ignoreTransform: true, ignoreRect: true));
+    expect(semantics, hasSemantics(expected, ignoreId: true, ignoreTransform: true, ignoreRect: true));
+
+    semantics.dispose();
+  });
+
+  testWidgets('BottomNavigationBar.shifting semantics', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+
+    await tester.pumpWidget(
+      boilerplate(
+        textDirection: TextDirection.ltr,
+        bottomNavigationBar: new BottomNavigationBar(
+          type: BottomNavigationBarType.shifting,
+          items: const <BottomNavigationBarItem>[
+            const BottomNavigationBarItem(
+              icon: const Icon(Icons.ac_unit),
+              title: const Text('AC'),
+            ),
+            const BottomNavigationBarItem(
+              icon: const Icon(Icons.access_alarm),
+              title: const Text('Alarm'),
+            ),
+            const BottomNavigationBarItem(
+              icon: const Icon(Icons.hot_tub),
+              title: const Text('Hot Tub'),
+            ),
+          ],
+        ),
+      ),
+    );
+
+    final TestSemantics expected = new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics(
+          children: <TestSemantics>[
+            new TestSemantics(
+              children: <TestSemantics>[
+                new TestSemantics(
+                  flags: <SemanticsFlag>[
+                    SemanticsFlag.isSelected,
+                    SemanticsFlag.isHeader,
+                  ],
+                  actions: <SemanticsAction>[SemanticsAction.tap],
+                  label: 'AC\nTab 1 of 3',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  flags: <SemanticsFlag>[
+                    SemanticsFlag.isHeader,
+                  ],
+                  actions: <SemanticsAction>[SemanticsAction.tap],
+                  label: 'Alarm\nTab 2 of 3',
+                  textDirection: TextDirection.ltr,
+                ),
+                new TestSemantics(
+                  flags: <SemanticsFlag>[
+                    SemanticsFlag.isHeader,
+                  ],
+                  actions: <SemanticsAction>[SemanticsAction.tap],
+                  label: 'Hot Tub\nTab 3 of 3',
+                  textDirection: TextDirection.ltr,
+                ),
+              ],
+            ),
+          ],
+        ),
+      ],
+    );
+    expect(semantics, hasSemantics(expected, ignoreId: true, ignoreTransform: true, ignoreRect: true));
 
     semantics.dispose();
   });

packages/flutter/test/rendering/proxy_box_test.dart

@@ -219,6 +219,7 @@ void main() {
     )..value = 0.0;
 
     final RenderAnimatedOpacity renderAnimatedOpacity = new RenderAnimatedOpacity(
+      alwaysIncludeSemantics: false,
       opacity: opacityAnimation,
       child: new RenderSizedBox(const Size(1.0, 1.0)), // size doesn't matter
     );
@@ -233,6 +234,7 @@ void main() {
     )..value = 1.0;
 
     final RenderAnimatedOpacity renderAnimatedOpacity = new RenderAnimatedOpacity(
+      alwaysIncludeSemantics: false,
       opacity: opacityAnimation,
       child: new RenderSizedBox(const Size(1.0, 1.0)), // size doesn't matter
     );

packages/flutter/test/rendering/semantics_and_children_test.dart

@@ -43,6 +43,7 @@ void main() {
   test('RenderOpacity and children and semantics', () {
     final AnimationController controller = new AnimationController(vsync: const TestVSync());
     final RenderAnimatedOpacity box = new RenderAnimatedOpacity(
+      alwaysIncludeSemantics: false,
       opacity: controller,
       child: new RenderParagraph(
         const TextSpan(),