Toggle whether label or hint contribute to text field semantics when unfocused/focused (#19790)

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

@@ -540,12 +540,14 @@ class _RenderDecoration extends RenderBox {
     @required _Decoration decoration,
     @required TextDirection textDirection,
     @required TextBaseline textBaseline,
+    @required bool isFocused,
   }) : assert(decoration != null),
        assert(textDirection != null),
        assert(textBaseline != null),
        _decoration = decoration,
        _textDirection = textDirection,
-       _textBaseline = textBaseline;
+       _textBaseline = textBaseline,
+       _isFocused = isFocused;
 
   final Map<_DecorationSlot, RenderBox> slotToChild = <_DecorationSlot, RenderBox>{};
   final Map<RenderBox, _DecorationSlot> childToSlot = <RenderBox, _DecorationSlot>{};
@@ -686,6 +688,16 @@ class _RenderDecoration extends RenderBox {
     markNeedsLayout();
   }
 
+  bool get isFocused => _isFocused;
+  bool _isFocused;
+  set isFocused(bool value) {
+    assert(value != null);
+    if (_isFocused == value)
+      return;
+    _isFocused = value;
+    markNeedsSemanticsUpdate();
+  }
+
   @override
   void attach(PipelineOwner owner) {
     super.attach(owner);
@@ -710,6 +722,35 @@ class _RenderDecoration extends RenderBox {
     _children.forEach(visitor);
   }
 
+  @override
+  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
+    if (icon != null)
+      visitor(icon);
+    if (prefix != null)
+      visitor(prefix);
+    if (prefixIcon != null)
+      visitor(prefixIcon);
+    if (isFocused && hint != null) {
+      // Bypass opacity to always read hint when focused. This prevents the
+      // label from changing when text is entered.
+      final RenderProxyBox typedHint = hint;
+      visitor(typedHint.child);
+    } else if (!isFocused && label != null)
+      visitor(label);
+    if (input != null)
+      visitor(input);
+    if (suffixIcon != null)
+      visitor(suffixIcon);
+    if (suffix != null)
+      visitor(suffix);
+    if (container != null)
+      visitor(container);
+    if (helperError != null)
+      visitor(helperError);
+    if (counter != null)
+      visitor(counter);
+  }
+
   @override
   List<DiagnosticsNode> debugDescribeChildren() {
     final List<DiagnosticsNode> value = <DiagnosticsNode>[];
@@ -1301,6 +1342,7 @@ class _Decorator extends RenderObjectWidget {
     @required this.decoration,
     @required this.textDirection,
     @required this.textBaseline,
+    @required this.isFocused,
   }) : assert(decoration != null),
        assert(textDirection != null),
        assert(textBaseline != null),
@@ -1309,6 +1351,7 @@ class _Decorator extends RenderObjectWidget {
   final _Decoration decoration;
   final TextDirection textDirection;
   final TextBaseline textBaseline;
+  final bool isFocused;
 
   @override
   _RenderDecorationElement createElement() => new _RenderDecorationElement(this);
@@ -1319,6 +1362,7 @@ class _Decorator extends RenderObjectWidget {
       decoration: decoration,
       textDirection: textDirection,
       textBaseline: textBaseline,
+      isFocused: isFocused,
     );
   }
 
@@ -1327,7 +1371,8 @@ class _Decorator extends RenderObjectWidget {
     renderObject
      ..decoration = decoration
      ..textDirection = textDirection
-     ..textBaseline = textBaseline;
+     ..textBaseline = textBaseline
+     ..isFocused = isFocused;
   }
 }
 
@@ -1784,6 +1829,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
         ? const EdgeInsets.fromLTRB(12.0, 20.0, 12.0, 12.0)
         : const EdgeInsets.fromLTRB(12.0, 24.0, 12.0, 16.0));
     }
+
     return new _Decorator(
       decoration: new _Decoration(
         contentPadding: contentPadding,
@@ -1803,9 +1849,10 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
         helperError: helperError,
         counter: counter,
         container: container,
-      ),
-      textDirection: textDirection,
-      textBaseline: textBaseline,
+    ),
+    textDirection: textDirection,
+    textBaseline: textBaseline,
+    isFocused: isFocused,
     );
   }
 }

packages/flutter/test/material/text_field_test.dart

@@ -2250,4 +2250,68 @@ void main() {
     expect(focusNode.hasFocus, isFalse);
   });
 
+  testWidgets('TextField semantics', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    final TextEditingController controller = new TextEditingController();
+    final Key key = new UniqueKey();
+
+    await tester.pumpWidget(
+      overlay(
+        child: new TextField(
+          key: key,
+          controller: controller,
+          decoration: const InputDecoration(
+            labelText: 'label',
+            hintText: 'hint',
+            helperText: 'helper',
+            counterText: 'counter',
+          ),
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics.rootChild(
+          label: 'label\nhelper\ncounter',
+          id: 1,
+          textDirection: TextDirection.ltr,
+          actions: <SemanticsAction>[
+            SemanticsAction.tap,
+          ],
+          flags: <SemanticsFlag>[
+            SemanticsFlag.isTextField,
+          ],
+        ),
+      ],
+    ), ignoreTransform: true, ignoreRect: true));
+
+    await tester.tap(find.byType(TextField));
+    await tester.pump();
+
+    expect(semantics, hasSemantics(new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics.rootChild(
+          label: 'hint\nhelper\ncounter',
+          id: 1,
+          textDirection: TextDirection.ltr,
+          textSelection: const TextSelection(baseOffset: 0, extentOffset: 0),
+          actions: <SemanticsAction>[
+            SemanticsAction.tap,
+            SemanticsAction.setSelection,
+            SemanticsAction.paste,
+          ],
+          flags: <SemanticsFlag>[
+            SemanticsFlag.isTextField,
+            SemanticsFlag.isFocused,
+          ],
+        ),
+      ],
+    ), ignoreTransform: true, ignoreRect: true));
+
+    controller.text = 'hello';
+    await tester.pump();
+    semantics.dispose();
+  });
+
 }