CupertinoPicker semantics (#23551)

packages/flutter/lib/src/cupertino/picker.dart

@@ -161,6 +161,32 @@ class CupertinoPicker extends StatefulWidget {
 
 class _CupertinoPickerState extends State<CupertinoPicker> {
   int _lastHapticIndex;
+  FixedExtentScrollController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+    if (widget.scrollController == null) {
+      _controller = FixedExtentScrollController();
+    }
+  }
+
+  @override
+  void didUpdateWidget(CupertinoPicker oldWidget) {
+    if (widget.scrollController != null && oldWidget.scrollController == null) {
+      _controller = null;
+    } else if (widget.scrollController == null && oldWidget.scrollController != null) {
+      assert(_controller == null);
+      _controller = FixedExtentScrollController();
+    }
+    super.didUpdateWidget(oldWidget);
+  }
+
+  @override
+  void dispose() {
+    _controller?.dispose();
+    super.dispose();
+  }
 
   void _handleSelectedItemChanged(int index) {
     // Only the haptic engine hardware on iOS devices would produce the
@@ -246,17 +272,20 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
     Widget result = Stack(
       children: <Widget>[
         Positioned.fill(
-          child: ListWheelScrollView.useDelegate(
-            controller: widget.scrollController,
-            physics: const FixedExtentScrollPhysics(),
-            diameterRatio: widget.diameterRatio,
-            perspective: _kDefaultPerspective,
-            offAxisFraction: widget.offAxisFraction,
-            useMagnifier: widget.useMagnifier,
-            magnification: widget.magnification,
-            itemExtent: widget.itemExtent,
-            onSelectedItemChanged: _handleSelectedItemChanged,
-            childDelegate: widget.childDelegate,
+          child: _CupertinoPickerSemantics(
+            scrollController: widget.scrollController ?? _controller,
+            child: ListWheelScrollView.useDelegate(
+              controller: widget.scrollController ?? _controller,
+              physics: const FixedExtentScrollPhysics(),
+              diameterRatio: widget.diameterRatio,
+              perspective: _kDefaultPerspective,
+              offAxisFraction: widget.offAxisFraction,
+              useMagnifier: widget.useMagnifier,
+              magnification: widget.magnification,
+              itemExtent: widget.itemExtent,
+              onSelectedItemChanged: _handleSelectedItemChanged,
+              childDelegate: widget.childDelegate,
+            ),
           ),
         ),
         _buildGradientScreen(),
@@ -274,3 +303,110 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
     return result;
   }
 }
+
+// Turns the scroll semantics of the ListView into a single adjustable semantics
+// node. This is done by removing all of the child semantics of the scroll
+// wheel and using the scroll indexes to look up the current, previous, and
+// next semantic label. This label is then turned into the value of a new
+// adjustable semantic node, with adjustment callbacks wired to move the
+// scroll controller.
+class _CupertinoPickerSemantics extends SingleChildRenderObjectWidget {
+  const _CupertinoPickerSemantics({
+    Key key,
+    Widget child,
+    @required this.scrollController,
+  }) : super(key: key, child: child);
+
+  final FixedExtentScrollController scrollController;
+
+  @override
+  RenderObject createRenderObject(BuildContext context) => _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context));
+
+  @override
+  void updateRenderObject(BuildContext context, covariant _RenderCupertinoPickerSemantics renderObject) {
+    renderObject
+      ..textDirection = Directionality.of(context)
+      ..controller = scrollController;
+  }
+}
+
+class _RenderCupertinoPickerSemantics extends RenderProxyBox {
+  _RenderCupertinoPickerSemantics(FixedExtentScrollController controller, this._textDirection) {
+    this.controller = controller;
+  }
+
+  FixedExtentScrollController get controller => _controller;
+  FixedExtentScrollController _controller;
+  set controller(FixedExtentScrollController value) {
+    if (value == _controller)
+      return;
+    if (_controller != null)
+      _controller.removeListener(_handleScrollUpdate);
+    else
+      _currentIndex = value.initialItem ?? 0;
+    value.addListener(_handleScrollUpdate);
+    _controller = value;
+  }
+
+  TextDirection get textDirection => _textDirection;
+  TextDirection _textDirection;
+  set textDirection(TextDirection value) {
+    if (textDirection == value)
+      return;
+    _textDirection = value;
+    markNeedsSemanticsUpdate();
+  }
+
+  int _currentIndex = 0;
+
+  void _handleIncrease() {
+    controller.jumpToItem(_currentIndex + 1);
+  }
+
+  void _handleDecrease() {
+    if (_currentIndex == 0)
+      return;
+    controller.jumpToItem(_currentIndex - 1);
+   }
+
+  void _handleScrollUpdate() {
+    if (controller.selectedItem == _currentIndex)
+      return;
+    _currentIndex = controller.selectedItem;
+    markNeedsSemanticsUpdate();
+  }
+   @override
+  void describeSemanticsConfiguration(SemanticsConfiguration config) {
+    super.describeSemanticsConfiguration(config);
+    config.isSemanticBoundary = true;
+    config.textDirection = textDirection;
+  }
+
+  @override
+  void assembleSemanticsNode(SemanticsNode node, SemanticsConfiguration config, Iterable<SemanticsNode> children) {
+    if (children.isEmpty)
+      return super.assembleSemanticsNode(node, config, children);
+    final SemanticsNode scrollable = children.first;
+    final Map<int, SemanticsNode> indexedChildren = <int, SemanticsNode>{};
+    scrollable.visitChildren((SemanticsNode child) {
+      assert(child.indexInParent != null);
+      indexedChildren[child.indexInParent] = child;
+      return true;
+    });
+    if (indexedChildren[_currentIndex] == null) {
+      return node.updateWith(config: config);
+    }
+    config.value = indexedChildren[_currentIndex].label;
+    final SemanticsNode previousChild = indexedChildren[_currentIndex - 1];
+    final SemanticsNode nextChild = indexedChildren[_currentIndex + 1];
+    if (nextChild != null) {
+      config.increasedValue = nextChild.label;
+      config.onIncrease = _handleIncrease;
+    }
+    if (previousChild != null) {
+      config.decreasedValue = previousChild.label;
+      config.onDecrease = _handleDecrease;
+    }
+    node.updateWith(config: config);
+  }
+}

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

@@ -4032,7 +4032,6 @@ class RenderSemanticsAnnotations extends RenderProxyBox {
     super.visitChildrenForSemantics(visitor);
   }
 
-
   @override
   void describeSemanticsConfiguration(SemanticsConfiguration config) {
     super.describeSemanticsConfiguration(config);

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

@@ -10,6 +10,7 @@ import 'package:flutter/physics.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 
+import 'basic.dart';
 import 'framework.dart';
 import 'notification_listener.dart';
 import 'scroll_context.dart';
@@ -91,7 +92,7 @@ class ListWheelChildListDelegate extends ListWheelChildDelegate {
   Widget build(BuildContext context, int index) {
     if (index < 0 || index >= children.length)
       return null;
-    return children[index];
+    return IndexedSemantics(child: children[index], index: index);
   }
 
   @override
@@ -137,7 +138,7 @@ class ListWheelChildLoopingListDelegate extends ListWheelChildDelegate {
   Widget build(BuildContext context, int index) {
     if (children.isEmpty)
       return null;
-    return children[index % children.length];
+    return IndexedSemantics(child: children[index % children.length], index: index);
   }
 
   @override
@@ -178,11 +179,13 @@ class ListWheelChildBuilderDelegate extends ListWheelChildDelegate {
 
   @override
   Widget build(BuildContext context, int index) {
-    if (childCount == null)
-      return builder(context, index);
+    if (childCount == null) {
+      final Widget child = builder(context, index);
+      return child == null ? null : IndexedSemantics(child: child, index: index);
+    }
     if (index < 0 || index >= childCount)
       return null;
-    return builder(context, index);
+    return IndexedSemantics(child: builder(context, index), index: index);
   }
 
   @override

packages/flutter/test/cupertino/date_picker_test.dart

 // Copyright 2017 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';
+
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/semantics.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -581,4 +584,98 @@ void main() {
       expect(date, DateTime(2018, 1, 1, 15, 59));
     });
   });
-}
\ No newline at end of file
+
+  testWidgets('scrollController can be removed or added', (WidgetTester tester) async {
+    final SemanticsHandle handle = tester.ensureSemantics();
+    int lastSelectedItem;
+    void onSelectedItemChanged(int index) {
+      lastSelectedItem = index;
+    }
+    await tester.pumpWidget(_buildPicker(
+      controller: FixedExtentScrollController(),
+      onSelectedItemChanged: onSelectedItemChanged,
+    ));
+
+    tester.binding.pipelineOwner.semanticsOwner.performAction(1, SemanticsAction.increase);
+    await tester.pumpAndSettle();
+    expect(lastSelectedItem, 1);
+
+    await tester.pumpWidget(_buildPicker(
+      onSelectedItemChanged: onSelectedItemChanged,
+    ));
+
+    tester.binding.pipelineOwner.semanticsOwner.performAction(1, SemanticsAction.increase);
+    await tester.pumpAndSettle();
+    expect(lastSelectedItem, 2);
+
+    await tester.pumpWidget(_buildPicker(
+      controller: FixedExtentScrollController(),
+      onSelectedItemChanged: onSelectedItemChanged,
+    ));
+
+    tester.binding.pipelineOwner.semanticsOwner.performAction(1, SemanticsAction.increase);
+    await tester.pumpAndSettle();
+    expect(lastSelectedItem, 3);
+
+    handle.dispose();
+  });
+
+  testWidgets('picker exports semantics', (WidgetTester tester) async {
+    final SemanticsHandle handle = tester.ensureSemantics();
+    debugResetSemanticsIdCounter();
+    int lastSelectedItem;
+    await tester.pumpWidget(_buildPicker(onSelectedItemChanged: (int index) {
+      lastSelectedItem = index;
+    }));
+
+    expect(tester.getSemantics(find.byType(CupertinoPicker)), matchesSemantics(
+      children: <Matcher>[
+        matchesSemantics(
+          hasIncreaseAction: true,
+          hasDecreaseAction: false,
+          increasedValue: '1',
+          value: '0',
+          textDirection: TextDirection.ltr,
+        ),
+      ],
+    ));
+
+    tester.binding.pipelineOwner.semanticsOwner.performAction(1, SemanticsAction.increase);
+    await tester.pumpAndSettle();
+
+    expect(tester.getSemantics(find.byType(CupertinoPicker)), matchesSemantics(
+      children: <Matcher>[
+        matchesSemantics(
+          hasIncreaseAction: true,
+          hasDecreaseAction: true,
+          increasedValue: '2',
+          decreasedValue: '0',
+          value: '1',
+          textDirection: TextDirection.ltr,
+        ),
+      ],
+    ));
+    expect(lastSelectedItem, 1);
+    handle.dispose();
+  });
+}
+
+Widget _buildPicker({FixedExtentScrollController controller, ValueChanged<int> onSelectedItemChanged}) {
+  return Directionality(
+    textDirection: TextDirection.ltr,
+    child: CupertinoPicker(
+      scrollController: controller,
+      itemExtent: 100.0,
+      onSelectedItemChanged: onSelectedItemChanged,
+      children: List<Widget>.generate(100, (int index) {
+        return Center(
+          child: Container(
+            width: 400.0,
+            height: 100.0,
+            child: Text(index.toString()),
+          ),
+        );
+      }),
+    ),
+  );
+}

packages/flutter/test/widgets/list_wheel_scroll_view_test.dart

@@ -331,7 +331,7 @@ void main() {
         )
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Text)).parent;
+      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Text)).parent.parent;
 
       // Item 0 is in the middle. There are 3 children visible after it, so the
       // value of childCount should be 4.
@@ -515,7 +515,7 @@ void main() {
         ),
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent;
+      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent.parent;
       expect(viewport, paints..transform(
         matrix4: equals(<dynamic>[
           1.0, 0.0, 0.0, 0.0,
@@ -573,7 +573,7 @@ void main() {
         ),
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent;
+      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent.parent;
       expect(viewport, paints..transform(
         matrix4: equals(<dynamic>[
           1.0, 0.0, 0.0, 0.0,
@@ -694,7 +694,7 @@ void main() {
         ),
       );
 
-      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent;
+      final RenderListWheelViewport viewport = tester.firstRenderObject(find.byType(Container)).parent.parent;
       expect(viewport, paints
         ..transform(
           matrix4: equals(<dynamic>[