[web:a11y] add platform view role

lib/web_ui/lib/src/engine/platform_views/content_manager.dart

@@ -108,7 +108,7 @@ class PlatformViewManager {
   /// The resulting DOM for the `contents` of a Platform View looks like this:
   ///
   /// ```html
-  /// <flt-platform-view slot="...">
+  /// <flt-platform-view id="flt-platform-view-VIEW_ID" slot="...">
   ///   <arbitrary-html-elements />
   /// </flt-platform-view-slot>
   /// ```
@@ -134,6 +134,7 @@ class PlatformViewManager {
     return _contents.putIfAbsent(viewId, () {
       final DomElement wrapper = domDocument
           .createElement('flt-platform-view')
+            ..id = getPlatformViewDomId(viewId)
             ..setAttribute('slot', slotName);
 
       final Function factoryFunction = _factories[viewType]!;

lib/web_ui/lib/src/engine/platform_views/slots.dart

@@ -13,6 +13,12 @@ String getPlatformViewSlotName(int viewId) {
   return 'flt-pv-slot-$viewId';
 }
 
+/// Returns the value of the HTML "id" attribute set on the wrapper element that
+/// hosts the platform view content.
+String getPlatformViewDomId(int viewId) {
+  return 'flt-pv-$viewId';
+}
+
 /// Creates the HTML markup for the `slot` of a Platform View.
 ///
 /// The resulting DOM for a `slot` looks like this:

lib/web_ui/lib/src/engine/semantics/platform_view.dart

@@ -0,0 +1,42 @@
+// Copyright 2013 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 '../dom.dart';
+import '../platform_views/slots.dart';
+import 'semantics.dart';
+
+/// Manages the semantic element corresponding to a platform view.
+///
+/// The element in the semantics tree exists only to supply the ARIA traversal
+/// order. The actual content of the platform view is managed by
+/// [PlatformViewManager].
+///
+/// The traversal order is established using "aria-owns", by pointing to the
+/// element that hosts the view contents. As of this writing, Safari on macOS
+/// and on iOS does not support "aria-owns". All other browsers on all operating
+/// systems support it.
+///
+/// See also:
+///   * https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-owns
+///   * https://bugs.webkit.org/show_bug.cgi?id=223798
+class PlatformViewRoleManager extends PrimaryRoleManager {
+  PlatformViewRoleManager(SemanticsObject semanticsObject)
+      : super.withBasics(PrimaryRole.platformView, semanticsObject);
+
+  @override
+  void update() {
+    super.update();
+
+    if (semanticsObject.isPlatformView) {
+      if (semanticsObject.isPlatformViewIdDirty) {
+        semanticsObject.element.setAttribute(
+          'aria-owns',
+          getPlatformViewDomId(semanticsObject.platformViewId),
+        );
+      }
+    } else {
+      semanticsObject.element.removeAttribute('aria-owns');
+    }
+  }
+}

lib/web_ui/lib/src/engine/semantics/semantics.dart

@@ -25,6 +25,7 @@ import 'image.dart';
 import 'incrementable.dart';
 import 'label_and_value.dart';
 import 'live_region.dart';
+import 'platform_view.dart';
 import 'scrollable.dart';
 import 'semantics_helper.dart';
 import 'tappable.dart';
@@ -332,12 +333,6 @@ class SemanticsNodeUpdate {
 /// Each value corresponds to the most specific role a semantics node plays in
 /// the semantics tree.
 enum PrimaryRole {
-  /// A role used when a more specific role cannot be assigend to
-  /// a [SemanticsObject].
-  ///
-  /// Provides a label or a value.
-  generic,
-
   /// Supports incrementing and/or decrementing its value.
   incrementable,
 
@@ -378,6 +373,15 @@ enum PrimaryRole {
   ///   children. For example, a modal barrier has `scopesRoute` set but marking
   ///   it as a dialog would be wrong.
   dialog,
+
+  /// The node's primary role is to host a platform view.
+  platformView,
+
+  /// A role used when a more specific role cannot be assigend to
+  /// a [SemanticsObject].
+  ///
+  /// Provides a label or a value.
+  generic,
 }
 
 /// Identifies one of the secondary [RoleManager]s of a [PrimaryRoleManager].
@@ -964,6 +968,9 @@ class SemanticsObject {
 
   static const int _platformViewIdIndex = 1 << 23;
 
+  /// Whether the [platformViewId] field has been updated but has not been
+  /// applied to the DOM yet.
+  bool get isPlatformViewIdDirty => _isDirty(_platformViewIdIndex);
   void _markPlatformViewIdDirty() {
     _dirtyFields |= _platformViewIdIndex;
   }
@@ -1462,7 +1469,9 @@ class SemanticsObject {
 
   PrimaryRole _getPrimaryRoleIdentifier() {
     // The most specific role should take precedence.
-    if (isTextField) {
+    if (isPlatformView) {
+      return PrimaryRole.platformView;
+    } else if (isTextField) {
       return PrimaryRole.textField;
     } else if (isIncrementable) {
       return PrimaryRole.incrementable;
@@ -1490,6 +1499,7 @@ class SemanticsObject {
       PrimaryRole.checkable => Checkable(this),
       PrimaryRole.dialog => Dialog(this),
       PrimaryRole.image => ImageRoleManager(this),
+      PrimaryRole.platformView => PlatformViewRoleManager(this),
       PrimaryRole.generic => GenericRole(this),
     };
   }

lib/web_ui/test/engine/platform_views/content_manager_test.dart

@@ -109,7 +109,8 @@ void testMain() {
       test('rendered markup contains required attributes', () async {
         final DomElement content =
             contentManager.renderContent(viewType, viewId, null);
-        expect(content.getAttribute('slot'), contains('$viewId'));
+        expect(content.getAttribute('slot'), getPlatformViewSlotName(viewId));
+        expect(content.getAttribute('id'), getPlatformViewDomId(viewId));
 
         final DomElement userContent = content.querySelector('div')!;
         expect(userContent.style.height, '100%');

lib/web_ui/test/engine/platform_views/slots_test.dart

@@ -35,4 +35,12 @@ void testMain() {
       });
     });
   });
+
+  test('getPlatformViewSlotName', () {
+    expect(getPlatformViewSlotName(42), 'flt-pv-slot-42');
+  });
+
+  test('getPlatformViewDomId', () {
+    expect(getPlatformViewDomId(42), 'flt-pv-42');
+  });
 }

lib/web_ui/test/engine/semantics/semantics_test.dart

@@ -2277,6 +2277,38 @@ void _testLiveRegion() {
 }
 
 void _testPlatformView() {
+  test('sets and updates aria-owns', () async {
+    semantics()
+      ..debugOverrideTimestampFunction(() => _testTime)
+      ..semanticsEnabled = true;
+
+    // Set.
+    {
+      final ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
+      updateNode(
+        builder,
+        platformViewId: 5,
+        rect: const ui.Rect.fromLTRB(0, 0, 100, 50),
+      );
+      semantics().updateSemantics(builder.build());
+      expectSemanticsTree('<sem aria-owns="flt-pv-5" style="$rootSemanticStyle"></sem>');
+    }
+
+    // Update.
+    {
+      final ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
+      updateNode(
+        builder,
+        platformViewId: 42,
+        rect: const ui.Rect.fromLTRB(0, 0, 100, 50),
+      );
+      semantics().updateSemantics(builder.build());
+      expectSemanticsTree('<sem aria-owns="flt-pv-42" style="$rootSemanticStyle"></sem>');
+    }
+
+    semantics().semanticsEnabled = false;
+  });
+
   test('is transparent w.r.t. hit testing', () async {
     semantics()
       ..debugOverrideTimestampFunction(() => _testTime)
@@ -2290,7 +2322,7 @@ void _testPlatformView() {
     );
     semantics().updateSemantics(builder.build());
 
-    expectSemanticsTree('<sem style="$rootSemanticStyle"></sem>');
+    expectSemanticsTree('<sem aria-owns="flt-pv-5" style="$rootSemanticStyle"></sem>');
     final DomElement element = appHostNode.querySelector('flt-semantics')!;
     expect(element.style.pointerEvents, 'none');
 
@@ -2375,7 +2407,7 @@ void _testPlatformView() {
 <sem style="$rootSemanticStyle">
   <sem-c>
     <sem style="z-index: 3"></sem>
-    <sem style="z-index: 2"></sem>
+    <sem style="z-index: 2" aria-owns="flt-pv-0"></sem>
     <sem style="z-index: 1"></sem>
   </sem-c>
 </sem>''');

lib/web_ui/test/engine/semantics/semantics_tester.dart

@@ -301,7 +301,7 @@ class SemanticsTester {
       currentValueLength: currentValueLength ?? 0,
       textSelectionBase: textSelectionBase ?? 0,
       textSelectionExtent: textSelectionExtent ?? 0,
-      platformViewId: platformViewId ?? 0,
+      platformViewId: platformViewId ?? -1,
       scrollChildren: scrollChildren ?? 0,
       scrollIndex: scrollIndex ?? 0,
       scrollPosition: scrollPosition ?? 0,