Mirror Android platform views a11y tree in the Flutter a11y tree. (fl…

…utter#8237)

This PR mirrors virtual a11y tree of embedded platform views in the Flutter
a11y tree.

Non virtual hierarchies are not currently supported.

Only works on Android versions earlier than Android P as it relies on
reflection access to hidden system APIs which cannot be done starting
Android P.

A11y is not yet working as we also need to delegate a11y events from the
platform view to the FlutterView. This will be done in a following PR to
keep the change size a little saner.

ci/licenses_golden/licenses_flutter

@@ -524,6 +524,7 @@ FILE: ../../../flutter/shell/platform/android/io/flutter/util/PathUtils.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/util/Preconditions.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/util/Predicate.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/view/AccessibilityBridge.java
+FILE: ../../../flutter/shell/platform/android/io/flutter/view/AccessibilityViewEmbedder.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/view/FlutterCallbackInformation.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/view/FlutterMain.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/view/FlutterNativeView.java

shell/platform/android/BUILD.gn

@@ -166,6 +166,7 @@ java_library("flutter_shell_java") {
     "io/flutter/util/Preconditions.java",
     "io/flutter/util/Predicate.java",
     "io/flutter/view/AccessibilityBridge.java",
+    "io/flutter/view/AccessibilityViewEmbedder.java",
     "io/flutter/view/FlutterCallbackInformation.java",
     "io/flutter/view/FlutterMain.java",
     "io/flutter/view/FlutterNativeView.java",

shell/platform/android/io/flutter/plugin/platform/PlatformViewsAccessibilityDelegate.java

@@ -4,14 +4,19 @@
 
 package io.flutter.plugin.platform;
 
+import android.view.View;
 import io.flutter.view.AccessibilityBridge;
 
 /**
  * Facilitates interaction between the accessibility bridge and embedded platform views.
  */
 public interface PlatformViewsAccessibilityDelegate {
-    // TODO(amirh): add a View getViewById(int id) here.
-    // not filing a tracking issue as this is going to be done in the next PR.
+
+    /**
+     * Returns the root of the view hierarchy for the platform view with the requested id, or null if there is no
+     * corresponding view.
+     */
+    View getPlatformViewById(Integer id);
 
     /**
      * Attaches an accessibility bridge for this platform views accessibility delegate.

shell/platform/android/io/flutter/plugin/platform/PlatformViewsController.java

@@ -120,9 +120,7 @@ public void onPreEngineRestart() {
         flushAllViews();
     }
 
-    /**
-     * Returns the embedded view with id, or null if no view with this id is registered.
-     */
+    @Override
     public View getPlatformViewById(Integer id) {
         VirtualDisplayController controller = vdControllers.get(id);
         if (controller == null) {

shell/platform/android/io/flutter/view/AccessibilityBridge.java

@@ -75,6 +75,16 @@ public class AccessibilityBridge extends AccessibilityNodeProvider {
     private static final float SCROLL_POSITION_CAP_FOR_INFINITY = 70000.0f;
     private static final int ROOT_NODE_ID = 0;
 
+    // The minimal ID for an engine generated AccessibilityNodeInfo.
+    //
+    // The AccessibilityNodeInfo node IDs are generated by the framework for most Flutter semantic nodes.
+    // When embedding platform views, the framework does not have the accessibility information for the embedded view;
+    // in this case the engine generates AccessibilityNodeInfo that mirrors the a11y information exposed by the platform
+    // view. To avoid the need of synchronizing the framework and engine mechanisms for generating the next ID, we split
+    // the 32bit range of virtual node IDs into 2. The least significant 16 bits are used for framework generated IDs
+    // and the most significant 16 bits are used for engine generated IDs.
+    private static final int MIN_ENGINE_GENERATED_NODE_ID = 1<<16;
+
     /// Value is derived from ACTION_TYPE_MASK in AccessibilityNodeInfo.java
     private static int FIRST_RESOURCE_ID = 267386881;
 
@@ -92,6 +102,9 @@ public class AccessibilityBridge extends AccessibilityNodeProvider {
     @NonNull
     private final AccessibilityManager accessibilityManager;
 
+    @NonNull
+    private final AccessibilityViewEmbedder accessibilityViewEmbedder;
+
     // The delegate for interacting with embedded platform views. Used to embed accessibility data for an embedded
     // view in the accessibility tree.
     @NonNull
@@ -363,6 +376,7 @@ public void onTouchExplorationStateChanged(boolean isTouchExplorationEnabled) {
         if (platformViewsAccessibilityDelegate != null) {
             platformViewsAccessibilityDelegate.attachAccessibilityBridge(this);
         }
+        accessibilityViewEmbedder = new AccessibilityViewEmbedder(rootAccessibilityView, MIN_ENGINE_GENERATED_NODE_ID);
     }
 
     /**
@@ -456,6 +470,11 @@ private boolean shouldSetCollectionInfo(final SemanticsNode semanticsNode) {
     @Override
     @SuppressWarnings("deprecation")
     public AccessibilityNodeInfo createAccessibilityNodeInfo(int virtualViewId) {
+        if (virtualViewId >= MIN_ENGINE_GENERATED_NODE_ID) {
+            // The node is in the engine generated range, and is provided by the accessibility view embedder.
+            return accessibilityViewEmbedder.createAccessibilityNodeInfo(virtualViewId);
+        }
+
         if (virtualViewId == View.NO_ID) {
             AccessibilityNodeInfo result = AccessibilityNodeInfo.obtain(rootAccessibilityView);
             rootAccessibilityView.onInitializeAccessibilityNodeInfo(result);
@@ -472,6 +491,13 @@ public AccessibilityNodeInfo createAccessibilityNodeInfo(int virtualViewId) {
             return null;
         }
 
+        if (semanticsNode.platformViewId != -1) {
+            // For platform views we delegate the node creation to the accessibility view embedder.
+            View embeddedView = platformViewsAccessibilityDelegate.getPlatformViewById(semanticsNode.platformViewId);
+            Rect bounds = semanticsNode.getGlobalRect();
+            return accessibilityViewEmbedder.getRootNode(embeddedView, semanticsNode.id, bounds);
+        }
+
         AccessibilityNodeInfo result = AccessibilityNodeInfo.obtain(rootAccessibilityView, virtualViewId);
         // Work around for https://github.com/flutter/flutter/issues/2101
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR2) {