Send AccessibilityEvent.TYPE_VIEW_FOCUSED when input focus is set. (f…

…lutter#12746)

When the focus changes, we should be sending a TYPE_VIEW_FOCUSED event. This enables that.

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

@@ -190,6 +190,14 @@ public class AccessibilityBridge extends AccessibilityNodeProvider {
     @Nullable
     private SemanticsNode inputFocusedSemanticsNode;
 
+    // Keeps track of the last semantics node that had the input focus.
+    //
+    // This is used to determine if the input focus has changed since the last time the
+    // {@code inputFocusSemanticsNode} has been set, so that we can send a {@code TYPE_VIEW_FOCUSED}
+    // event when it changes.
+    @Nullable
+    private SemanticsNode lastInputFocusedSemanticsNode;
+
     // The widget within Flutter that currently sits beneath a cursor, e.g,
     // beneath a stylus or mouse cursor.
     @Nullable
@@ -1377,6 +1385,20 @@ void updateSemantics(@NonNull ByteBuffer buffer, @NonNull String[] strings) {
                 event.getText().add(object.label);
                 sendAccessibilityEvent(event);
             }
+
+            // If the object is the input-focused node, then tell the reader about it, but only if
+            // it has changed since the last update.
+            if (inputFocusedSemanticsNode != null && inputFocusedSemanticsNode.id == object.id &&
+                    (lastInputFocusedSemanticsNode == null || lastInputFocusedSemanticsNode.id != inputFocusedSemanticsNode.id)) {
+                lastInputFocusedSemanticsNode = inputFocusedSemanticsNode;
+                sendAccessibilityEvent(obtainAccessibilityEvent(object.id, AccessibilityEvent.TYPE_VIEW_FOCUSED));
+            } else if (inputFocusedSemanticsNode == null) {
+                // There's no TYPE_VIEW_CLEAR_FOCUSED event, so if the current input focus becomes
+                // null, then we just set the last one to null too, so that it sends the event again
+                // when something regains focus.
+                lastInputFocusedSemanticsNode = null;
+            }
+
             if (inputFocusedSemanticsNode != null && inputFocusedSemanticsNode.id == object.id
                     && object.hadFlag(Flag.IS_TEXT_FIELD) && object.hasFlag(Flag.IS_TEXT_FIELD)
                     // If we have a TextField that has InputFocus, we should avoid announcing it if something