Adds a11y action for selecting text (and moving cursor) (#4589)

See https://github.com/flutter/flutter/pull/14275 for framework side change.

Also includes some minor clean-ups for consistency. 

Required for https://github.com/flutter/flutter/issues/13469.

lib/ui/semantics.dart

@@ -17,9 +17,10 @@ class SemanticsAction {
   static const int _kScrollDownIndex = 1 << 5;
   static const int _kIncreaseIndex = 1 << 6;
   static const int _kDecreaseIndex = 1 << 7;
-  static const int _kShowOnScreen = 1 << 8;
-  static const int _kMoveCursorForwardByCharacter = 1 << 9;
-  static const int _kMoveCursorBackwardByCharacter = 1 << 10;
+  static const int _kShowOnScreenIndex = 1 << 8;
+  static const int _kMoveCursorForwardByCharacterIndex = 1 << 9;
+  static const int _kMoveCursorBackwardByCharacterIndex = 1 << 10;
+  static const int _kSetSelectionIndex = 1 << 11;
 
   /// The numerical value for this action.
   ///
@@ -76,18 +77,35 @@ class SemanticsAction {
   ///
   /// For example, this action might be send to a node in a scrollable list that
   /// is partially off screen to bring it on screen.
-  static const SemanticsAction showOnScreen = const SemanticsAction._(_kShowOnScreen);
+  static const SemanticsAction showOnScreen = const SemanticsAction._(_kShowOnScreenIndex);
 
 
   /// Move the cursor forward by one character.
   ///
   /// This is for example used by the cursor control in text fields.
-  static const SemanticsAction moveCursorForwardByCharacter = const SemanticsAction._(_kMoveCursorForwardByCharacter);
+  ///
+  /// The action includes a boolean argument, which indicates whether the cursor
+  /// movement should extend (or start) a selection.
+  static const SemanticsAction moveCursorForwardByCharacter = const SemanticsAction._(_kMoveCursorForwardByCharacterIndex);
 
   /// Move the cursor backward by one character.
   ///
   /// This is for example used by the cursor control in text fields.
-  static const SemanticsAction moveCursorBackwardByCharacter = const SemanticsAction._(_kMoveCursorBackwardByCharacter);
+  ///
+  /// The action includes a boolean argument, which indicates whether the cursor
+  /// movement should extend (or start) a selection.
+  static const SemanticsAction moveCursorBackwardByCharacter = const SemanticsAction._(_kMoveCursorBackwardByCharacterIndex);
+
+  /// Set the text selection to the given range.
+  ///
+  /// The provided argument is a Map<String, int> which includes the keys `base`
+  /// and `extent` indicating where the selection within the `value` of the
+  /// semantics node should start and where it should end. Values for both
+  /// keys can range from 0 to length of `value` (inclusive).
+  ///
+  /// Setting `base` and `extent` to the same value will move the cursor to
+  /// that position (without selecting anything).
+  static const SemanticsAction setSelection = const SemanticsAction._(_kSetSelectionIndex);
 
   /// The possible semantics actions.
   ///
@@ -102,9 +120,10 @@ class SemanticsAction {
     _kScrollDownIndex: scrollDown,
     _kIncreaseIndex: increase,
     _kDecreaseIndex: decrease,
-    _kShowOnScreen: showOnScreen,
-    _kMoveCursorForwardByCharacter: moveCursorForwardByCharacter,
-    _kMoveCursorBackwardByCharacter: moveCursorBackwardByCharacter,
+    _kShowOnScreenIndex: showOnScreen,
+    _kMoveCursorForwardByCharacterIndex: moveCursorForwardByCharacter,
+    _kMoveCursorBackwardByCharacterIndex: moveCursorBackwardByCharacter,
+    _kSetSelectionIndex: setSelection,
   };
 
   @override
@@ -126,12 +145,14 @@ class SemanticsAction {
         return 'SemanticsAction.increase';
       case _kDecreaseIndex:
         return 'SemanticsAction.decrease';
-      case _kShowOnScreen:
+      case _kShowOnScreenIndex:
         return 'SemanticsAction.showOnScreen';
-      case _kMoveCursorForwardByCharacter:
+      case _kMoveCursorForwardByCharacterIndex:
         return 'SemanticsAction.moveCursorForwardByCharacter';
-      case _kMoveCursorBackwardByCharacter:
+      case _kMoveCursorBackwardByCharacterIndex:
         return 'SemanticsAction.moveCursorBackwardByCharacter';
+      case _kSetSelectionIndex:
+        return 'SemanticsAction.setSelection';
     }
     return null;
   }

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

@@ -31,6 +31,11 @@ import java.util.Set;
 class AccessibilityBridge extends AccessibilityNodeProvider implements BasicMessageChannel.MessageHandler<Object> {
     private static final String TAG = "FlutterView";
 
+    // Constants from higher API levels.
+    // TODO(goderbauer): Get these from Android Support Library when
+    // https://github.com/flutter/flutter/issues/11099 is resolved.
+    public static final int ACTION_SHOW_ON_SCREEN = 16908342; // API level 23
+
     private Map<Integer, SemanticsObject> mObjects;
     private final FlutterView mOwner;
     private boolean mAccessibilityEnabled = false;
@@ -51,7 +56,8 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements BasicMess
         DECREASE(1 << 7),
         SHOW_ON_SCREEN(1 << 8),
         MOVE_CURSOR_FORWARD_BY_CHARACTER(1 << 9),
-        MOVE_CURSOR_BACKWARD_BY_CHARACTER(1 << 10);
+        MOVE_CURSOR_BACKWARD_BY_CHARACTER(1 << 10),
+        SET_SELECTION(1 << 11);
 
         Action(int value) {
             this.value = value;
@@ -142,6 +148,9 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements BasicMess
             }
             result.setMovementGranularities(granularities);
         }
+        if (object.hasAction(Action.SET_SELECTION)) {
+            result.addAction(AccessibilityNodeInfo.ACTION_SET_SELECTION);
+        }
 
         if (object.hasFlag(Flag.IS_BUTTON)) {
           result.setClassName("android.widget.Button");
@@ -317,12 +326,30 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements BasicMess
 
                 return true;
             }
-            // TODO(goderbauer): Use ACTION_SHOW_ON_SCREEN from Android Support Library after
-            //     https://github.com/flutter/flutter/issues/11099 is resolved.
-            case 16908342: { // ACTION_SHOW_ON_SCREEN, added in API level 23
+            case ACTION_SHOW_ON_SCREEN: {
                 mOwner.dispatchSemanticsAction(virtualViewId, Action.SHOW_ON_SCREEN);
                 return true;
             }
+            case AccessibilityNodeInfo.ACTION_SET_SELECTION: {
+                final Map<String, Integer> selection = new HashMap<String, Integer>();
+                final boolean hasSelection = arguments != null
+                    && arguments.containsKey(
+                            AccessibilityNodeInfo.ACTION_ARGUMENT_SELECTION_START_INT)
+                    && arguments.containsKey(
+                            AccessibilityNodeInfo.ACTION_ARGUMENT_SELECTION_END_INT);
+                if (hasSelection) {
+                    selection.put("base", arguments.getInt(
+                        AccessibilityNodeInfo.ACTION_ARGUMENT_SELECTION_START_INT));
+                    selection.put("extent", arguments.getInt(
+                        AccessibilityNodeInfo.ACTION_ARGUMENT_SELECTION_END_INT));
+                } else {
+                    // Clear the selection
+                    selection.put("base", object.textSelectionExtent);
+                    selection.put("extent", object.textSelectionExtent);
+                }
+                mOwner.dispatchSemanticsAction(virtualViewId, Action.SET_SELECTION, selection);
+                return true;
+            }
         }
         return false;
     }
@@ -467,7 +494,7 @@ class AccessibilityBridge extends AccessibilityNodeProvider implements BasicMess
                         object.id, AccessibilityEvent.TYPE_VIEW_TEXT_SELECTION_CHANGED);
                     selectionEvent.getText().add(newValue);
                     selectionEvent.setFromIndex(object.textSelectionBase);
-                    selectionEvent.setToIndex(object.previousTextSelectionExtent);
+                    selectionEvent.setToIndex(object.textSelectionExtent);
                     selectionEvent.setItemCount(newValue.length());
                     sendAccessibilityEvent(selectionEvent);
                 }