Add shortcuts and actions for default focus traversal (#40186)

This adds the default shortcuts and actions for keyboard-based focus traversal of apps.

This list of shortcuts includes shortcuts for TAB, SHIFT TAB, RIGHT_ARROW, LEFT_ARROW, UP_ARROW, DOWN_ARROW, and the four DPAD keys for game controllers (because the DPAD produces arrow key events).

It doesn't yet include functionality for triggering a control (e.g. SPACE, ENTER, or controller buttons), because that involves restructuring some of the Flutter controls to trigger animations differently, and so will be done in another PR (#41220)

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

@@ -1199,11 +1199,21 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
 
     return Shortcuts(
       shortcuts: <LogicalKeySet, Intent>{
+        LogicalKeySet(LogicalKeyboardKey.tab): const Intent(NextFocusAction.key),
+        LogicalKeySet(LogicalKeyboardKey.shift, LogicalKeyboardKey.tab): const Intent(PreviousFocusAction.key),
+        LogicalKeySet(LogicalKeyboardKey.arrowLeft): const DirectionalFocusIntent(TraversalDirection.left),
+        LogicalKeySet(LogicalKeyboardKey.arrowRight): const DirectionalFocusIntent(TraversalDirection.right),
+        LogicalKeySet(LogicalKeyboardKey.arrowDown): const DirectionalFocusIntent(TraversalDirection.down),
+        LogicalKeySet(LogicalKeyboardKey.arrowUp): const DirectionalFocusIntent(TraversalDirection.up),
         LogicalKeySet(LogicalKeyboardKey.enter): const Intent(ActivateAction.key),
       },
       child: Actions(
         actions: <LocalKey, ActionFactory>{
           DoNothingAction.key: () => const DoNothingAction(),
+          RequestFocusAction.key: () => RequestFocusAction(),
+          NextFocusAction.key: () => NextFocusAction(),
+          PreviousFocusAction.key: () => PreviousFocusAction(),
+          DirectionalFocusAction.key: () => DirectionalFocusAction(),
         },
         child: DefaultFocusTraversal(
           policy: ReadingOrderTraversalPolicy(),

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

@@ -2,9 +2,12 @@
 // 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/foundation.dart';
 import 'package:flutter/painting.dart';
 
+import 'actions.dart';
 import 'basic.dart';
 import 'binding.dart';
 import 'focus_manager.dart';
@@ -790,3 +793,138 @@ class DefaultFocusTraversal extends InheritedWidget {
   @override
   bool updateShouldNotify(DefaultFocusTraversal oldWidget) => policy != oldWidget.policy;
 }
+
+// A base class for all of the default actions that request focus for a node.
+class _RequestFocusActionBase extends Action {
+  _RequestFocusActionBase(LocalKey name) : super(name);
+
+  FocusNode _previousFocus;
+
+  @override
+  void invoke(FocusNode node, Intent tag) {
+    _previousFocus = WidgetsBinding.instance.focusManager.primaryFocus;
+    node.requestFocus();
+  }
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(DiagnosticsProperty<FocusNode>('previous', _previousFocus));
+  }
+}
+
+/// An [Action] that requests the focus on the node it is invoked on.
+///
+/// This action can be used to request focus for a particular node, by calling
+/// [Action.invoke] like so:
+///
+/// ```dart
+/// Actions.invoke(context, const Intent(RequestFocusAction.key), focusNode: _focusNode);
+/// ```
+///
+/// Where the `_focusNode` is the node for which the focus will be requested.
+///
+/// The difference between requesting focus in this way versus calling
+/// [_focusNode.requestFocus] directly is that it will use the [Action]
+/// registered in the nearest [Actions] widget associated with [key] to make the
+/// request, rather than just requesting focus directly. This allows the action
+/// to have additional side effects, like logging, or undo and redo
+/// functionality.
+///
+/// However, this [RequestFocusAction] is the default action associated with the
+/// [key] in the [WidgetsApp], and it simply requests focus and has no side
+/// effects.
+class RequestFocusAction extends _RequestFocusActionBase {
+  /// Creates a [RequestFocusAction] with a fixed [key].
+  RequestFocusAction() : super(key);
+
+  /// The [LocalKey] that uniquely identifies this action to an [Intent].
+  static const LocalKey key = ValueKey<Type>(RequestFocusAction);
+
+  @override
+  void invoke(FocusNode node, Intent tag) {
+    super.invoke(node, tag);
+    node.requestFocus();
+  }
+}
+
+/// An [Action] that moves the focus to the next focusable node in the focus
+/// order.
+///
+/// This action is the default action registered for the [key], and by default
+/// is bound to the [LogicalKeyboardKey.tab] key in the [WidgetsApp].
+class NextFocusAction extends _RequestFocusActionBase {
+  /// Creates a [NextFocusAction] with a fixed [key];
+  NextFocusAction() : super(key);
+
+  /// The [LocalKey] that uniquely identifies this action to an [Intent].
+  static const LocalKey key = ValueKey<Type>(NextFocusAction);
+
+  @override
+  void invoke(FocusNode node, Intent tag) {
+    super.invoke(node, tag);
+    node.nextFocus();
+  }
+}
+
+/// An [Action] that moves the focus to the previous focusable node in the focus
+/// order.
+///
+/// This action is the default action registered for the [key], and by default
+/// is bound to a combination of the [LogicalKeyboardKey.tab] key and the
+/// [LogicalKeyboardKey.shift] key in the [WidgetsApp].
+class PreviousFocusAction extends _RequestFocusActionBase {
+  /// Creates a [PreviousFocusAction] with a fixed [key];
+  PreviousFocusAction() : super(key);
+
+  /// The [LocalKey] that uniquely identifies this action to an [Intent].
+  static const LocalKey key = ValueKey<Type>(PreviousFocusAction);
+
+  @override
+  void invoke(FocusNode node, Intent tag) {
+    super.invoke(node, tag);
+    node.previousFocus();
+  }
+}
+
+/// An [Intent] that represents moving to the next focusable node in the given
+/// [direction].
+///
+/// This is the [Intent] bound by default to the [LogicalKeyboardKey.arrowUp],
+/// [LogicalKeyboardKey.arrowDown], [LogicalKeyboardKey.arrowLeft], and
+/// [LogicalKeyboardKey.arrowRight] keys in the [WidgetsApp], with the
+/// appropriate associated directions.
+class DirectionalFocusIntent extends Intent {
+  /// Creates a [DirectionalFocusIntent] with a fixed [key], and the given
+  /// [direction].
+  const DirectionalFocusIntent(this.direction) : super(DirectionalFocusAction.key);
+
+  /// The direction in which to look for the next focusable node when the
+  /// associated [DirectionalFocusAction] is invoked.
+  final TraversalDirection direction;
+}
+
+/// An [Action] that moves the focus to the focusable node in the given
+/// [direction] configured by the associated [DirectionalFocusIntent].
+///
+/// This is the [Action] associated with the [key] and bound by default to the
+/// [LogicalKeyboardKey.arrowUp], [LogicalKeyboardKey.arrowDown],
+/// [LogicalKeyboardKey.arrowLeft], and [LogicalKeyboardKey.arrowRight] keys in
+/// the [WidgetsApp], with the appropriate associated directions.
+class DirectionalFocusAction extends _RequestFocusActionBase {
+  /// Creates a [DirectionalFocusAction] with a fixed [key];
+  DirectionalFocusAction() : super(key);
+
+  /// The [LocalKey] that uniquely identifies this action to [DirectionalFocusIntent].
+  static const LocalKey key = ValueKey<Type>(DirectionalFocusAction);
+
+  /// The direction in which to look for the next focusable node when invoked.
+  TraversalDirection direction;
+
+  @override
+  void invoke(FocusNode node, DirectionalFocusIntent tag) {
+    super.invoke(node, tag);
+    final DirectionalFocusIntent args = tag;
+    node.focusInDirection(args.direction);
+  }
+}

packages/flutter/test/widgets/focus_traversal_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/painting.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
@@ -914,5 +916,112 @@ void main() {
       expect(focusCenter.hasFocus, isFalse);
       expect(focusTop.hasFocus, isTrue);
     });
+    testWidgets('Focus traversal actions are invoked when shortcuts are used.', (WidgetTester tester) async {
+      final GlobalKey upperLeftKey = GlobalKey(debugLabel: 'upperLeftKey');
+      final GlobalKey upperRightKey = GlobalKey(debugLabel: 'upperRightKey');
+      final GlobalKey lowerLeftKey = GlobalKey(debugLabel: 'lowerLeftKey');
+      final GlobalKey lowerRightKey = GlobalKey(debugLabel: 'lowerRightKey');
+
+      await tester.pumpWidget(
+        WidgetsApp(
+          color: const Color(0xFFFFFFFF),
+          onGenerateRoute: (RouteSettings settings) {
+            return TestRoute(
+              child: Directionality(
+                textDirection: TextDirection.ltr,
+                child: FocusScope(
+                  debugLabel: 'scope',
+                  child: Column(
+                    children: <Widget>[
+                      Row(
+                        children: <Widget>[
+                          Focus(
+                            autofocus: true,
+                            debugLabel: 'upperLeft',
+                            child: Container(width: 100, height: 100, key: upperLeftKey),
+                          ),
+                          Focus(
+                            debugLabel: 'upperRight',
+                            child: Container(width: 100, height: 100, key: upperRightKey),
+                          ),
+                        ],
+                      ),
+                      Row(
+                        children: <Widget>[
+                          Focus(
+                            debugLabel: 'lowerLeft',
+                            child: Container(width: 100, height: 100, key: lowerLeftKey),
+                          ),
+                          Focus(
+                            debugLabel: 'lowerRight',
+                            child: Container(width: 100, height: 100, key: lowerRightKey),
+                          ),
+                        ],
+                      ),
+                    ],
+                  ),
+                ),
+              ),
+            );
+          },
+        ),
+      );
+
+      // Initial focus happens.
+      expect(Focus.of(upperLeftKey.currentContext).hasPrimaryFocus, isTrue);
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      expect(Focus.of(upperRightKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      expect(Focus.of(lowerLeftKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      expect(Focus.of(lowerRightKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      expect(Focus.of(upperLeftKey.currentContext).hasPrimaryFocus, isTrue);
+
+      await tester.sendKeyDownEvent(LogicalKeyboardKey.shift);
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      await tester.sendKeyUpEvent(LogicalKeyboardKey.shift);
+      expect(Focus.of(lowerRightKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyDownEvent(LogicalKeyboardKey.shift);
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      await tester.sendKeyUpEvent(LogicalKeyboardKey.shift);
+      expect(Focus.of(lowerLeftKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyDownEvent(LogicalKeyboardKey.shift);
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      await tester.sendKeyUpEvent(LogicalKeyboardKey.shift);
+      expect(Focus.of(upperRightKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyDownEvent(LogicalKeyboardKey.shift);
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      await tester.sendKeyUpEvent(LogicalKeyboardKey.shift);
+      expect(Focus.of(upperLeftKey.currentContext).hasPrimaryFocus, isTrue);
+
+      // Traverse in a direction
+      await tester.sendKeyEvent(LogicalKeyboardKey.arrowRight);
+      expect(Focus.of(upperRightKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyEvent(LogicalKeyboardKey.arrowDown);
+      expect(Focus.of(lowerRightKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyEvent(LogicalKeyboardKey.arrowLeft);
+      expect(Focus.of(lowerLeftKey.currentContext).hasPrimaryFocus, isTrue);
+      // Initial focus happens.
+      await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
+      expect(Focus.of(upperLeftKey.currentContext).hasPrimaryFocus, isTrue);
+    });
   });
 }
+
+class TestRoute extends PageRouteBuilder<void> {
+  TestRoute({Widget child})
+      : super(
+          pageBuilder: (BuildContext _, Animation<double> __, Animation<double> ___) {
+            return child;
+          },
+        );
+}