Optimize focus operations by caching descendants and ancestors. (#42683)

This optimizes certain paths in the FocusManager, FocusNode, and FocusScopeNode classes in order to fix a regression in stock_animation_open_first_frame_average when I added more focus nodes to the tree to do focus traversal.

Mainly I removed some remaining sync* iterators, and also started caching the computation of descendants and ancestors, since those are iterated over fairly often.

This improves stock_animation_open_first_frame_average by about 2.8% overall (so about half of a 4.9% regression, both averaged over 10 runs).

Addresses #42564

packages/flutter/lib/src/material/ink_well.dart

@@ -489,20 +489,20 @@ class _InkResponseState<T extends InkResponse> extends State<T> with AutomaticKe
 
   bool get highlightsExist => _highlights.values.where((InkHighlight highlight) => highlight != null).isNotEmpty;
 
+  Action _createAction() {
+    return CallbackAction(
+      ActivateAction.key,
+      onInvoke: (FocusNode node, Intent intent) {
+        _startSplash(context: node.context);
+        _handleTap(node.context);
+      },
+    );
+  }
+
   @override
   void initState() {
     super.initState();
-    _actionMap = <LocalKey, ActionFactory>{
-      ActivateAction.key: () {
-        return CallbackAction(
-          ActivateAction.key,
-          onInvoke: (FocusNode node, Intent intent) {
-            _startSplash(context: node.context);
-            _handleTap(node.context);
-          },
-        );
-      },
-    };
+    _actionMap = <LocalKey, ActionFactory>{ ActivateAction.key: _createAction };
     WidgetsBinding.instance.focusManager.addHighlightModeListener(_handleFocusHighlightModeChange);
   }
 

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

@@ -119,8 +119,8 @@ class FocusAttachment {
     assert(_node != null);
     if (isAttached) {
       assert(_node.context != null);
-      parent ??= Focus.of(_node.context, nullOk: true);
-      parent ??= FocusScope.of(_node.context);
+      parent ??= Focus.of(_node.context, nullOk: true, scopeOk: true);
+      parent ??= _node.context.owner.focusManager.rootScope;
       assert(parent != null);
       parent._reparent(_node);
     }
@@ -421,7 +421,10 @@ class FocusNode with DiagnosticableTreeMixin, ChangeNotifier {
   ///     its descendants.
   ///   - [FocusTraversalPolicy], a class that can be extended to describe a
   ///     traversal policy.
-  bool get canRequestFocus => _canRequestFocus && (enclosingScope == null || enclosingScope.canRequestFocus);
+  bool get canRequestFocus {
+    final FocusScopeNode scope = enclosingScope;
+    return _canRequestFocus && (scope == null || scope.canRequestFocus);
+  }
   bool _canRequestFocus;
   @mustCallSuper
   set canRequestFocus(bool value) {
@@ -450,6 +453,8 @@ class FocusNode with DiagnosticableTreeMixin, ChangeNotifier {
   FocusOnKeyCallback _onKey;
 
   FocusManager _manager;
+  List<FocusNode> _ancestors;
+  List<FocusNode> _descendants;
   bool _hasKeyboardToken = false;
 
   /// Returns the parent node for this object.
@@ -471,7 +476,7 @@ class FocusNode with DiagnosticableTreeMixin, ChangeNotifier {
       return const <FocusNode>[];
     }
     return children.where(
-      (FocusNode node) => !node.skipTraversal && node._canRequestFocus,
+      (FocusNode node) => !node.skipTraversal && node.canRequestFocus,
     );
   }
 
@@ -492,11 +497,16 @@ class FocusNode with DiagnosticableTreeMixin, ChangeNotifier {
 
   /// An [Iterable] over the hierarchy of children below this one, in
   /// depth-first order.
-  Iterable<FocusNode> get descendants sync* {
-    for (FocusNode child in _children) {
-      yield* child.descendants;
-      yield child;
+  Iterable<FocusNode> get descendants {
+    if (_descendants == null) {
+      final List<FocusNode> result = <FocusNode>[];
+      for (FocusNode child in _children) {
+        result.addAll(child.descendants);
+        result.add(child);
+      }
+      _descendants = result;
     }
+    return _descendants;
   }
 
   /// Returns all descendants which do not have the [skipTraversal] flag set.
@@ -507,12 +517,17 @@ class FocusNode with DiagnosticableTreeMixin, ChangeNotifier {
   /// Iterates the ancestors of this node starting at the parent and iterating
   /// over successively more remote ancestors of this node, ending at the root
   /// [FocusScope] ([FocusManager.rootScope]).
-  Iterable<FocusNode> get ancestors sync* {
-    FocusNode parent = _parent;
-    while (parent != null) {
-      yield parent;
-      parent = parent._parent;
+  Iterable<FocusNode> get ancestors {
+    if (_ancestors == null) {
+      final List<FocusNode> result = <FocusNode>[];
+      FocusNode parent = _parent;
+      while (parent != null) {
+        result.add(parent);
+        parent = parent._parent;
+      }
+      _ancestors = result;
     }
+    return _ancestors;
   }
 
   /// Whether this node has input focus.
@@ -710,6 +725,10 @@ class FocusNode with DiagnosticableTreeMixin, ChangeNotifier {
 
     node._parent = null;
     _children.remove(node);
+    for (FocusNode ancestor in ancestors) {
+      ancestor._descendants = null;
+    }
+    _descendants = null;
     assert(_manager == null || !_manager.rootScope.descendants.contains(node));
   }
 
@@ -717,6 +736,7 @@ class FocusNode with DiagnosticableTreeMixin, ChangeNotifier {
     _manager = manager;
     for (FocusNode descendant in descendants) {
       descendant._manager = manager;
+      descendant._ancestors = null;
     }
   }
 
@@ -737,7 +757,11 @@ class FocusNode with DiagnosticableTreeMixin, ChangeNotifier {
     child._parent?._removeChild(child, removeScopeFocus: oldScope != nearestScope);
     _children.add(child);
     child._parent = this;
+    child._ancestors = null;
     child._updateManager(_manager);
+    for (FocusNode ancestor in child.ancestors) {
+      ancestor._descendants = null;
+    }
     if (hadFocus) {
       // Update the focus chain for the current focus without changing it.
       _manager?.primaryFocus?._setAsFocusedChild();
@@ -1266,15 +1290,8 @@ class FocusManager with DiagnosticableTreeMixin {
       assert(_focusDebug('No primary focus for key event, ignored: $event'));
       return;
     }
-    Iterable<FocusNode> allNodes(FocusNode node) sync* {
-      yield node;
-      for (FocusNode ancestor in node.ancestors) {
-        yield ancestor;
-      }
-    }
-
     bool handled = false;
-    for (FocusNode node in allNodes(_primaryFocus)) {
+    for (FocusNode node in <FocusNode>[_primaryFocus, ..._primaryFocus.ancestors]) {
       if (node.onKey != null && node.onKey(node, event)) {
         assert(_focusDebug('Node $node handled key event $event.'));
         handled = true;
@@ -1338,7 +1355,7 @@ class FocusManager with DiagnosticableTreeMixin {
     final FocusNode previousFocus = _primaryFocus;
     if (_primaryFocus == null && _nextFocus == null) {
       // If we don't have any current focus, and nobody has asked to focus yet,
-      // then pick a first one using widget order as a default.
+      // then revert to the root scope.
       _nextFocus = rootScope;
     }
     if (_nextFocus != null && _nextFocus != _primaryFocus) {

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

@@ -262,33 +262,34 @@ class Focus extends StatefulWidget {
   /// [nullOk].
   ///
   /// The [context] and [nullOk] arguments must not be null.
-  static FocusNode of(BuildContext context, { bool nullOk = false }) {
+  static FocusNode of(BuildContext context, { bool nullOk = false, bool scopeOk = false }) {
     assert(context != null);
     assert(nullOk != null);
+    assert(scopeOk != null);
     final _FocusMarker marker = context.inheritFromWidgetOfExactType(_FocusMarker);
     final FocusNode node = marker?.notifier;
-    if (node is FocusScopeNode) {
+    if (node == null) {
       if (!nullOk) {
         throw FlutterError(
-            'Focus.of() was called with a context that does not contain a Focus between the given '
-            'context and the nearest FocusScope widget.\n'
-            'No Focus ancestor could be found starting from the context that was passed to '
-            'Focus.of() to the point where it found the nearest FocusScope widget. This can happen '
-            'because you are using a widget that looks for a Focus ancestor, and do not have a '
-            'Focus widget ancestor in the current FocusScope.\n'
-            'The context used was:\n'
-            '  $context'
+            'Focus.of() was called with a context that does not contain a Focus widget.\n'
+                'No Focus widget ancestor could be found starting from the context that was passed to '
+                'Focus.of(). This can happen because you are using a widget that looks for a Focus '
+                'ancestor, and do not have a Focus widget descendant in the nearest FocusScope.\n'
+                'The context used was:\n'
+                '  $context'
         );
       }
       return null;
     }
-    if (node == null) {
+    if (!scopeOk && node is FocusScopeNode) {
       if (!nullOk) {
         throw FlutterError(
-            'Focus.of() was called with a context that does not contain a Focus widget.\n'
-            'No Focus widget ancestor could be found starting from the context that was passed to '
-            'Focus.of(). This can happen because you are using a widget that looks for a Focus '
-            'ancestor, and do not have a Focus widget descendant in the nearest FocusScope.\n'
+            'Focus.of() was called with a context that does not contain a Focus between the given '
+            'context and the nearest FocusScope widget.\n'
+            'No Focus ancestor could be found starting from the context that was passed to '
+            'Focus.of() to the point where it found the nearest FocusScope widget. This can happen '
+            'because you are using a widget that looks for a Focus ancestor, and do not have a '
+            'Focus widget ancestor in the current FocusScope.\n'
             'The context used was:\n'
             '  $context'
         );

packages/flutter/test/widgets/focus_manager_test.dart

@@ -300,6 +300,41 @@ void main() {
       expect(scope1.focusedChild, isNull);
       expect(parent2.children.contains(child1), isTrue);
     });
+    testWidgets('ancestors and descendants are computed and recomputed properly', (WidgetTester tester) async {
+      final BuildContext context = await setupWidget(tester);
+      final FocusScopeNode scope1 = FocusScopeNode(debugLabel: 'scope1');
+      final FocusAttachment scope1Attachment = scope1.attach(context);
+      final FocusScopeNode scope2 = FocusScopeNode(debugLabel: 'scope2');
+      final FocusAttachment scope2Attachment = scope2.attach(context);
+      final FocusNode parent1 = FocusNode(debugLabel: 'parent1');
+      final FocusAttachment parent1Attachment = parent1.attach(context);
+      final FocusNode parent2 = FocusNode(debugLabel: 'parent2');
+      final FocusAttachment parent2Attachment = parent2.attach(context);
+      final FocusNode child1 = FocusNode(debugLabel: 'child1');
+      final FocusAttachment child1Attachment = child1.attach(context);
+      final FocusNode child2 = FocusNode(debugLabel: 'child2');
+      final FocusAttachment child2Attachment = child2.attach(context);
+      final FocusNode child3 = FocusNode(debugLabel: 'child3');
+      final FocusAttachment child3Attachment = child3.attach(context);
+      final FocusNode child4 = FocusNode(debugLabel: 'child4');
+      final FocusAttachment child4Attachment = child4.attach(context);
+      scope1Attachment.reparent(parent: tester.binding.focusManager.rootScope);
+      scope2Attachment.reparent(parent: tester.binding.focusManager.rootScope);
+      parent1Attachment.reparent(parent: scope1);
+      parent2Attachment.reparent(parent: scope2);
+      child1Attachment.reparent(parent: parent1);
+      child2Attachment.reparent(parent: parent1);
+      child3Attachment.reparent(parent: parent2);
+      child4Attachment.reparent(parent: parent2);
+      child4.requestFocus();
+      await tester.pump();
+      expect(child4.ancestors, equals(<FocusNode>[parent2, scope2, tester.binding.focusManager.rootScope]));
+      expect(tester.binding.focusManager.rootScope.descendants, equals(<FocusNode>[child1, child2, parent1, scope1, child3, child4, parent2, scope2]));
+      scope2Attachment.reparent(parent: child2);
+      await tester.pump();
+      expect(child4.ancestors, equals(<FocusNode>[parent2, scope2, child2, parent1, scope1, tester.binding.focusManager.rootScope]));
+      expect(tester.binding.focusManager.rootScope.descendants, equals(<FocusNode>[child1, child3, child4, parent2, scope2, child2, parent1, scope1]));
+    });
     testWidgets('Can move focus between scopes and keep focus', (WidgetTester tester) async {
       final BuildContext context = await setupWidget(tester);
       final FocusScopeNode scope1 = FocusScopeNode();

packages/flutter/test/widgets/focus_scope_test.dart

@@ -504,6 +504,7 @@ void main() {
 
       FocusScope.of(keyA.currentContext).requestFocus(keyA.currentState.focusNode);
       expect(FocusScope.of(keyA.currentContext), equals(childFocusScope));
+      expect(Focus.of(keyA.currentContext, scopeOk: true), equals(childFocusScope));
       WidgetsBinding.instance.focusManager.rootScope.setFirstFocus(FocusScope.of(keyA.currentContext));
 
       await tester.pumpAndSettle();