Improve performance of collectAllElements (#68065)

dev/benchmarks/microbenchmarks/lib/foundation/all_elements_bench.dart

@@ -0,0 +1,77 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/scheduler.dart';
+// ignore: implementation_imports
+import 'package:flutter_test/src/all_elements.dart';
+
+import '../common.dart';
+
+const int _kNumIters = 10000;
+
+Future<void> main() async {
+  assert(false, "Don't run benchmarks in debug mode! Use 'flutter run --release'.");
+  runApp(MaterialApp(
+    home: Scaffold(
+      body: GridView.count(
+        crossAxisCount: 5,
+        children: List<Widget>.generate(100, (int index) {
+          return Center(
+            child: Scaffold(
+              appBar: AppBar(
+                title: Text('App $index'),
+                actions: const <Widget>[
+                  Icon(Icons.help),
+                  Icon(Icons.add),
+                  Icon(Icons.ac_unit),
+                ],
+              ),
+              body: Column(
+                children: const <Widget>[
+                  Text('Item 1'),
+                  Text('Item 2'),
+                  Text('Item 3'),
+                  Text('Item 4'),
+                ],
+              ),
+            ),
+          );
+        }),
+      ),
+    ),
+  ));
+
+  await SchedulerBinding.instance.endOfFrame;
+
+  final Stopwatch watch = Stopwatch();
+
+  print('flutter_test allElements benchmark... (${WidgetsBinding.instance.renderViewElement})');
+  // Make sure we get enough elements to process for consistent benchmark runs
+  int elementCount = collectAllElementsFrom(WidgetsBinding.instance.renderViewElement, skipOffstage: false).length;
+  while (elementCount < 6242) {
+    await Future<void>.delayed(Duration.zero);
+    elementCount = collectAllElementsFrom(WidgetsBinding.instance.renderViewElement, skipOffstage: false).length;
+  }
+  print('element count: $elementCount');
+
+  watch.start();
+  for (int i = 0; i < _kNumIters; i += 1) {
+    final List<Element> allElements = collectAllElementsFrom(
+      WidgetsBinding.instance.renderViewElement,
+      skipOffstage: false,
+    ).toList();
+    allElements.clear();
+  }
+  watch.stop();
+
+  final BenchmarkResultPrinter printer = BenchmarkResultPrinter();
+  printer.addResult(
+    description: 'All elements iterate',
+    value: watch.elapsedMicroseconds / _kNumIters,
+    unit: 'µs per iteration',
+    name: 'all_elements_iteration',
+  );
+  printer.printToStdout();
+}

dev/devicelab/lib/tasks/microbenchmarks.dart

@@ -55,6 +55,7 @@ TaskFunction createMicrobenchmarkTask() {
       ...await _runMicrobench('lib/stocks/animation_bench.dart'),
       ...await _runMicrobench('lib/language/sync_star_bench.dart'),
       ...await _runMicrobench('lib/language/sync_star_semantics_bench.dart'),
+      ...await _runMicrobench('lib/foundation/all_elements_bench.dart'),
       ...await _runMicrobench('lib/foundation/change_notifier_bench.dart'),
  };
 

packages/flutter_test/lib/src/all_elements.dart

@@ -23,15 +23,39 @@ Iterable<Element> collectAllElementsFrom(
   return CachingIterable<Element>(_DepthFirstChildIterator(rootElement, skipOffstage));
 }
 
+/// Provides a recursive, efficient, depth first search of an element tree.
+///
+/// [Element.visitChildren] does not guarnatee order, but does guarnatee stable
+/// order. This iterator also guarantees stable order, and iterates in a left
+/// to right order:
+///
+///       1
+///     /   \
+///    2     3
+///   / \   / \
+///  4   5 6   7
+///
+/// Will iterate in order 2, 4, 5, 3, 6, 7.
+///
+/// Performance is important here because this method is on the critical path
+/// for flutter_driver and package:integration_test performance tests.
+/// Performance is measured in the all_elements_bench microbenchmark.
+/// Any changes to this implementation should check the before and after numbers
+/// on that benchmark to avoid regressions in general performance test overhead.
+///
+/// If we could use RTL order, we could save on performance, but numerous tests
+/// have been written (and developers clearly expect) that LTR order will be
+/// respected.
 class _DepthFirstChildIterator implements Iterator<Element> {
-  _DepthFirstChildIterator(Element rootElement, this.skipOffstage)
+  _DepthFirstChildIterator(Element rootElement, this.skipOffstage) {
-    : _stack = _reverseChildrenOf(rootElement, skipOffstage).toList();
+    _fillChildren(rootElement);
+  }
 
   final bool skipOffstage;
 
   late Element _current;
 
-  final List<Element> _stack;
+  final List<Element> _stack = <Element>[];
 
   @override
   Element get current => _current;
@@ -42,20 +66,26 @@ class _DepthFirstChildIterator implements Iterator<Element> {
       return false;
 
     _current = _stack.removeLast();
-    // Stack children in reverse order to traverse first branch first
+    _fillChildren(_current);
-    _stack.addAll(_reverseChildrenOf(_current, skipOffstage));
 
     return true;
   }
 
-  static Iterable<Element> _reverseChildrenOf(Element element, bool skipOffstage) {
+  void _fillChildren(Element element) {
     assert(element != null);
-    final List<Element> children = <Element>[];
+    // If we did not have to follow LTR order and could instead use RTL,
+    // we could avoid reversing this and the operation would be measurably
+    // faster. Unfortunately, a lot of tests depend on LTR order.
+    final List<Element> reversed = <Element>[];
     if (skipOffstage) {
-      element.debugVisitOnstageChildren(children.add);
+      element.debugVisitOnstageChildren(reversed.add);
     } else {
-      element.visitChildren(children.add);
+      element.visitChildren(reversed.add);
+    }
+    // This is faster than _stack.addAll(reversed.reversed), presumably since
+    // we don't actually care about maintaining an iteration pointer.
+    while (reversed.isNotEmpty) {
+      _stack.add(reversed.removeLast());
     }
-    return children.reversed;
   }
 }

packages/flutter_test/test/all_elements_test.dart

@@ -0,0 +1,34 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('collectAllElements goes in LTR DFS', (WidgetTester tester) async {
+    final GlobalKey key = GlobalKey();
+    await tester.pumpWidget(Directionality(
+      key: key,
+      textDirection: TextDirection.ltr,
+      child: Row(
+        children: <Widget>[
+          RichText(text: const TextSpan(text: 'a')),
+          RichText(text: const TextSpan(text: 'b')),
+        ],
+      ),
+    ));
+
+    final List<Element> elements = collectAllElementsFrom(
+      key.currentContext! as Element,
+      skipOffstage: false,
+    ).toList();
+
+    expect(elements.length, 3);
+    expect(elements[0].widget, isA<Row>());
+    expect(elements[1].widget, isA<RichText>());
+    expect(((elements[1].widget as RichText).text as TextSpan).text, 'a');
+    expect(elements[2].widget, isA<RichText>());
+    expect(((elements[2].widget as RichText).text as TextSpan).text, 'b');
+  });
+}