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const vs. non-const widget build benchmark (flutter#148261)
This benchmark illustrates the impact that const'ness has on widget build times by building a widget tree with all const-widgets and then building the same widget tree without any const'ness. Local testing with this benchmark running on a Moto G4 shows that const'ness is 13-16% faster. I'd like to check this benchmark in because the question of how important widget const'ness is comes up every so often. With this benchmark we have up-to-date data to point people to.
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‎dev/benchmarks/microbenchmarks/lib/building/const_vs_non_const_bench.dart‎

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// Copyright 2014 The Flutter Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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import 'package:flutter/material.dart';
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import 'package:flutter_test/flutter_test.dart';
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import '../common.dart';
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const Duration kBenchmarkTime = Duration(seconds: 15);
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Future<List<double>> runBuildBenchmark(ValueGetter<Widget> buildApp) async {
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assert(false, "Don't run benchmarks in debug mode! Use 'flutter run --release'.");
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// We control the framePolicy below to prevent us from scheduling frames in
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// the engine, so that the engine does not interfere with our timings.
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final LiveTestWidgetsFlutterBinding binding = TestWidgetsFlutterBinding.ensureInitialized() as LiveTestWidgetsFlutterBinding;
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final Stopwatch watch = Stopwatch();
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int iterations = 0;
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final List<double> values = <double>[];
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await benchmarkWidgets((WidgetTester tester) async {
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await tester.pumpWidget(buildApp());
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await tester.pump();
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await tester.pumpAndSettle(const Duration(seconds: 1));
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// Cannot use expects/asserts here since this is running outside of a test
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// in release mode.
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final int numberOfSizedBoxes = find.byType(SizedBox).evaluate().length;
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if (numberOfSizedBoxes != 30) {
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throw StateError('Expected 30 SizedBox widgets, but only found $numberOfSizedBoxes.');
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}
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if (find.text('testToken').evaluate().length != 1) {
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throw StateError('Did not find expected leaf widget.');
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}
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final Element rootWidget = tester.element(find.byKey(rootKey));
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Duration elapsed = Duration.zero;
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final LiveTestWidgetsFlutterBindingFramePolicy defaultPolicy = binding.framePolicy;
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binding.framePolicy = LiveTestWidgetsFlutterBindingFramePolicy.benchmark;
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while (elapsed < kBenchmarkTime) {
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watch.reset();
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watch.start();
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rootWidget.markNeedsBuild();
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await tester.pumpBenchmark(Duration(milliseconds: iterations * 16));
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watch.stop();
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iterations += 1;
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elapsed += Duration(microseconds: watch.elapsedMicroseconds);
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values.add(watch.elapsedMicroseconds.toDouble());
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}
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binding.framePolicy = defaultPolicy;
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});
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return values;
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}
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double calculateMean(List<double> values) {
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return values.reduce((double x, double y) => x + y) / values.length;
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}
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Future<void> main() async {
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final BenchmarkResultPrinter printer = BenchmarkResultPrinter();
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final double constMean = calculateMean(await runBuildBenchmark(() => ConstApp(key: rootKey)));
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final double nonConstMean = calculateMean(await runBuildBenchmark(() => NonConstApp(key: rootKey)));
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printer.addResult(
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description: 'const app build',
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value: constMean,
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unit: 'µs per iteration',
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name: 'const_app_build_iteration',
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);
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printer.addResult(
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description: 'non-const app build',
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value: nonConstMean,
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unit: 'µs per iteration',
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name: 'non_const_app_build_iteration',
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);
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printer.addResult(
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description: 'const speed-up (vs. non-const)',
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value: ((nonConstMean - constMean) / constMean) * 100,
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unit: '%',
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name: 'const_speed_up',
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);
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printer.printToStdout();
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}
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final Key rootKey = UniqueKey();
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class ConstApp extends StatelessWidget {
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const ConstApp({super.key});
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@override
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Widget build(BuildContext context) {
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return const SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: SizedBox(
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child: Text('testToken', textDirection: TextDirection.ltr),
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),
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),
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),
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);
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}
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}
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class NonConstApp extends StatelessWidget {
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const NonConstApp({super.key});
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@override
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Widget build(BuildContext context) {
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// The explicit goal is to test the performance of non-const widgets,
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// hence all these ignores.
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return SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: SizedBox( // ignore: prefer_const_constructors
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child: Text('testToken', textDirection: TextDirection.ltr), // ignore: prefer_const_constructors
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),
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);
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}
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}

‎dev/devicelab/lib/tasks/microbenchmarks.dart‎

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...await runMicrobench('lib/stocks/layout_bench.dart'),
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...await runMicrobench('lib/ui/image_bench.dart'),
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...await runMicrobench('lib/layout/text_intrinsic_bench.dart'),
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...await runMicrobench('lib/building/const_vs_non_const_bench.dart'),
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};
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return TaskResult.success(allResults,

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