[web] Profile text layout and forward data to macrobenchmarks

ci/licenses_golden/licenses_flutter

@@ -448,6 +448,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/platform_views.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/plugins.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/pointer_binding.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/pointer_converter.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/profiler.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/render_vertices.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/rrect_renderer.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/semantics/accessibility.dart

lib/web_ui/lib/src/engine.dart

@@ -69,6 +69,7 @@ part 'engine/platform_views.dart';
 part 'engine/plugins.dart';
 part 'engine/pointer_binding.dart';
 part 'engine/pointer_converter.dart';
+part 'engine/profiler.dart';
 part 'engine/render_vertices.dart';
 part 'engine/rrect_renderer.dart';
 part 'engine/semantics/accessibility.dart';
@@ -164,6 +165,10 @@ void webOnlyInitializeEngine() {
 
   WebExperiments.ensureInitialized();
 
+  if (Profiler.isBenchmarkMode) {
+    Profiler.ensureInitialized();
+  }
+
   bool waitingForAnimation = false;
   ui.webOnlyScheduleFrameCallback = () {
     // We're asked to schedule a frame and call `frameHandler` when the frame

lib/web_ui/lib/src/engine/profiler.dart

@@ -0,0 +1,72 @@
+// Copyright 2013 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.
+
+// @dart = 2.6
+part of engine;
+
+typedef OnBenchmark = void Function(String name, num value);
+
+/// The purpose of this class is to facilitate communication of
+/// profiling/benchmark data to the outside world (e.g. a macrobenchmark that's
+/// running a flutter app).
+///
+/// To use the [Profiler]:
+///
+/// 1. Set the environment variable `FLUTTER_WEB_ENABLE_PROFILING` to true.
+///
+/// 2. Using JS interop, assign a listener function to
+///    `window._flutter_internal_on_benchmark` in the browser.
+///
+/// The listener function will be called every time a new benchmark number is
+/// calculated. The signature is `Function(String name, num value)`.
+class Profiler {
+  Profiler._() {
+    _checkBenchmarkMode();
+  }
+
+  static bool isBenchmarkMode = const bool.fromEnvironment(
+    'FLUTTER_WEB_ENABLE_PROFILING',
+    defaultValue: true,
+  );
+
+  static Profiler ensureInitialized() {
+    _checkBenchmarkMode();
+    return Profiler._instance ??= Profiler._();
+  }
+
+  static Profiler get instance {
+    _checkBenchmarkMode();
+    if (_instance == null) {
+      throw Exception(
+        'Profiler has not been properly initialized. '
+        'Make sure Profiler.ensureInitialized() is being called before you '
+        'access Profiler.instance',
+      );
+    }
+    return _instance;
+  }
+
+  static Profiler _instance;
+
+  static void _checkBenchmarkMode() {
+    if (!isBenchmarkMode) {
+      throw Exception(
+        'Cannot use Profiler unless benchmark mode is enabled. '
+        'You can enable it by setting the `FLUTTER_WEB_ENABLE_PROFILING` '
+        'environment variable to true.',
+      );
+    }
+  }
+
+  /// Used to send benchmark data to whoever is listening to them.
+  void benchmark(String name, num value) {
+    _checkBenchmarkMode();
+
+    final OnBenchmark onBenchmark =
+        js_util.getProperty(html.window, '_flutter_internal_on_benchmark');
+    if (onBenchmark != null) {
+      onBenchmark(name, value);
+    }
+  }
+}

lib/web_ui/lib/src/engine/text/paragraph.dart

@@ -255,7 +255,16 @@ class EngineParagraph implements ui.Paragraph {
       return;
     }
 
+    Stopwatch stopwatch;
+    if (Profiler.isBenchmarkMode) {
+      stopwatch = Stopwatch()..start();
+    }
     _measurementResult = _measurementService.measure(this, constraints);
+    if (Profiler.isBenchmarkMode) {
+      stopwatch.stop();
+      Profiler.instance.benchmark('text_layout', stopwatch.elapsedMicroseconds);
+    }
+
     _lastUsedConstraints = constraints;
 
     if (_geometricStyle.maxLines != null) {

lib/web_ui/test/canvas_test.dart

@@ -13,6 +13,7 @@ import 'mock_engine_canvas.dart';
 void main() {
   setUpAll(() {
     WebExperiments.ensureInitialized();
+    Profiler.ensureInitialized();
   });
 
   group('EngineCanvas', () {

lib/web_ui/test/engine/profiler_test.dart

@@ -0,0 +1,99 @@
+// Copyright 2013 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.
+
+// @dart = 2.6
+import 'dart:html' as html;
+import 'dart:js_util' as js_util;
+
+import 'package:test/test.dart';
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart';
+
+void main() {
+  setUp(() {
+    Profiler.isBenchmarkMode = true;
+    Profiler.ensureInitialized();
+  });
+
+  tearDown(() {
+    jsOnBenchmark(null);
+    Profiler.isBenchmarkMode = false;
+  });
+
+  test('works when there is no listener', () {
+    expect(() => Profiler.instance.benchmark('foo', 123), returnsNormally);
+  });
+
+  test('can listen to benchmarks', () {
+    final List<BenchmarkDatapoint> data = <BenchmarkDatapoint>[];
+    jsOnBenchmark((String name, num value) {
+      data.add(BenchmarkDatapoint(name, value));
+    });
+
+    Profiler.instance.benchmark('foo', 123);
+    expect(data, <BenchmarkDatapoint>[BenchmarkDatapoint('foo', 123)]);
+    data.clear();
+
+    Profiler.instance.benchmark('bar', 0.0125);
+    expect(data, <BenchmarkDatapoint>[BenchmarkDatapoint('bar', 0.0125)]);
+    data.clear();
+
+    // Remove listener and make sure nothing breaks and the data isn't being
+    // sent to the old callback anymore.
+    jsOnBenchmark(null);
+    expect(() => Profiler.instance.benchmark('baz', 99.999), returnsNormally);
+    expect(data, isEmpty);
+  });
+
+  test('throws on wrong listener type', () {
+    final List<BenchmarkDatapoint> data = <BenchmarkDatapoint>[];
+
+    // Wrong callback signature.
+    jsOnBenchmark((num value) {
+      data.add(BenchmarkDatapoint('bad', value));
+    });
+    expect(
+      () => Profiler.instance.benchmark('foo', 123),
+      throwsA(isA<TypeError>()),
+    );
+    expect(data, isEmpty);
+
+    // Not even a callback.
+    jsOnBenchmark('string');
+    expect(
+      () => Profiler.instance.benchmark('foo', 123),
+      throwsA(isA<TypeError>()),
+    );
+  });
+}
+
+class BenchmarkDatapoint {
+  BenchmarkDatapoint(this.name, this.value);
+
+  final String name;
+  final num value;
+
+  @override
+  int get hashCode => hashValues(name, value);
+
+  @override
+  operator ==(dynamic other) {
+    if (identical(this, other)) {
+      return true;
+    }
+    if (other.runtimeType != runtimeType) {
+      return false;
+    }
+    return name == other.name && value == other.value;
+  }
+
+  @override
+  String toString() {
+    return '$runtimeType("$name", $value)';
+  }
+}
+
+void jsOnBenchmark(dynamic listener) {
+  js_util.setProperty(html.window, '_flutter_internal_on_benchmark', listener);
+}

lib/web_ui/test/paragraph_builder_test.dart

@@ -11,6 +11,7 @@ import 'package:test/test.dart';
 void main() {
   setUpAll(() {
     WebExperiments.ensureInitialized();
+    Profiler.ensureInitialized();
   });
 
   test('Should be able to build and layout a paragraph', () {