[web] Introduce js interop to enable experimental flags on web (flutter/engine#17099)

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -495,6 +495,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/text_editing/text_editing.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/util.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/validators.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/vector_math.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/web_experiments.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/window.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/annotations.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/canvas.dart

engine/src/flutter/lib/web_ui/lib/src/engine.dart

@@ -117,6 +117,7 @@ part 'engine/text_editing/text_editing.dart';
 part 'engine/util.dart';
 part 'engine/validators.dart';
 part 'engine/vector_math.dart';
+part 'engine/web_experiments.dart';
 part 'engine/window.dart';
 
 bool _engineInitialized = false;
@@ -161,6 +162,8 @@ void webOnlyInitializeEngine() {
   // initialize framework bindings.
   domRenderer;
 
+  WebExperiments.ensureInitialized();
+
   bool waitingForAnimation = false;
   ui.webOnlyScheduleFrameCallback = () {
     // We're asked to schedule a frame and call `frameHandler` when the frame

engine/src/flutter/lib/web_ui/lib/src/engine/text/measurement.dart

@@ -187,13 +187,6 @@ abstract class TextMeasurementService {
   static TextMeasurementService get canvasInstance =>
       CanvasTextMeasurementService.instance;
 
-  /// Whether the new experimental implementation of canvas-based text
-  /// measurement is enabled or not.
-  ///
-  /// This is only used for testing at the moment. Once the implementation is
-  /// complete and production-ready, we'll get rid of this flag.
-  static bool enableExperimentalCanvasImplementation = const bool.fromEnvironment('FLUTTER_WEB_USE_EXPERIMENTAL_CANVAS_TEXT', defaultValue: false);
-
   /// Gets the appropriate [TextMeasurementService] instance for the given
   /// [paragraph].
   static TextMeasurementService forParagraph(ui.Paragraph paragraph) {
@@ -206,7 +199,7 @@ abstract class TextMeasurementService {
     // Skip using canvas measurements until the iframe becomes visible.
     // see: https://github.com/flutter/flutter/issues/36341
     if (!window.physicalSize.isEmpty &&
-        enableExperimentalCanvasImplementation &&
+        WebExperiments.instance.useCanvasText &&
         _canUseCanvasMeasurement(paragraph)) {
       return canvasInstance;
     }

engine/src/flutter/lib/web_ui/lib/src/engine/web_experiments.dart

@@ -0,0 +1,53 @@
+// 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;
+
+/// A bag of all experiment flags in the web engine.
+///
+/// This class also handles platform messages that can be sent to enable/disable
+/// certain experiments at runtime without the need to access engine internals.
+class WebExperiments {
+  WebExperiments._() {
+    js.context['_flutter_internal_update_experiment'] = updateExperiment;
+    registerHotRestartListener(() {
+      js.context['_flutter_internal_update_experiment'] = null;
+    });
+  }
+
+  static WebExperiments ensureInitialized() {
+    if (WebExperiments.instance == null) {
+      WebExperiments.instance = WebExperiments._();
+    }
+    return WebExperiments.instance;
+  }
+
+  static WebExperiments instance;
+
+  /// Experiment flag for using canvas-based text measurement.
+  bool get useCanvasText => _useCanvasText ?? false;
+  set useCanvasText(bool enabled) {
+    _useCanvasText = enabled;
+  }
+
+  bool _useCanvasText = const bool.fromEnvironment(
+    'FLUTTER_WEB_USE_EXPERIMENTAL_CANVAS_TEXT',
+    defaultValue: null,
+  );
+
+  /// Reset all experimental flags to their default values.
+  void reset() {
+    _useCanvasText = null;
+  }
+
+  /// Used to enable/disable experimental flags in the web engine.
+  void updateExperiment(String name, bool enabled) {
+    switch (name) {
+      case 'useCanvasText':
+        _useCanvasText = enabled;
+        break;
+    }
+  }
+}

engine/src/flutter/lib/web_ui/test/canvas_test.dart

@@ -11,6 +11,10 @@ import 'package:test/test.dart';
 import 'mock_engine_canvas.dart';
 
 void main() {
+  setUpAll(() {
+    WebExperiments.ensureInitialized();
+  });
+
   group('EngineCanvas', () {
     MockEngineCanvas mockCanvas;
     ui.Paragraph paragraph;

engine/src/flutter/lib/web_ui/test/engine/web_experiments_test.dart

@@ -0,0 +1,80 @@
+// 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';
+
+void main() {
+  setUp(() {
+    WebExperiments.ensureInitialized();
+  });
+
+  tearDown(() {
+    WebExperiments.instance.reset();
+  });
+
+  test('default web experiment values', () {
+    expect(WebExperiments.instance.useCanvasText, false);
+  });
+
+  test('can turn on/off web experiments', () {
+    WebExperiments.instance.updateExperiment('useCanvasText', true);
+    expect(WebExperiments.instance.useCanvasText, true);
+
+    WebExperiments.instance.updateExperiment('useCanvasText', false);
+    expect(WebExperiments.instance.useCanvasText, false);
+
+    WebExperiments.instance.updateExperiment('useCanvasText', null);
+    // Goes back to default value.
+    expect(WebExperiments.instance.useCanvasText, false);
+  });
+
+  test('ignores unknown experiments', () {
+    expect(WebExperiments.instance.useCanvasText, false);
+    WebExperiments.instance.updateExperiment('foobarbazqux', true);
+    expect(WebExperiments.instance.useCanvasText, false);
+    WebExperiments.instance.updateExperiment('foobarbazqux', false);
+    expect(WebExperiments.instance.useCanvasText, false);
+  });
+
+  test('can reset web experiments', () {
+    WebExperiments.instance.updateExperiment('useCanvasText', true);
+    WebExperiments.instance.reset();
+    expect(WebExperiments.instance.useCanvasText, false);
+
+    WebExperiments.instance.updateExperiment('useCanvasText', true);
+    WebExperiments.instance.updateExperiment('foobarbazqux', true);
+    WebExperiments.instance.reset();
+    expect(WebExperiments.instance.useCanvasText, false);
+  });
+
+  test('js interop also works', () {
+    expect(WebExperiments.instance.useCanvasText, false);
+
+    expect(() => jsUpdateExperiment('useCanvasText', true), returnsNormally);
+    expect(WebExperiments.instance.useCanvasText, true);
+
+    expect(() => jsUpdateExperiment('useCanvasText', null), returnsNormally);
+    expect(WebExperiments.instance.useCanvasText, false);
+  });
+
+  test('js interop throws on wrong type', () {
+    expect(() => jsUpdateExperiment(123, true), throwsA(anything));
+    expect(() => jsUpdateExperiment('foo', 123), throwsA(anything));
+    expect(() => jsUpdateExperiment('foo', 'bar'), throwsA(anything));
+    expect(() => jsUpdateExperiment(false, 'foo'), throwsA(anything));
+  });
+}
+
+void jsUpdateExperiment(dynamic name, dynamic enabled) {
+  js_util.callMethod(
+    html.window,
+    '_flutter_internal_update_experiment',
+    <dynamic>[name, enabled],
+  );
+}

engine/src/flutter/lib/web_ui/test/golden_tests/engine/scuba.dart

@@ -71,7 +71,7 @@ class EngineScubaTester {
       sceneElement.append(canvas.rootElement);
       html.document.body.append(sceneElement);
       String screenshotName = '${fileName}_${canvas.runtimeType}';
-      if (TextMeasurementService.enableExperimentalCanvasImplementation) {
+      if (WebExperiments.instance.useCanvasText) {
         screenshotName += '+canvas_measurement';
       }
       await diffScreenshot(
@@ -96,18 +96,20 @@ void testEachCanvas(String description, CanvasTest body,
   test('$description (bitmap)', () {
     try {
       TextMeasurementService.initialize(rulerCacheCapacity: 2);
+      WebExperiments.instance.useCanvasText = false;
       return body(BitmapCanvas(bounds));
     } finally {
+      WebExperiments.instance.useCanvasText = null;
       TextMeasurementService.clearCache();
     }
   });
   test('$description (bitmap + canvas measurement)', () async {
     try {
       TextMeasurementService.initialize(rulerCacheCapacity: 2);
-      TextMeasurementService.enableExperimentalCanvasImplementation = true;
+      WebExperiments.instance.useCanvasText = true;
       await body(BitmapCanvas(bounds));
     } finally {
-      TextMeasurementService.enableExperimentalCanvasImplementation = false;
+      WebExperiments.instance.useCanvasText = null;
       TextMeasurementService.clearCache();
     }
   });

engine/src/flutter/lib/web_ui/test/golden_tests/engine/text_overflow_golden_test.dart

@@ -5,7 +5,7 @@
 // @dart = 2.6
 import 'dart:async';
 
-import 'package:ui/ui.dart';
+import 'package:ui/ui.dart' hide window;
 import 'package:ui/src/engine.dart';
 
 import 'scuba.dart';
@@ -89,7 +89,7 @@ void main() async {
     offset = offset.translate(0, p.height + 10);
 
     // Only the first line is rendered with an ellipsis.
-    if (!TextMeasurementService.enableExperimentalCanvasImplementation) {
+    if (!WebExperiments.instance.useCanvasText) {
       // This is now correct with the canvas-based measurement, so we shouldn't
       // print the "(wrong)" warning.
       p = warning('(wrong)');
@@ -106,7 +106,7 @@ void main() async {
 
     // Only the first two lines are rendered and the ellipsis appears on the 2nd
     // line.
-    if (!TextMeasurementService.enableExperimentalCanvasImplementation) {
+    if (!WebExperiments.instance.useCanvasText) {
       // This is now correct with the canvas-based measurement, so we shouldn't
       // print the "(wrong)" warning.
       p = warning('(wrong)');

engine/src/flutter/lib/web_ui/test/paragraph_builder_test.dart

@@ -3,11 +3,16 @@
 // found in the LICENSE file.
 
 // @dart = 2.6
+import 'package:ui/src/engine.dart';
 import 'package:ui/ui.dart';
 
 import 'package:test/test.dart';
 
 void main() {
+  setUpAll(() {
+    WebExperiments.ensureInitialized();
+  });
+
   test('Should be able to build and layout a paragraph', () {
     final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle());
     builder.addText('Hello');

engine/src/flutter/lib/web_ui/test/paragraph_test.dart

@@ -4,7 +4,7 @@
 
 // @dart = 2.6
 import 'package:ui/src/engine.dart';
-import 'package:ui/ui.dart';
+import 'package:ui/ui.dart' hide window;
 
 import 'package:test/test.dart';
 
@@ -12,18 +12,20 @@ void testEachMeasurement(String description, VoidCallback body, {bool skip}) {
   test('$description (dom measurement)', () async {
     try {
       TextMeasurementService.initialize(rulerCacheCapacity: 2);
+      WebExperiments.instance.useCanvasText = false;
       return body();
     } finally {
+      WebExperiments.instance.useCanvasText = null;
       TextMeasurementService.clearCache();
     }
   }, skip: skip);
   test('$description (canvas measurement)', () async {
     try {
       TextMeasurementService.initialize(rulerCacheCapacity: 2);
-      TextMeasurementService.enableExperimentalCanvasImplementation = true;
+      WebExperiments.instance.useCanvasText = true;
       return body();
     } finally {
-      TextMeasurementService.enableExperimentalCanvasImplementation = false;
+      WebExperiments.instance.useCanvasText = null;
       TextMeasurementService.clearCache();
     }
   }, skip: skip);
@@ -184,7 +186,7 @@ void main() async {
   test('getPositionForOffset multi-line', () {
     // [Paragraph.getPositionForOffset] for multi-line text doesn't work well
     // with dom-based measurement.
-    TextMeasurementService.enableExperimentalCanvasImplementation = true;
+    WebExperiments.instance.useCanvasText = true;
     TextMeasurementService.initialize(rulerCacheCapacity: 2);
 
     final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
@@ -280,11 +282,11 @@ void main() async {
     );
 
     TextMeasurementService.clearCache();
-    TextMeasurementService.enableExperimentalCanvasImplementation = false;
+    WebExperiments.instance.useCanvasText = null;
   });
 
   test('getPositionForOffset multi-line centered', () {
-    TextMeasurementService.enableExperimentalCanvasImplementation = true;
+    WebExperiments.instance.useCanvasText = true;
     TextMeasurementService.initialize(rulerCacheCapacity: 2);
 
     final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
@@ -387,7 +389,7 @@ void main() async {
     );
 
     TextMeasurementService.clearCache();
-    TextMeasurementService.enableExperimentalCanvasImplementation = false;
+    WebExperiments.instance.useCanvasText = null;
   });
 
   testEachMeasurement('getBoxesForRange returns a box', () {
@@ -782,7 +784,7 @@ void main() async {
 
   test('longestLine', () {
     // [Paragraph.longestLine] is only supported by canvas-based measurement.
-    TextMeasurementService.enableExperimentalCanvasImplementation = true;
+    WebExperiments.instance.useCanvasText = true;
     TextMeasurementService.initialize(rulerCacheCapacity: 2);
 
     final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
@@ -797,7 +799,7 @@ void main() async {
     expect(paragraph.longestLine, 50.0);
 
     TextMeasurementService.clearCache();
-    TextMeasurementService.enableExperimentalCanvasImplementation = false;
+    WebExperiments.instance.useCanvasText = null;
   });
 
   testEachMeasurement('getLineBoundary (single-line)', () {
@@ -824,7 +826,7 @@ void main() async {
   test('getLineBoundary (multi-line)', () {
     // [Paragraph.getLineBoundary] for multi-line paragraphs is only supported
     // by canvas-based measurement.
-    TextMeasurementService.enableExperimentalCanvasImplementation = true;
+    WebExperiments.instance.useCanvasText = true;
     TextMeasurementService.initialize(rulerCacheCapacity: 2);
 
     final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
@@ -867,7 +869,7 @@ void main() async {
     }
 
     TextMeasurementService.clearCache();
-    TextMeasurementService.enableExperimentalCanvasImplementation = false;
+    WebExperiments.instance.useCanvasText = null;
   });
 
   testEachMeasurement('width should be a whole integer', () {