[web] Simplify handling of custom url strategy (flutter/engine#42043)

Issue: https://github.com/flutter/flutter/issues/126831

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

@@ -8,6 +8,7 @@ import 'dart:js_interop';
 
 import 'package:ui/src/engine.dart';
 import 'package:ui/ui.dart' as ui;
+import 'package:ui/ui_web/src/ui_web.dart' as ui_web;
 import 'package:web_test_fonts/web_test_fonts.dart';
 
 /// The mode the app is running in.
@@ -265,7 +266,7 @@ Future<void> _downloadAssetFonts() async {
 void _addUrlStrategyListener() {
   jsSetUrlStrategy = allowInterop((JsUrlStrategy? jsStrategy) {
     if (jsStrategy == null) {
-      customUrlStrategy = null;
+      ui_web.urlStrategy = null;
     } else {
       // Because `JSStrategy` could be anything, we check for the
       // `addPopStateListener` property and throw if it is missing.
@@ -274,7 +275,7 @@ void _addUrlStrategyListener() {
             'Unexpected JsUrlStrategy: $jsStrategy is missing '
             '`addPopStateListener` property');
       }
-      customUrlStrategy = CustomUrlStrategy.fromJs(jsStrategy);
+      ui_web.urlStrategy = CustomUrlStrategy.fromJs(jsStrategy);
     }
   });
   registerHotRestartListener(() {

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

@@ -19,7 +19,6 @@ import 'navigation/history.dart';
 import 'navigation/js_url_strategy.dart';
 import 'platform_dispatcher.dart';
 import 'services.dart';
-import 'test_embedding.dart';
 import 'util.dart';
 
 typedef _HandleMessageCallBack = Future<bool> Function();
@@ -30,20 +29,6 @@ const bool debugPrintPlatformMessages = false;
 /// The view ID for the implicit flutter view provided by the platform.
 const int kImplicitViewId = 0;
 
-/// Whether [_customUrlStrategy] has been set or not.
-///
-/// It is valid to set [_customUrlStrategy] to null, so we can't use a null
-/// check to determine whether it was set or not. We need an extra boolean.
-bool _isUrlStrategySet = false;
-
-/// A custom URL strategy set by the app before running.
-ui_web.UrlStrategy? _customUrlStrategy;
-set customUrlStrategy(ui_web.UrlStrategy? strategy) {
-  assert(!_isUrlStrategySet, 'Cannot set URL strategy more than once.');
-  _isUrlStrategySet = true;
-  _customUrlStrategy = strategy;
-}
-
 class EngineFlutterDisplay extends ui.Display {
   EngineFlutterDisplay({
     required this.id,
@@ -69,8 +54,8 @@ class EngineFlutterWindow extends ui.SingletonFlutterWindow {
         platformDispatcher as EnginePlatformDispatcher;
     engineDispatcher.viewData[viewId] = this;
     engineDispatcher.windowConfigurations[viewId] = const ViewConfiguration();
-    if (_isUrlStrategySet) {
-      _browserHistory = createHistoryForExistingState(_customUrlStrategy);
+    if (ui_web.isCustomUrlStrategySet) {
+      _browserHistory = createHistoryForExistingState(ui_web.urlStrategy);
     }
     registerHotRestartListener(() {
       _browserHistory?.dispose();
@@ -103,11 +88,9 @@ class EngineFlutterWindow extends ui.SingletonFlutterWindow {
   }
 
   ui_web.UrlStrategy? get _urlStrategyForInitialization {
-    final ui_web.UrlStrategy? urlStrategy =
-        _isUrlStrategySet ? _customUrlStrategy : _createDefaultUrlStrategy();
     // Prevent any further customization of URL strategy.
-    _isUrlStrategySet = true;
-    return urlStrategy;
+    ui_web.preventCustomUrlStrategy();
+    return ui_web.urlStrategy;
   }
 
   BrowserHistory?
@@ -166,9 +149,9 @@ class EngineFlutterWindow extends ui.SingletonFlutterWindow {
     ui_web.UrlStrategy? strategy, {
     required bool useSingle,
   }) async {
-    // Prevent any further customization of URL strategy.
-    _isUrlStrategySet = true;
     await _browserHistory?.tearDown();
+
+    ui_web.urlStrategy = strategy;
     if (useSingle) {
       _browserHistory = SingleEntryBrowserHistory(urlStrategy: strategy);
     } else {
@@ -179,9 +162,7 @@ class EngineFlutterWindow extends ui.SingletonFlutterWindow {
   Future<void> resetHistory() async {
     await _browserHistory?.tearDown();
     _browserHistory = null;
-    // Reset the globals too.
-    _isUrlStrategySet = false;
-    _customUrlStrategy = null;
+    ui_web.debugResetCustomUrlStrategy();
   }
 
   Future<void> _endOfTheLine = Future<void>.value();
@@ -370,19 +351,12 @@ typedef _JsSetUrlStrategy = void Function(JsUrlStrategy?);
 
 /// A JavaScript hook to customize the URL strategy of a Flutter app.
 //
-// Keep this js name in sync with flutter_web_plugins. Find it at:
-// https://github.com/flutter/flutter/blob/custom_location_strategy/packages/flutter_web_plugins/lib/src/navigation/js_url_strategy.dart
+// DO NOT CHANGE THE JS NAME, IT IS PUBLIC API AT THIS POINT.
 //
 // TODO(mdebbar): Add integration test https://github.com/flutter/flutter/issues/66852
 @JS('_flutter_web_set_location_strategy')
 external set jsSetUrlStrategy(_JsSetUrlStrategy? newJsSetUrlStrategy);
 
-ui_web.UrlStrategy? _createDefaultUrlStrategy() {
-  return ui.debugEmulateFlutterTesterEnvironment
-      ? TestUrlStrategy.fromEntry(const TestHistoryEntry('default', null, '/'))
-      : const ui_web.HashUrlStrategy();
-}
-
 /// The Web implementation of [ui.SingletonFlutterWindow].
 class EngineSingletonFlutterWindow extends EngineFlutterWindow {
   EngineSingletonFlutterWindow(

engine/src/flutter/lib/web_ui/lib/ui_web/src/ui_web/navigation/url_strategy.dart

@@ -4,11 +4,47 @@
 
 import 'dart:async';
 
+import 'package:meta/meta.dart';
 import 'package:ui/src/engine.dart';
 import 'package:ui/ui.dart' as ui;
 
 import 'platform_location.dart';
 
+UrlStrategy _realDefaultUrlStrategy = ui.debugEmulateFlutterTesterEnvironment
+      ? TestUrlStrategy.fromEntry(const TestHistoryEntry('default', null, '/'))
+      : const HashUrlStrategy();
+
+UrlStrategy get _defaultUrlStrategy => debugDefaultUrlStrategyOverride ?? _realDefaultUrlStrategy;
+
+/// Overrides the default URL strategy.
+///
+/// Setting this to null allows the real default URL strategy to be used.
+///
+/// This is intended to be used for testing and debugging only.
+@visibleForTesting
+UrlStrategy? debugDefaultUrlStrategyOverride;
+
+/// Whether a custom URL strategy has been set or not.
+//
+// It is valid to set [_customUrlStrategy] to null, so we can't use a null
+// check to determine whether it was set or not. We need an extra boolean.
+bool isCustomUrlStrategySet = false;
+
+/// Whether a custom URL strategy can be set or not.
+///
+/// This is used to prevent setting a custom URL strategy for a second time or
+/// after the app has already started running.
+bool _customUrlStrategyCanBeSet = true;
+
+/// A custom URL strategy set by the app before running.
+UrlStrategy? _customUrlStrategy;
+
+/// Returns the present [UrlStrategy] for handling the browser URL.
+///
+/// Returns null when the browser integration has been manually disabled.
+UrlStrategy? get urlStrategy =>
+    isCustomUrlStrategySet ? _customUrlStrategy : _defaultUrlStrategy;
+
 /// Sets a custom URL strategy instead of the default one.
 ///
 /// Passing null disables browser history integration altogether.
@@ -16,7 +52,27 @@ import 'platform_location.dart';
 /// This setter can only be called once. Subsequent calls will throw an error
 /// in debug mode.
 set urlStrategy(UrlStrategy? strategy) {
-  customUrlStrategy = strategy;
+  assert(
+    _customUrlStrategyCanBeSet,
+    'Cannot set URL strategy a second time or after the app has been initialized.',
+  );
+  preventCustomUrlStrategy();
+  isCustomUrlStrategySet = true;
+  _customUrlStrategy = strategy;
+}
+
+/// From this point on, prevents setting a custom URL strategy.
+void preventCustomUrlStrategy() {
+  _customUrlStrategyCanBeSet = false;
+}
+
+/// Resets everything to do with custom URL strategy.
+///
+/// This should only be used in tests to reset things back after each test.
+void debugResetCustomUrlStrategy() {
+  isCustomUrlStrategySet = false;
+  _customUrlStrategyCanBeSet = true;
+  _customUrlStrategy = null;
 }
 
 /// Callback that receives the new state of the browser history entry.

engine/src/flutter/lib/web_ui/test/ui/url_strategy_test.dart

@@ -0,0 +1,61 @@
+// 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.
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart' hide TypeMatcher, isInstanceOf;
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui_web/src/ui_web.dart' as ui_web;
+
+import '../common/matchers.dart';
+import '../common/test_initialization.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+Future<void> testMain() async {
+  setUpUnitTests();
+
+  setUp(() {
+    ui_web.debugResetCustomUrlStrategy();
+  });
+  tearDown(() {
+    ui_web.debugResetCustomUrlStrategy();
+  });
+
+  test('uses the default if no custom URL strategy is set', () {
+    final ui_web.UrlStrategy defaultUrlStrategy = TestUrlStrategy();
+    ui_web.debugDefaultUrlStrategyOverride = defaultUrlStrategy;
+
+    expect(ui_web.urlStrategy, defaultUrlStrategy);
+    expect(ui_web.isCustomUrlStrategySet, isFalse);
+  });
+
+  test('can set a custom URL strategy', () {
+    final TestUrlStrategy customUrlStrategy = TestUrlStrategy();
+    ui_web.urlStrategy = customUrlStrategy;
+
+    expect(ui_web.urlStrategy, customUrlStrategy);
+    expect(ui_web.isCustomUrlStrategySet, isTrue);
+    // Does not allow custom URL strategy to be set again.
+    expect(
+      () {
+        ui_web.urlStrategy = customUrlStrategy;
+      },
+      throwsAssertionError,
+    );
+  });
+
+  test('custom URL strategy can be prevented manually', () {
+    ui_web.preventCustomUrlStrategy();
+
+    expect(ui_web.isCustomUrlStrategySet, isFalse);
+    expect(
+      () {
+        ui_web.urlStrategy = TestUrlStrategy();
+      },
+      throwsAssertionError,
+    );
+  });
+}