[dart:ui] Introduce PlatformDispatcher.implicitView (flutter/engine#39553)

This introduces `PlatformDispatcher.implicitView`, a low-level primitive for the framework's bootstrapping. Most code, including the framework after bootstrapping, will use `View.of(context)` instead of this new API. This new primitive will let us deprecate the `window` global.

Goals:

1. **Enable multi-window**. The `PlatformDispatcher.implicitView` is nullable. If `null`, the app must create a window to get a view it can draw into.
2. **Backwards compatibility**. For "single window" apps, `PlatformDispatcher.instance.implicitView` should behave as similar to `window` as possible.
    1. The `PlatformDispatcher.instance.implicitView.viewId` should be `0`.
    1. The `PlatformDispatcher.instance.implicitView` must be available synchronously at root isolate startup. This allows the framework to determine if it can make single window assumptions at startup.
    2. The `PlatformDispatcher.instance.implicitView` reference must not change after startup: if it is null at startup, it must always be null; if it is non-null at startup, it must always be non-null. If "single window" app enters headless mode, the implicit view must remain non-null.

In the future, the embedder will control whether an implicit view is created: mobile & legacy desktop apps will have an implicit view, multi-window desktop apps won't have an implicit view. This requires updating the engine's embedder API and is out-of-scope for this change. For now, all apps will have an implicit view.

Part of https://github.com/flutter/flutter/issues/120306

engine/src/flutter/lib/ui/dart_ui.cc

@@ -95,6 +95,7 @@ typedef CanvasPath Path;
   V(IsolateNameServerNatives::RemovePortNameMapping, 1)               \
   V(NativeStringAttribute::initLocaleStringAttribute, 4)              \
   V(NativeStringAttribute::initSpellOutStringAttribute, 3)            \
+  V(PlatformConfigurationNativeApi::ImplicitViewEnabled, 0)           \
   V(PlatformConfigurationNativeApi::DefaultRouteName, 0)              \
   V(PlatformConfigurationNativeApi::ScheduleFrame, 0)                 \
   V(PlatformConfigurationNativeApi::Render, 1)                        \

engine/src/flutter/lib/ui/platform_dispatcher.dart

@@ -168,6 +168,36 @@ class PlatformDispatcher {
   // A map of opaque platform view identifiers to view configurations.
   final Map<Object, ViewConfiguration> _viewConfigurations = <Object, ViewConfiguration>{};
 
+  /// The [FlutterView] provided by the engine if the platform is unable to
+  /// create windows, or, for backwards compatibility.
+  ///
+  /// If the platform provides an implicit view, it can be used to bootstrap
+  /// the framework. This is common for platforms designed for single-view
+  /// applications like mobile devices with a single display.
+  ///
+  /// Applications and libraries must not rely on this property being set
+  /// as it may be null depending on the engine's configuration. Instead,
+  /// consider using [View.of] to lookup the [FlutterView] the current
+  /// [BuildContext] is drawing into.
+  ///
+  /// While the properties on the referenced [FlutterView] may change,
+  /// the reference itself is guaranteed to never change over the lifetime
+  /// of the application: if this property is null at startup, it will remain
+  /// so throughout the entire lifetime of the application. If it points to a
+  /// specific [FlutterView], it will continue to point to the same view until
+  /// the application is shut down (although the engine may replace or remove
+  /// the underlying backing surface of the view at its discretion).
+  ///
+  /// See also:
+  ///
+  /// * [View.of], for accessing the current view.
+  /// * [PlatformDisptacher.views] for a list of all [FlutterView]s provided
+  ///   by the platform.
+  FlutterView? get implicitView => _implicitViewEnabled() ? _views[0] : null;
+
+  @Native<Handle Function()>(symbol: 'PlatformConfigurationNativeApi::ImplicitViewEnabled')
+  external static bool _implicitViewEnabled();
+
   /// A callback that is invoked whenever the [ViewConfiguration] of any of the
   /// [views] changes.
   ///

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

@@ -896,6 +896,7 @@ enum Brightness {
 /// * [PlatformDispatcher.views], contains the current list of Flutter windows
 ///   belonging to the application, including top level application windows like
 ///   this one.
+/// * [PlatformDispatcher.implicitView], this window's view.
 final SingletonFlutterWindow window = SingletonFlutterWindow._(0, PlatformDispatcher.instance);
 
 /// Additional data available on each flutter frame.

engine/src/flutter/lib/ui/window/platform_configuration.cc

@@ -22,6 +22,8 @@
 namespace flutter {
 namespace {
 
+constexpr int kImplicitViewId = 0;
+
 Dart_Handle ToByteData(const fml::Mapping& buffer) {
   return tonic::DartByteData::Create(buffer.GetMapping(), buffer.GetSize());
 }
@@ -67,8 +69,13 @@ void PlatformConfiguration::DidCreateIsolate() {
                   Dart_GetField(library, tonic::ToDart("_drawFrame")));
   report_timings_.Set(tonic::DartState::Current(),
                       Dart_GetField(library, tonic::ToDart("_reportTimings")));
-  windows_.insert(std::make_pair(
+
-      0, std::make_unique<Window>(0, ViewportMetrics{1.0, 0.0, 0.0, -1})));
+  // TODO(loicsharma): This should only be created if the embedder enables the
+  // implicit view.
+  // See: https://github.com/flutter/flutter/issues/120306
+  windows_.emplace(kImplicitViewId,
+                   std::make_unique<Window>(
+                       kImplicitViewId, ViewportMetrics{1.0, 0.0, 0.0, -1}));
 }
 
 void PlatformConfiguration::UpdateLocales(
@@ -427,6 +434,16 @@ Dart_Handle PlatformConfigurationNativeApi::ComputePlatformResolvedLocale(
   return tonic::DartConverter<std::vector<std::string>>::ToDart(results);
 }
 
+Dart_Handle PlatformConfigurationNativeApi::ImplicitViewEnabled() {
+  UIDartState::ThrowIfUIOperationsProhibited();
+  bool enabled = UIDartState::Current()
+                     ->platform_configuration()
+                     ->client()
+                     ->ImplicitViewEnabled();
+
+  return Dart_NewBoolean(enabled);
+}
+
 std::string PlatformConfigurationNativeApi::DefaultRouteName() {
   UIDartState::ThrowIfUIOperationsProhibited();
   return UIDartState::Current()

engine/src/flutter/lib/ui/window/platform_configuration.h

@@ -49,6 +49,16 @@ enum class AccessibilityFeatureFlag : int32_t {
 ///
 class PlatformConfigurationClient {
  public:
+  //--------------------------------------------------------------------------
+  /// @brief      Whether the platform provides an implicit view. If true,
+  ///             the Framework may assume that it can always render into
+  ///             the view with ID 0.
+  ///
+  ///             This value must not change for the lifetime of the
+  ///             application.
+  ///
+  virtual bool ImplicitViewEnabled() = 0;
+
   //--------------------------------------------------------------------------
   /// @brief      The route or path that the embedder requested when the
   ///             application was launched.
@@ -71,7 +81,7 @@ class PlatformConfigurationClient {
   virtual void Render(Scene* scene) = 0;
 
   //--------------------------------------------------------------------------
-  /// @brief      Receives a updated semantics tree from the Framework.
+  /// @brief      Receives an updated semantics tree from the Framework.
   ///
   /// @param[in] update The updated semantic tree to apply.
   ///
@@ -469,6 +479,8 @@ class PlatformMessageHandlerStorage {
 //----------------------------------------------------------------------------
 class PlatformConfigurationNativeApi {
  public:
+  static Dart_Handle ImplicitViewEnabled();
+
   static std::string DefaultRouteName();
 
   static void ScheduleFrame();

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

@@ -33,6 +33,8 @@ abstract class PlatformDispatcher {
 
   Iterable<FlutterView> get views;
 
+  FlutterView? get implicitView;
+
   VoidCallback? get onMetricsChanged;
   set onMetricsChanged(VoidCallback? callback);
 

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

@@ -148,6 +148,34 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   final Map<Object, ui.ViewConfiguration> _windowConfigurations =
       <Object, ui.ViewConfiguration>{};
 
+  /// The [FlutterView] provided by the engine if the platform is unable to
+  /// create windows, or, for backwards compatibility.
+  ///
+  /// If the platform provides an implicit view, it can be used to bootstrap
+  /// the framework. This is common for platforms designed for single-view
+  /// applications like mobile devices with a single display.
+  ///
+  /// Applications and libraries must not rely on this property being set
+  /// as it may be null depending on the engine's configuration. Instead,
+  /// consider using [View.of] to lookup the [FlutterView] the current
+  /// [BuildContext] is drawing into.
+  ///
+  /// While the properties on the referenced [FlutterView] may change,
+  /// the reference itself is guaranteed to never change over the lifetime
+  /// of the application: if this property is null at startup, it will remain
+  /// so throughout the entire lifetime of the application. If it points to a
+  /// specific [FlutterView], it will continue to point to the same view until
+  /// the application is shut down (although the engine may replace or remove
+  /// the underlying backing surface of the view at its discretion).
+  ///
+  /// See also:
+  ///
+  /// * [View.of], for accessing the current view.
+  /// * [PlatformDisptacher.views] for a list of all [FlutterView]s provided
+  ///   by the platform.
+  @override
+  ui.FlutterView? get implicitView => viewData[kImplicitViewId];
+
   /// A callback that is invoked whenever the platform's [devicePixelRatio],
   /// [physicalSize], [padding], [viewInsets], or [systemGestureInsets]
   /// values change, for example when the device is rotated or when the
@@ -474,7 +502,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
             // TODO(a-wallen): As multi-window support expands, the pop call
             // will need to include the view ID. Right now only one view is
             // supported.
-            (viewData[kSingletonViewId]! as EngineFlutterWindow)
+            (viewData[kImplicitViewId]! as EngineFlutterWindow)
                 .browserHistory
                 .exit()
                 .then((_) {
@@ -579,7 +607,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
         // TODO(a-wallen): As multi-window support expands, the navigation call
         // will need to include the view ID. Right now only one view is
         // supported.
-        (viewData[kSingletonViewId]! as EngineFlutterWindow)
+        (viewData[kImplicitViewId]! as EngineFlutterWindow)
             .handleNavigationMessage(data)
             .then((bool handled) {
           if (handled) {
@@ -1179,7 +1207,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   @override
   String get defaultRouteName {
     return _defaultRouteName ??=
-        (viewData[kSingletonViewId]! as EngineFlutterWindow).browserHistory.currentPath;
+        (viewData[kImplicitViewId]! as EngineFlutterWindow).browserHistory.currentPath;
   }
 
   /// Lazily initialized when the `defaultRouteName` getter is invoked.

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

@@ -27,8 +27,8 @@ typedef _HandleMessageCallBack = Future<bool> Function();
 /// When set to true, all platform messages will be printed to the console.
 const bool debugPrintPlatformMessages = false;
 
-/// The view ID for a singleton flutter window.
+/// The view ID for the implicit flutter view provided by the platform.
-const int kSingletonViewId = 0;
+const int kImplicitViewId = 0;
 
 /// Whether [_customUrlStrategy] has been set or not.
 ///
@@ -349,7 +349,7 @@ class EngineSingletonFlutterWindow extends EngineFlutterWindow {
 /// API surface, providing Web-specific functionality that the standard
 /// `dart:ui` version does not.
 final EngineSingletonFlutterWindow window =
-    EngineSingletonFlutterWindow(kSingletonViewId, EnginePlatformDispatcher.instance);
+    EngineSingletonFlutterWindow(kImplicitViewId, EnginePlatformDispatcher.instance);
 
 /// The Web implementation of [ui.WindowPadding].
 class WindowPadding implements ui.WindowPadding {

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

@@ -41,6 +41,13 @@ void testMain() {
     await window.resetHistory();
   });
 
+  // For now, web always has an implicit view provided by the web engine.
+  test('EnginePlatformDispatcher.instance.implicitView should be non-null', () async {
+    expect(EnginePlatformDispatcher.instance.implicitView, isNotNull);
+    expect(EnginePlatformDispatcher.instance.implicitView?.viewId, 0);
+    expect(window.viewId, 0);
+  });
+
   test('window.defaultRouteName should work with JsUrlStrategy', () async {
     dynamic state = <dynamic, dynamic>{};
     final JsUrlStrategy jsUrlStrategy = JsUrlStrategy(

engine/src/flutter/runtime/runtime_controller.cc

@@ -298,6 +298,11 @@ RuntimeController::GetPlatformConfigurationIfAvailable() {
   return root_isolate ? root_isolate->platform_configuration() : nullptr;
 }
 
+// |PlatformConfigurationClient|
+bool RuntimeController::ImplicitViewEnabled() {
+  return client_.ImplicitViewEnabled();
+}
+
 // |PlatformConfigurationClient|
 std::string RuntimeController::DefaultRouteName() {
   return client_.DefaultRouteName();

engine/src/flutter/runtime/runtime_controller.h

@@ -610,6 +610,9 @@ class RuntimeController : public PlatformConfigurationClient {
 
   bool FlushRuntimeStateToIsolate();
 
+  // |PlatformConfigurationClient|
+  bool ImplicitViewEnabled() override;
+
   // |PlatformConfigurationClient|
   std::string DefaultRouteName() override;
 

engine/src/flutter/runtime/runtime_delegate.h

@@ -21,6 +21,8 @@ namespace flutter {
 
 class RuntimeDelegate {
  public:
+  virtual bool ImplicitViewEnabled() = 0;
+
   virtual std::string DefaultRouteName() = 0;
 
   virtual void ScheduleFrame(bool regenerate_layer_tree = true) = 0;

engine/src/flutter/shell/common/engine.cc

@@ -431,6 +431,14 @@ void Engine::SetAccessibilityFeatures(int32_t flags) {
   runtime_controller_->SetAccessibilityFeatures(flags);
 }
 
+bool Engine::ImplicitViewEnabled() {
+  // TODO(loicsharma): This value should be provided by the embedder
+  // when it launches the engine. For now, assume the embedder always creates a
+  // view.
+  // See: https://github.com/flutter/flutter/issues/120306
+  return true;
+}
+
 std::string Engine::DefaultRouteName() {
   if (!initial_route_.empty()) {
     return initial_route_;

engine/src/flutter/shell/common/engine.h

@@ -888,6 +888,9 @@ class Engine final : public RuntimeDelegate, PointerDataDispatcher::Delegate {
   const std::weak_ptr<VsyncWaiter> GetVsyncWaiter() const;
 
  private:
+  // |RuntimeDelegate|
+  bool ImplicitViewEnabled() override;
+
   // |RuntimeDelegate|
   std::string DefaultRouteName() override;
 

engine/src/flutter/shell/common/engine_unittests.cc

@@ -48,6 +48,7 @@ class MockResponse : public PlatformMessageResponse {
 
 class MockRuntimeDelegate : public RuntimeDelegate {
  public:
+  MOCK_METHOD0(ImplicitViewEnabled, bool());
   MOCK_METHOD0(DefaultRouteName, std::string());
   MOCK_METHOD1(ScheduleFrame, void(bool));
   MOCK_METHOD1(Render, void(std::shared_ptr<flutter::LayerTree>));

engine/src/flutter/shell/platform/embedder/fixtures/main.dart

@@ -45,8 +45,15 @@ void executableNameNotNull() {
   notifyStringValue(Platform.executable);
 }
 
+@pragma('vm:entry-point')
+void implicitViewNotNull() {
+  notifyBoolValue(PlatformDispatcher.instance.implicitView != null);
+}
+
 @pragma('vm:external-name', 'NotifyStringValue')
 external void notifyStringValue(String value);
+@pragma('vm:external-name', 'NotifyBoolValue')
+external void notifyBoolValue(bool value);
 
 @pragma('vm:entry-point')
 void invokePlatformTaskRunner() {

engine/src/flutter/shell/platform/embedder/tests/embedder_unittests.cc

@@ -174,6 +174,30 @@ TEST_F(EmbedderTest, ExecutableNameNotNull) {
   latch.Wait();
 }
 
+TEST_F(EmbedderTest, ImplicitViewNotNull) {
+  // TODO(loicsharma): Update this test when embedders can opt-out
+  // of the implicit view.
+  // See: https://github.com/flutter/flutter/issues/120306
+  auto& context = GetEmbedderContext(EmbedderTestContextType::kSoftwareContext);
+
+  bool implicitViewNotNull = false;
+  fml::AutoResetWaitableEvent latch;
+  context.AddNativeCallback(
+      "NotifyBoolValue", CREATE_NATIVE_ENTRY([&](Dart_NativeArguments args) {
+        implicitViewNotNull = tonic::DartConverter<bool>::FromDart(
+            Dart_GetNativeArgument(args, 0));
+        latch.Signal();
+      }));
+
+  EmbedderConfigBuilder builder(context);
+  builder.SetSoftwareRendererConfig();
+  builder.SetDartEntrypoint("implicitViewNotNull");
+  auto engine = builder.LaunchEngine();
+  latch.Wait();
+
+  EXPECT_TRUE(implicitViewNotNull);
+}
+
 std::atomic_size_t EmbedderTestTaskRunner::sEmbedderTaskRunnerIdentifiers = {};
 
 TEST_F(EmbedderTest, CanSpecifyCustomPlatformTaskRunner) {