[Windows] Make the engine own a map of views (#51017)

Updates Windows engine's data structures to use a map of views.

This is a refactoring with no semantic changes. Flutter Windows's APIs restrict users to 0 or 1 view only - it is not possible to create multiple views yet.

_One small step for Windows. One giant leap for multi-view._

Part of flutter/flutter#143765
Part of flutter/flutter#142845

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: loic-sharma

shell/platform/windows/flutter_windows_engine.cc

@@ -496,7 +496,7 @@ std::unique_ptr<FlutterWindowsView> FlutterWindowsEngine::CreateView(
   auto view = std::make_unique<FlutterWindowsView>(
       kImplicitViewId, this, std::move(window), windows_proc_table_);
 
-  view_ = view.get();
+  views_[kImplicitViewId] = view.get();
   InitializeKeyboard();
 
   return std::move(view);
@@ -531,9 +531,12 @@ std::chrono::nanoseconds FlutterWindowsEngine::FrameInterval() {
 }
 
 FlutterWindowsView* FlutterWindowsEngine::view(FlutterViewId view_id) const {
-  FML_DCHECK(view_id == kImplicitViewId);
+  auto iterator = views_.find(view_id);
+  if (iterator == views_.end()) {
+    return nullptr;
+  }
 
-  return view_;
+  return iterator->second;
 }
 
 // Returns the currently configured Plugin Registrar.
@@ -672,7 +675,7 @@ void FlutterWindowsEngine::SendSystemLocales() {
 }
 
 void FlutterWindowsEngine::InitializeKeyboard() {
-  if (view_ == nullptr) {
+  if (views_.empty()) {
     FML_LOG(ERROR) << "Cannot initialize keyboard on Windows headless mode.";
   }
 
@@ -762,15 +765,15 @@ void FlutterWindowsEngine::UpdateSemanticsEnabled(bool enabled) {
   if (engine_ && semantics_enabled_ != enabled) {
     semantics_enabled_ = enabled;
     embedder_api_.UpdateSemanticsEnabled(engine_, enabled);
-    if (view_) {
-      view_->UpdateSemanticsEnabled(enabled);
+    for (auto iterator = views_.begin(); iterator != views_.end(); iterator++) {
+      iterator->second->UpdateSemanticsEnabled(enabled);
     }
   }
 }
 
 void FlutterWindowsEngine::OnPreEngineRestart() {
   // Reset the keyboard's state on hot restart.
-  if (view_) {
+  if (!views_.empty()) {
     InitializeKeyboard();
   }
 }
@@ -827,7 +830,9 @@ void FlutterWindowsEngine::OnQuit(std::optional<HWND> hwnd,
 }
 
 void FlutterWindowsEngine::OnDwmCompositionChanged() {
-  view_->OnDwmCompositionChanged();
+  for (auto iterator = views_.begin(); iterator != views_.end(); iterator++) {
+    iterator->second->OnDwmCompositionChanged();
+  }
 }
 
 void FlutterWindowsEngine::OnWindowStateEvent(HWND hwnd,

shell/platform/windows/flutter_windows_engine.h

@@ -11,6 +11,7 @@
 #include <optional>
 #include <string>
 #include <string_view>
+#include <unordered_map>
 #include <vector>
 
 #include "flutter/fml/closure.h"
@@ -119,12 +120,13 @@ class FlutterWindowsEngine {
   // Returns false if stopping the engine fails, or if it was not running.
   virtual bool Stop();
 
-  // Create the view that is displaying this engine's content.
+  // Create a view that can display this engine's content.
   std::unique_ptr<FlutterWindowsView> CreateView(
       std::unique_ptr<WindowBindingHandler> window);
 
-  // The view displaying this engine's content, if any. This will be null for
-  // headless engines.
+  // Get a view that displays this engine's content.
+  //
+  // Returns null if the view does not exist.
   FlutterWindowsView* view(FlutterViewId view_id) const;
 
   // Returns the currently configured Plugin Registrar.
@@ -349,8 +351,8 @@ class FlutterWindowsEngine {
   // AOT data, if any.
   UniqueAotDataPtr aot_data_;
 
-  // The view displaying the content running in this engine, if any.
-  FlutterWindowsView* view_ = nullptr;
+  // The views displaying the content running in this engine, if any.
+  std::unordered_map<FlutterViewId, FlutterWindowsView*> views_;
 
   // Task runner for tasks posted from the engine.
   std::unique_ptr<TaskRunner> task_runner_;

shell/platform/windows/flutter_windows_engine_unittests.cc

@@ -34,6 +34,19 @@ using ::testing::Return;
 
 class FlutterWindowsEngineTest : public WindowsTest {};
 
+// The engine can be run without any views.
+TEST_F(FlutterWindowsEngineTest, RunHeadless) {
+  FlutterWindowsEngineBuilder builder{GetContext()};
+  std::unique_ptr<FlutterWindowsEngine> engine = builder.Build();
+
+  EngineModifier modifier(engine.get());
+  modifier.embedder_api().RunsAOTCompiledDartCode = []() { return false; };
+
+  ASSERT_TRUE(engine->Run());
+  ASSERT_EQ(engine->view(kImplicitViewId), nullptr);
+  ASSERT_EQ(engine->view(123), nullptr);
+}
+
 TEST_F(FlutterWindowsEngineTest, RunDoesExpectedInitialization) {
   FlutterWindowsEngineBuilder builder{GetContext()};
   builder.AddDartEntrypointArgument("arg1");

shell/platform/windows/testing/engine_modifier.h

@@ -39,7 +39,9 @@ class EngineModifier {
 
   // Override the engine's implicit view. This is the "default" view
   // that Flutter apps render to.
-  void SetImplicitView(FlutterWindowsView* view) { engine_->view_ = view; }
+  void SetImplicitView(FlutterWindowsView* view) {
+    engine_->views_[kImplicitViewId] = view;
+  }
 
   /// Reset the start_time field that is used to align vsync events.
   void SetStartTime(uint64_t start_time_nanos) {