Add an Info.plist flag to enable the embedded iOS views preview. (#6756)

When the flag is true, we currently use a single thread configuration,
and disabled the raster cache for opacity layers.

The flag's name is 'io.flutter_embedded_views_preview'.

flow/layers/opacity_layer.cc

@@ -39,7 +39,12 @@ void OpacityLayer::Paint(PaintContext& context) const {
       RasterCache::GetIntegralTransCTM(context.canvas->getTotalMatrix()));
 #endif
 
-  if (layers().size() == 1 && context.raster_cache) {
+  // Embedded platform views are changing the canvas in the middle of the paint
+  // traversal. To make sure we paint on the right canvas, when the embedded
+  // platform views preview is enabled (context.view_embedded is not null) we
+  // don't use the cache.
+  if (context.view_embedder == nullptr && layers().size() == 1 &&
+      context.raster_cache) {
     const SkMatrix& ctm = context.canvas->getTotalMatrix();
     RasterCacheResult child_cache =
         context.raster_cache->Get(layers()[0].get(), ctm);

shell/common/rasterizer.cc

@@ -162,16 +162,7 @@ bool Rasterizer::DrawToSurface(flow::LayerTree& layer_tree) {
 
   auto canvas = frame->SkiaCanvas();
 
-  // External view embedding required that the gpu and platform threads are the
-  // same. The dynamic merging of these threads is WIP so for now we don't
-  // populate the view embedder. Once we can merge the threads, we should
-  // populate the view embedded here with surface_->GetExternalViewEmbedder() if
-  // the scene contains an external view (and we can probably assert that the
-  // gpu and platform threads are the same).
-  //
-  // TODO(amirh): populate the view embedder once we dynamically merge the
-  // threads for embedded platform views.
-  auto external_view_embedder = nullptr;
+  auto external_view_embedder = surface_->GetExternalViewEmbedder();
 
   auto compositor_frame = compositor_context_->AcquireFrame(
       surface_->GetContext(), canvas, external_view_embedder,

shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -22,6 +22,7 @@
 #include "flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputDelegate.h"
 #include "flutter/shell/platform/darwin/ios/framework/Source/FlutterViewController_Internal.h"
 #include "flutter/shell/platform/darwin/ios/framework/Source/platform_message_response_darwin.h"
+#include "flutter/shell/platform/darwin/ios/ios_surface.h"
 #include "flutter/shell/platform/darwin/ios/platform_view_ios.h"
 
 @interface FlutterEngine () <FlutterTextInputDelegate>
@@ -295,13 +296,8 @@
       threadLabel.UTF8String,  // label
       shell::ThreadHost::Type::UI | shell::ThreadHost::Type::GPU | shell::ThreadHost::Type::IO};
 
-  blink::TaskRunners task_runners(threadLabel.UTF8String,                          // label
-                                  fml::MessageLoop::GetCurrent().GetTaskRunner(),  // platform
-                                  _threadHost.gpu_thread->GetTaskRunner(),         // gpu
-                                  _threadHost.ui_thread->GetTaskRunner(),          // ui
-                                  _threadHost.io_thread->GetTaskRunner()           // io
-  );
-
+  bool embedded_views_preview_enabled = [[[NSBundle mainBundle]
+      objectForInfoDictionaryKey:@(shell::kEmbeddedViewsPreview)] boolValue];
   // Lambda captures by pointers to ObjC objects are fine here because the
   // create call is
   // synchronous.
@@ -314,12 +310,41 @@
     return std::make_unique<shell::Rasterizer>(shell.GetTaskRunners());
   };
 
-  // Create the shell. This is a blocking operation.
-  _shell = shell::Shell::Create(std::move(task_runners),  // task runners
-                                std::move(settings),      // settings
-                                on_create_platform_view,  // platform view creation
-                                on_create_rasterizer      // rasterzier creation
-  );
+  if (embedded_views_preview_enabled) {
+    // Embedded views requires the gpu and the platform views to be the same.
+    // The plan is to eventually dynamically merge the threads when there's a
+    // platform view in the layer tree.
+    // For now we run in a single threaded configuration.
+    // TODO(amirh/chinmaygarde): merge only the gpu and platform threads.
+    // https://github.com/flutter/flutter/issues/23974
+    // TODO(amirh/chinmaygarde): remove this, and dynamically change the thread configuration.
+    // https://github.com/flutter/flutter/issues/23975
+    blink::TaskRunners task_runners(threadLabel.UTF8String,                          // label
+                                    fml::MessageLoop::GetCurrent().GetTaskRunner(),  // platform
+                                    fml::MessageLoop::GetCurrent().GetTaskRunner(),  // gpu
+                                    fml::MessageLoop::GetCurrent().GetTaskRunner(),  // ui
+                                    fml::MessageLoop::GetCurrent().GetTaskRunner()   // io
+    );
+    // Create the shell. This is a blocking operation.
+    _shell = shell::Shell::Create(std::move(task_runners),  // task runners
+                                  std::move(settings),      // settings
+                                  on_create_platform_view,  // platform view creation
+                                  on_create_rasterizer      // rasterzier creation
+    );
+  } else {
+    blink::TaskRunners task_runners(threadLabel.UTF8String,                          // label
+                                    fml::MessageLoop::GetCurrent().GetTaskRunner(),  // platform
+                                    _threadHost.gpu_thread->GetTaskRunner(),         // gpu
+                                    _threadHost.ui_thread->GetTaskRunner(),          // ui
+                                    _threadHost.io_thread->GetTaskRunner()           // io
+    );
+    // Create the shell. This is a blocking operation.
+    _shell = shell::Shell::Create(std::move(task_runners),  // task runners
+                                  std::move(settings),      // settings
+                                  on_create_platform_view,  // platform view creation
+                                  on_create_rasterizer      // rasterzier creation
+    );
+  }
 
   if (_shell == nullptr) {
     FML_LOG(ERROR) << "Could not start a shell FlutterEngine with entrypoint: "

shell/platform/darwin/ios/ios_surface.h

@@ -15,6 +15,9 @@
 
 namespace shell {
 
+// The name of the Info.plist flag to enable the embedded iOS views preview.
+const char* const kEmbeddedViewsPreview = "io.flutter_embedded_views_preview";
+
 class IOSSurface {
  public:
   IOSSurface(FlutterPlatformViewsController* platform_views_controller);

shell/platform/darwin/ios/ios_surface_gl.mm

@@ -67,7 +67,11 @@ bool IOSSurfaceGL::GLContextPresent() {
 }
 
 flow::ExternalViewEmbedder* IOSSurfaceGL::GetExternalViewEmbedder() {
-  return this;
+  if ([[[NSBundle mainBundle] objectForInfoDictionaryKey:@(kEmbeddedViewsPreview)] boolValue]) {
+    return this;
+  } else {
+    return nullptr;
+  }
 }
 
 SkCanvas* IOSSurfaceGL::CompositeEmbeddedView(int view_id, const flow::EmbeddedViewParams& params) {

shell/platform/darwin/ios/ios_surface_software.mm

@@ -131,7 +131,11 @@ bool IOSSurfaceSoftware::PresentBackingStore(sk_sp<SkSurface> backing_store) {
 }
 
 flow::ExternalViewEmbedder* IOSSurfaceSoftware::GetExternalViewEmbedder() {
-  return this;
+  if ([[[NSBundle mainBundle] objectForInfoDictionaryKey:@(kEmbeddedViewsPreview)] boolValue]) {
+    return this;
+  } else {
+    return nullptr;
+  }
 }
 
 SkCanvas* IOSSurfaceSoftware::CompositeEmbeddedView(int view_id,