Implement the render_to_surface flag in GPUSurfaceGLImpeller (flutter/engine#50669)

Without this, the EmbedderExternalViewEmbedder's call to SurfaceFrame::Submit will render an empty display list that will overwrite the output rendered by the external view embedder's layers.

See https://github.com/flutter/flutter/issues/143387

engine/src/flutter/shell/gpu/gpu_surface_gl_impeller.cc

@@ -14,7 +14,8 @@ namespace flutter {
 
 GPUSurfaceGLImpeller::GPUSurfaceGLImpeller(
     GPUSurfaceGLDelegate* delegate,
-    std::shared_ptr<impeller::Context> context)
+    std::shared_ptr<impeller::Context> context,
+    bool render_to_surface)
     : weak_factory_(this) {
   if (delegate == nullptr) {
     return;
@@ -38,6 +39,7 @@ GPUSurfaceGLImpeller::GPUSurfaceGLImpeller(
 
   delegate_ = delegate;
   impeller_context_ = std::move(context);
+  render_to_surface_ = render_to_surface;
   impeller_renderer_ = std::move(renderer);
   aiks_context_ = std::move(aiks_context);
   is_valid_ = true;
@@ -82,6 +84,15 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceGLImpeller::AcquireFrame(
     return nullptr;
   }
 
+  if (!render_to_surface_) {
+    return std::make_unique<SurfaceFrame>(
+        nullptr, SurfaceFrame::FramebufferInfo{.supports_readback = true},
+        [](const SurfaceFrame& surface_frame, DlCanvas* canvas) {
+          return true;
+        },
+        size);
+  }
+
   GLFrameInfo frame_info = {static_cast<uint32_t>(size.width()),
                             static_cast<uint32_t>(size.height())};
   const GLFBOInfo fbo_info = delegate_->GLContextFBO(frame_info);

engine/src/flutter/shell/gpu/gpu_surface_gl_impeller.h

@@ -18,7 +18,8 @@ namespace flutter {
 class GPUSurfaceGLImpeller final : public Surface {
  public:
   explicit GPUSurfaceGLImpeller(GPUSurfaceGLDelegate* delegate,
-                                std::shared_ptr<impeller::Context> context);
+                                std::shared_ptr<impeller::Context> context,
+                                bool render_to_surface);
 
   // |Surface|
   ~GPUSurfaceGLImpeller() override;
@@ -29,6 +30,7 @@ class GPUSurfaceGLImpeller final : public Surface {
  private:
   GPUSurfaceGLDelegate* delegate_ = nullptr;
   std::shared_ptr<impeller::Context> impeller_context_;
+  bool render_to_surface_ = true;
   std::shared_ptr<impeller::Renderer> impeller_renderer_;
   std::shared_ptr<impeller::AiksContext> aiks_context_;
   bool is_valid_ = false;

engine/src/flutter/shell/platform/android/android_surface_gl_impeller.cc

@@ -36,8 +36,9 @@ bool AndroidSurfaceGLImpeller::IsValid() const {
 std::unique_ptr<Surface> AndroidSurfaceGLImpeller::CreateGPUSurface(
     GrDirectContext* gr_context) {
   auto surface = std::make_unique<GPUSurfaceGLImpeller>(
-      this,                                   // delegate
-      android_context_->GetImpellerContext()  // context
+      this,                                    // delegate
+      android_context_->GetImpellerContext(),  // context
+      true                                     // render to surface
   );
   if (!surface->IsValid()) {
     return nullptr;

engine/src/flutter/shell/platform/embedder/BUILD.gn

@@ -242,6 +242,7 @@ test_fixtures("fixtures") {
     "fixtures/dpr_noxform.png",
     "fixtures/dpr_xform.png",
     "fixtures/gradient.png",
+    "fixtures/impeller_gl_gradient.png",
     "fixtures/vk_dpr_noxform.png",
     "fixtures/vk_gradient.png",
     "fixtures/gradient_metal.png",
@@ -250,7 +251,6 @@ test_fixtures("fixtures") {
     "fixtures/scene_without_custom_compositor.png",
     "fixtures/scene_without_custom_compositor_with_xform.png",
     "fixtures/snapshot_large_scene.png",
-    "fixtures/solid_red.png",
     "fixtures/verifyb143464703.png",
     "fixtures/verifyb143464703_soft_noxform.png",
   ]

engine/src/flutter/shell/platform/embedder/embedder_surface_gl_impeller.cc

@@ -179,8 +179,9 @@ EmbedderSurfaceGLImpeller::GLContextFramebufferInfo() const {
 // |EmbedderSurface|
 std::unique_ptr<Surface> EmbedderSurfaceGLImpeller::CreateGPUSurface() {
   return std::make_unique<GPUSurfaceGLImpeller>(
-      this,              // GPU surface GL delegate
-      impeller_context_  // render to surface
+      this,                     // GPU surface GL delegate
+      impeller_context_,        // Impeller context
+      !external_view_embedder_  // render to surface
   );
 }
 

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

@@ -4704,8 +4704,14 @@ TEST_F(EmbedderTest, CanRenderWithImpellerOpenGL) {
   auto& context = GetEmbedderContext(EmbedderTestContextType::kOpenGLContext);
   EmbedderConfigBuilder builder(context);
 
+  bool present_called = false;
+  static_cast<EmbedderTestContextGL&>(context).SetGLPresentCallback(
+      [&present_called](FlutterPresentInfo present_info) {
+        present_called = true;
+      });
+
   builder.AddCommandLineArgument("--enable-impeller");
-  builder.SetDartEntrypoint("draw_solid_red");
+  builder.SetDartEntrypoint("render_gradient");
   builder.SetOpenGLRendererConfig(SkISize::Make(800, 600));
   builder.SetCompositor();
   builder.SetRenderTargetType(
@@ -4727,8 +4733,12 @@ TEST_F(EmbedderTest, CanRenderWithImpellerOpenGL) {
 
   ASSERT_TRUE(ImageMatchesFixture(
       FixtureNameForBackend(EmbedderTestContextType::kOpenGLContext,
-                            "solid_red.png"),
+                            "impeller_gl_gradient.png"),
       rendered_scene));
+
+  // The scene will be rendered by the compositor, and the surface present
+  // callback should not be invoked.
+  ASSERT_FALSE(present_called);
 }
 
 INSTANTIATE_TEST_SUITE_P(