Reland "[Windows] Move to FlutterCompositor for rendering" (#49262)

## Reland

#48849 was reverted as it incorrectly expected to receive always 1 layer. However, the engine will present 0 layers on an ["empty" app](https://github.com/flutter/flutter/blob/6981fe6fd3aacb95bfc4a6c427ee5d493f43c5dc/dev/integration_tests/ui/lib/empty.dart#L8-L19). This pull request is split into two commits:

1. df604a1 is the original pull request, unchanged
2. c30b369 adds the ability to "clear" the view if the engine presents 0 layers

## Original pull request description

This migrates the Windows embedder to `FlutterCompositor` so that the engine renders off-screen to a framebuffer instead of directly onto the window's surface. This will allow us to support platform views and multiple views on Windows.

Addresses flutter/flutter#128904

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

Author: loic-sharma

ci/licenses_golden/excluded_files

@@ -369,6 +369,8 @@
 ../../../flutter/shell/platform/windows/client_wrapper/flutter_view_unittests.cc
 ../../../flutter/shell/platform/windows/client_wrapper/plugin_registrar_windows_unittests.cc
 ../../../flutter/shell/platform/windows/client_wrapper/testing
+../../../flutter/shell/platform/windows/compositor_opengl_unittests.cc
+../../../flutter/shell/platform/windows/compositor_software_unittests.cc
 ../../../flutter/shell/platform/windows/cursor_handler_unittests.cc
 ../../../flutter/shell/platform/windows/direct_manipulation_unittests.cc
 ../../../flutter/shell/platform/windows/dpi_utils_unittests.cc

ci/licenses_golden/licenses_flutter

@@ -7201,6 +7201,11 @@ ORIGIN: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/f
 ORIGIN: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/flutter_view.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/flutter_view_controller.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/plugin_registrar_windows.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/compositor.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/compositor_opengl.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/compositor_opengl.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/compositor_software.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/compositor_software.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/cursor_handler.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/cursor_handler.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/direct_manipulation.cc + ../../../flutter/LICENSE
@@ -10039,6 +10044,11 @@ FILE: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/flu
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/flutter_view.h
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/flutter_view_controller.h
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/plugin_registrar_windows.h
+FILE: ../../../flutter/shell/platform/windows/compositor.h
+FILE: ../../../flutter/shell/platform/windows/compositor_opengl.cc
+FILE: ../../../flutter/shell/platform/windows/compositor_opengl.h
+FILE: ../../../flutter/shell/platform/windows/compositor_software.cc
+FILE: ../../../flutter/shell/platform/windows/compositor_software.h
 FILE: ../../../flutter/shell/platform/windows/cursor_handler.cc
 FILE: ../../../flutter/shell/platform/windows/cursor_handler.h
 FILE: ../../../flutter/shell/platform/windows/direct_manipulation.cc

impeller/renderer/backend/gles/description_gles.cc

@@ -156,6 +156,10 @@ std::string DescriptionGLES::GetString() const {
   return stream.str();
 }
 
+Version DescriptionGLES::GetGlVersion() const {
+  return gl_version_;
+}
+
 bool DescriptionGLES::IsES() const {
   return is_es_;
 }

impeller/renderer/backend/gles/description_gles.h

@@ -27,6 +27,8 @@ class DescriptionGLES {
 
   std::string GetString() const;
 
+  Version GetGlVersion() const;
+
   bool HasExtension(const std::string& ext) const;
 
   /// @brief      Returns whether GLES includes the debug extension.

shell/platform/linux/fl_backing_store_provider.cc

@@ -82,7 +82,8 @@ uint32_t fl_backing_store_provider_get_gl_format(FlBackingStoreProvider* self) {
   // In Linux kN32_SkColorType is assumed to be kBGRA_8888_SkColorType.
   // So we must choose a valid gl format to be compatible with surface format
   // BGRA8.
-  // Following logics are copied from Skia GrGLCaps.cpp.
+  // Following logics are copied from Skia GrGLCaps.cpp:
+  // https://github.com/google/skia/blob/4738ed711e03212aceec3cd502a4adb545f38e63/src/gpu/ganesh/gl/GrGLCaps.cpp#L1963-L2116
 
   if (epoxy_is_desktop_gl()) {
     // For OpenGL.

shell/platform/windows/BUILD.gn

@@ -42,6 +42,11 @@ source_set("flutter_windows_source") {
     "accessibility_bridge_windows.h",
     "angle_surface_manager.cc",
     "angle_surface_manager.h",
+    "compositor.h",
+    "compositor_opengl.cc",
+    "compositor_opengl.h",
+    "compositor_software.cc",
+    "compositor_software.h",
     "cursor_handler.cc",
     "cursor_handler.h",
     "direct_manipulation.cc",
@@ -134,6 +139,7 @@ source_set("flutter_windows_source") {
   deps = [
     ":flutter_windows_headers",
     "//flutter/fml:fml",
+    "//flutter/impeller/renderer/backend/gles",
     "//flutter/shell/platform/common:common_cpp",
     "//flutter/shell/platform/common:common_cpp_input",
     "//flutter/shell/platform/common:common_cpp_switches",
@@ -175,6 +181,8 @@ executable("flutter_windows_unittests") {
   # Common Windows test sources.
   sources = [
     "accessibility_bridge_windows_unittests.cc",
+    "compositor_opengl_unittests.cc",
+    "compositor_software_unittests.cc",
     "cursor_handler_unittests.cc",
     "direct_manipulation_unittests.cc",
     "dpi_utils_unittests.cc",
@@ -235,6 +243,7 @@ executable("flutter_windows_unittests") {
     ":flutter_windows_fixtures",
     ":flutter_windows_headers",
     ":flutter_windows_source",
+    "//flutter/impeller/renderer/backend/gles",
     "//flutter/shell/platform/common:common_cpp",
     "//flutter/shell/platform/common/client_wrapper:client_wrapper",
     "//flutter/shell/platform/embedder:embedder_as_internal_library",

shell/platform/windows/compositor.h

@@ -0,0 +1,39 @@
+// 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.
+
+#ifndef FLUTTER_SHELL_PLATFORM_WINDOWS_COMPOSITOR_H_
+#define FLUTTER_SHELL_PLATFORM_WINDOWS_COMPOSITOR_H_
+
+#include "flutter/shell/platform/embedder/embedder.h"
+
+namespace flutter {
+
+// Enables the Flutter engine to render content on Windows.
+//
+// The engine uses this to:
+//
+// 1. Create backing stores used for rendering Flutter content
+// 2. Composite and present Flutter content and platform views onto a view
+//
+// Platform views are not yet supported.
+class Compositor {
+ public:
+  virtual ~Compositor() = default;
+
+  // Creates a backing store used for rendering Flutter content.
+  //
+  // The backing store's configuration is stored in |backing_store_out|.
+  virtual bool CreateBackingStore(const FlutterBackingStoreConfig& config,
+                                  FlutterBackingStore* backing_store_out) = 0;
+
+  // Destroys a backing store and releases its resources.
+  virtual bool CollectBackingStore(const FlutterBackingStore* store) = 0;
+
+  // Present Flutter content and platform views onto the view.
+  virtual bool Present(const FlutterLayer** layers, size_t layers_count) = 0;
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_WINDOWS_COMPOSITOR_H_

shell/platform/windows/compositor_opengl.cc

@@ -0,0 +1,188 @@
+// 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.
+
+#include "flutter/shell/platform/windows/compositor_opengl.h"
+
+#include "GLES3/gl3.h"
+#include "flutter/shell/platform/windows/flutter_windows_view.h"
+
+namespace flutter {
+
+namespace {
+
+constexpr uint32_t kWindowFrameBufferId = 0;
+
+// The metadata for an OpenGL framebuffer backing store.
+struct FramebufferBackingStore {
+  uint32_t framebuffer_id;
+  uint32_t texture_id;
+};
+
+// Based off Skia's logic:
+// https://github.com/google/skia/blob/4738ed711e03212aceec3cd502a4adb545f38e63/src/gpu/ganesh/gl/GrGLCaps.cpp#L1963-L2116
+int GetSupportedTextureFormat(const impeller::DescriptionGLES* description) {
+  if (description->HasExtension("GL_EXT_texture_format_BGRA8888")) {
+    return GL_BGRA8_EXT;
+  } else if (description->HasExtension("GL_APPLE_texture_format_BGRA8888") &&
+             description->GetGlVersion().IsAtLeast(impeller::Version(3, 0))) {
+    return GL_BGRA8_EXT;
+  } else {
+    return GL_RGBA8;
+  }
+}
+
+}  // namespace
+
+CompositorOpenGL::CompositorOpenGL(FlutterWindowsEngine* engine,
+                                   impeller::ProcTableGLES::Resolver resolver)
+    : engine_(engine), resolver_(resolver) {}
+
+bool CompositorOpenGL::CreateBackingStore(
+    const FlutterBackingStoreConfig& config,
+    FlutterBackingStore* result) {
+  if (!is_initialized_ && !Initialize()) {
+    return false;
+  }
+
+  auto store = std::make_unique<FramebufferBackingStore>();
+
+  gl_->GenTextures(1, &store->texture_id);
+  gl_->GenFramebuffers(1, &store->framebuffer_id);
+
+  gl_->BindFramebuffer(GL_FRAMEBUFFER, store->framebuffer_id);
+
+  gl_->BindTexture(GL_TEXTURE_2D, store->texture_id);
+  gl_->TexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
+  gl_->TexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
+  gl_->TexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+  gl_->TexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+  gl_->TexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, config.size.width,
+                  config.size.height, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
+  gl_->BindTexture(GL_TEXTURE_2D, 0);
+
+  gl_->FramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0_EXT,
+                            GL_TEXTURE_2D, store->texture_id, 0);
+
+  result->type = kFlutterBackingStoreTypeOpenGL;
+  result->open_gl.type = kFlutterOpenGLTargetTypeFramebuffer;
+  result->open_gl.framebuffer.name = store->framebuffer_id;
+  result->open_gl.framebuffer.target = format_;
+  result->open_gl.framebuffer.user_data = store.release();
+  result->open_gl.framebuffer.destruction_callback = [](void* user_data) {
+    // Backing store destroyed in `CompositorOpenGL::CollectBackingStore`, set
+    // on FlutterCompositor.collect_backing_store_callback during engine start.
+  };
+  return true;
+}
+
+bool CompositorOpenGL::CollectBackingStore(const FlutterBackingStore* store) {
+  FML_DCHECK(is_initialized_);
+  FML_DCHECK(store->type == kFlutterBackingStoreTypeOpenGL);
+  FML_DCHECK(store->open_gl.type == kFlutterOpenGLTargetTypeFramebuffer);
+
+  auto user_data = static_cast<FramebufferBackingStore*>(
+      store->open_gl.framebuffer.user_data);
+
+  gl_->DeleteFramebuffers(1, &user_data->framebuffer_id);
+  gl_->DeleteTextures(1, &user_data->texture_id);
+
+  delete user_data;
+  return true;
+}
+
+bool CompositorOpenGL::Present(const FlutterLayer** layers,
+                               size_t layers_count) {
+  if (!engine_->view()) {
+    return false;
+  }
+
+  // Clear the view if there are no layers to present.
+  if (layers_count == 0) {
+    // Normally the compositor is initialized when the first backing store is
+    // created. However, on an empty frame no backing stores are created and
+    // the present needs to initialize the compositor.
+    if (!is_initialized_ && !Initialize()) {
+      return false;
+    }
+
+    return ClearSurface();
+  }
+
+  // TODO: Support compositing layers and platform views.
+  // See: https://github.com/flutter/flutter/issues/31713
+  FML_DCHECK(is_initialized_);
+  FML_DCHECK(layers_count == 1);
+  FML_DCHECK(layers[0]->offset.x == 0 && layers[0]->offset.y == 0);
+  FML_DCHECK(layers[0]->type == kFlutterLayerContentTypeBackingStore);
+  FML_DCHECK(layers[0]->backing_store->type == kFlutterBackingStoreTypeOpenGL);
+  FML_DCHECK(layers[0]->backing_store->open_gl.type ==
+             kFlutterOpenGLTargetTypeFramebuffer);
+
+  auto width = layers[0]->size.width;
+  auto height = layers[0]->size.height;
+
+  // Check if this frame can be presented. This resizes the surface if a resize
+  // is pending and |width| and |height| match the target size.
+  if (!engine_->view()->OnFrameGenerated(width, height)) {
+    return false;
+  }
+
+  if (!engine_->surface_manager()->MakeCurrent()) {
+    return false;
+  }
+
+  auto source_id = layers[0]->backing_store->open_gl.framebuffer.name;
+  gl_->BindFramebuffer(GL_READ_FRAMEBUFFER, source_id);
+  gl_->BindFramebuffer(GL_DRAW_FRAMEBUFFER, kWindowFrameBufferId);
+
+  gl_->BlitFramebuffer(0,                    // srcX0
+                       0,                    // srcY0
+                       width,                // srcX1
+                       height,               // srcY1
+                       0,                    // dstX0
+                       0,                    // dstY0
+                       width,                // dstX1
+                       height,               // dstY1
+                       GL_COLOR_BUFFER_BIT,  // mask
+                       GL_NEAREST            // filter
+  );
+
+  return engine_->view()->SwapBuffers();
+}
+
+bool CompositorOpenGL::Initialize() {
+  FML_DCHECK(!is_initialized_);
+
+  if (!engine_->surface_manager()->MakeCurrent()) {
+    return false;
+  }
+
+  gl_ = std::make_unique<impeller::ProcTableGLES>(resolver_);
+  if (!gl_->IsValid()) {
+    gl_.reset();
+    return false;
+  }
+
+  format_ = GetSupportedTextureFormat(gl_->GetDescription());
+  is_initialized_ = true;
+  return true;
+}
+
+bool CompositorOpenGL::ClearSurface() {
+  FML_DCHECK(is_initialized_);
+
+  // Resize the surface if needed.
+  engine_->view()->OnEmptyFrameGenerated();
+
+  if (!engine_->surface_manager()->MakeCurrent()) {
+    return false;
+  }
+
+  gl_->ClearColor(0.0f, 0.0f, 0.0f, 0.0f);
+  gl_->Clear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
+
+  return engine_->view()->SwapBuffers();
+}
+
+}  // namespace flutter

shell/platform/windows/compositor_opengl.h

@@ -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.
+
+#ifndef FLUTTER_SHELL_PLATFORM_WINDOWS_COMPOSITOR_OPENGL_H_
+#define FLUTTER_SHELL_PLATFORM_WINDOWS_COMPOSITOR_OPENGL_H_
+
+#include <memory>
+
+#include "flutter/impeller/renderer/backend/gles/proc_table_gles.h"
+#include "flutter/shell/platform/embedder/embedder.h"
+#include "flutter/shell/platform/windows/compositor.h"
+#include "flutter/shell/platform/windows/flutter_windows_engine.h"
+
+namespace flutter {
+
+// Enables the Flutter engine to render content on Windows using OpenGL.
+class CompositorOpenGL : public Compositor {
+ public:
+  CompositorOpenGL(FlutterWindowsEngine* engine,
+                   impeller::ProcTableGLES::Resolver resolver);
+
+  /// |Compositor|
+  bool CreateBackingStore(const FlutterBackingStoreConfig& config,
+                          FlutterBackingStore* result) override;
+
+  /// |Compositor|
+  bool CollectBackingStore(const FlutterBackingStore* store) override;
+
+  /// |Compositor|
+  bool Present(const FlutterLayer** layers, size_t layers_count) override;
+
+ private:
+  // The Flutter engine that manages the views to render.
+  FlutterWindowsEngine* engine_;
+
+ private:
+  // The compositor initializes itself lazily once |CreateBackingStore| is
+  // called. True if initialization completed successfully.
+  bool is_initialized_ = false;
+
+  // Function used to resolve GLES functions.
+  impeller::ProcTableGLES::Resolver resolver_ = nullptr;
+
+  // Table of resolved GLES functions. Null until the compositor is initialized.
+  std::unique_ptr<impeller::ProcTableGLES> gl_ = nullptr;
+
+  // The OpenGL texture target format for backing stores. Invalid value until
+  // the compositor is initialized.
+  uint32_t format_ = 0;
+
+  // Initialize the compositor. This must run on the raster thread.
+  bool Initialize();
+
+  // Clear the view's surface and removes any previously presented layers.
+  bool ClearSurface();
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_WINDOWS_COMPOSITOR_OPENGL_H_