Document GPUSurfaceGLDelegate methods and move it to its own file. (flutter/engine#7755)

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -403,6 +403,7 @@ FILE: ../../../flutter/shell/common/vsync_waiter_fallback.cc
 FILE: ../../../flutter/shell/common/vsync_waiter_fallback.h
 FILE: ../../../flutter/shell/gpu/gpu_surface_gl.cc
 FILE: ../../../flutter/shell/gpu/gpu_surface_gl.h
+FILE: ../../../flutter/shell/gpu/gpu_surface_gl_delegate.h
 FILE: ../../../flutter/shell/gpu/gpu_surface_software.cc
 FILE: ../../../flutter/shell/gpu/gpu_surface_software.h
 FILE: ../../../flutter/shell/gpu/gpu_surface_vulkan.cc

engine/src/flutter/shell/gpu/BUILD.gn

@@ -28,6 +28,7 @@ source_set("gpu_surface_gl") {
   sources = [
     "$gpu_dir/gpu_surface_gl.cc",
     "$gpu_dir/gpu_surface_gl.h",
+    "$gpu_dir/gpu_surface_gl_delegate.h",
   ]
 
   deps = gpu_common_deps + [ "//third_party/skia" ]

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

@@ -12,33 +12,11 @@
 #include "flutter/fml/macros.h"
 #include "flutter/fml/memory/weak_ptr.h"
 #include "flutter/shell/common/surface.h"
+#include "flutter/shell/gpu/gpu_surface_gl_delegate.h"
 #include "third_party/skia/include/gpu/GrContext.h"
 
 namespace shell {
 
-class GPUSurfaceGLDelegate {
- public:
-  virtual bool GLContextMakeCurrent() = 0;
-
-  virtual bool GLContextClearCurrent() = 0;
-
-  virtual bool GLContextPresent() = 0;
-
-  virtual intptr_t GLContextFBO() const = 0;
-
-  virtual bool GLContextFBOResetAfterPresent() const;
-
-  virtual bool UseOffscreenSurface() const;
-
-  virtual SkMatrix GLContextSurfaceTransformation() const;
-
-  virtual flow::ExternalViewEmbedder* GetExternalViewEmbedder();
-
-  using GLProcResolver =
-      std::function<void* /* proc name */ (const char* /* proc address */)>;
-  virtual GLProcResolver GetGLProcResolver() const;
-};
-
 class GPUSurfaceGL : public Surface {
  public:
   GPUSurfaceGL(GPUSurfaceGLDelegate* delegate);

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

@@ -0,0 +1,60 @@
+// 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_GPU_GPU_SURFACE_GL_DELEGATE_H_
+#define FLUTTER_SHELL_GPU_GPU_SURFACE_GL_DELEGATE_H_
+
+#include "flutter/flow/embedded_views.h"
+#include "flutter/fml/macros.h"
+#include "third_party/skia/include/core/SkMatrix.h"
+
+namespace shell {
+
+class GPUSurfaceGLDelegate {
+ public:
+  // Called to make the main GL context current on the current thread.
+  virtual bool GLContextMakeCurrent() = 0;
+
+  // Called to clear the current GL context on the thread. This may be called on
+  // either the GPU or IO threads.
+  virtual bool GLContextClearCurrent() = 0;
+
+  // Called to present the main GL surface. This is only called for the main GL
+  // context and not any of the contexts dedicated for IO.
+  virtual bool GLContextPresent() = 0;
+
+  // The ID of the main window bound framebuffer. Typically FBO0.
+  virtual intptr_t GLContextFBO() const = 0;
+
+  // The rendering subsystem assumes that the ID of the main window bound
+  // framebuffer remains constant throughout. If this assumption in incorrect,
+  // embedders are required to return true from this method. In such cases,
+  // GLContextFBO() will be called again to acquire the new FBO ID for rendering
+  // subsequent frames.
+  virtual bool GLContextFBOResetAfterPresent() const;
+
+  // Create an offscreen surface to render into before onscreen composition.
+  virtual bool UseOffscreenSurface() const;
+
+  // A transformation applied to the onscreen surface before the canvas is
+  // flushed.
+  virtual SkMatrix GLContextSurfaceTransformation() const;
+
+  // Get a reference to the external views embedder. This happens on the same
+  // thread that the renderer is operating on.
+  virtual flow::ExternalViewEmbedder* GetExternalViewEmbedder();
+
+  using GLProcResolver =
+      std::function<void* /* proc name */ (const char* /* proc address */)>;
+  // Provide a custom GL proc resolver. If no such resolver is present, Skia
+  // will attempt to do GL proc address resolution on its own. Embedders that
+  // have specific opinions on GL API selection or need to add their own
+  // instrumentation to specific GL calls can specify custom GL functions
+  // here.
+  virtual GLProcResolver GetGLProcResolver() const;
+};
+
+}  // namespace shell
+
+#endif  // FLUTTER_SHELL_GPU_GPU_SURFACE_GL_DELEGATE_H_