Expose locks for CopyFromCompositingSurface/CopyFromBackingStore API

content/public/browser/render_widget_host.h

 // Copyright (c) 2012 The Chromium 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 CONTENT_PUBLIC_BROWSER_RENDER_WIDGET_HOST_H_
 #define CONTENT_PUBLIC_BROWSER_RENDER_WIDGET_HOST_H_
 
 #include "base/callback.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/native_web_keyboard_event.h"
@@ skipping 182 matching lines @@
   // source contents is used. |callback| is invoked with true on success, false
   // otherwise, along with a SkBitmap containing the copied pixel data.
   //
   // NOTE: |callback| is called synchronously if the backing store is available.
   // When accelerated compositing is active, |callback| may be called
   // asynchronously.
   virtual void CopyFromBackingStore(
       const gfx::Rect& src_rect,
       const gfx::Size& accelerated_dst_size,
       const base::Callback<void(bool, const SkBitmap&)>& callback) = 0;
+  virtual bool CanCopyFromBackingStore() = 0;

[jam, 2014/02/26 00:09:54]

so to be clear, this is in the public api since chrome on android, which isn't
in this repository, is calling these methods?

[no sievers, 2014/02/26 00:25:56]

Actually we don't have to add this one. It already exists in
content/public/render_widget_host_view.h:
 // Returns true is the current display surface is available.
  virtual bool IsSurfaceAvailableForCopy() const = 0;

[powei, 2014/02/26 18:42:38]

I thought it would make sense that the user would just interact with
RenderWidgetHost and not have to worry about whether the view is there or not. 
WDYT? 

+  // Ensures that the view does not drop the backing store even when hidden.
+  virtual void LockForCopyFromBackingStore() = 0;
+  virtual void UnlockForCopyFromBackingStore() = 0;

[jam, 2014/02/26 00:09:54]

I'm sure there's a reason why you're exposing these methods, but right now I'm
not seeing it.

Per the cl subject, you're doing this so that the content isn't evicted when
calling async CopyFromBackingStore/CopyFromCompositingSurface. But I don't
understand why you need public locking methods. Why can't the implementation of
CopyFromBackingStore/CopyFromCompositingSurface do this internally since they
know when there's an oustanding callback?

[no sievers, 2014/02/26 00:25:56]

The problem is that the application wants to replace the live tab with a
placeholder approximation which needs to happen asynchronously.

While this happens and until this is ready and added to the compositor tree, we
cannot let RWHV release the live surface.

It's sort of an override for 'frontbuffer' memory management which the app is
not really involved in.

[jam, 2014/02/26 17:37:29]

I'm not familiar with this part of the code, so I have more questions. When this
"replace the live tab with a placeholder" is happening, at that point a pending
CopyFromBackingStore/CopyFromCompositingSurface is happening? I'm just trying to
convince myself that we can't implement this internally without a public API.
It's hard to see the whole picture because I don't see the calling code. I also
don't understand why these lock methods are empty (see my other comment)

 #if defined(TOOLKIT_GTK)
   // Paint the backing store into the target's |dest_rect|.
   virtual bool CopyFromBackingStoreToGtkWindow(const gfx::Rect& dest_rect,
                                                GdkWindow* target) = 0;
 #elif defined(OS_MACOSX)
   virtual gfx::Size GetBackingStoreSize() = 0;
   virtual bool CopyFromBackingStoreToCGContext(const CGRect& dest_rect,
                                                CGContextRef target) = 0;
 #endif
 
@@ skipping 90 matching lines @@
   // within content/.  This method is necessary because
   // RenderWidgetHost is the root of a diamond inheritance pattern, so
   // subclasses inherit it virtually, which removes our ability to
   // static_cast to the subclass.
   virtual RenderWidgetHostImpl* AsRenderWidgetHostImpl() = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_RENDER_WIDGET_HOST_H_