Delegate root layer scroll offset to android_webview.

content/renderer/android/synchronous_compositor_impl.cc

 // Copyright 2013 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.
 
 #include "content/renderer/android/synchronous_compositor_impl.h"
 
 #include "base/message_loop.h"
 #include "cc/input/input_handler.h"
 #include "content/public/renderer/android/synchronous_compositor_client.h"
 #include "content/public/renderer/content_renderer_client.h"
@@ skipping 54 matching lines @@
   GetContentClient()->renderer()->DidCreateSynchronousCompositor(routing_id_,
                                                                  this);
 }
 
 void SynchronousCompositorImpl::DidDestroySynchronousOutputSurface() {
   DCHECK(CalledOnValidThread());
   output_surface_ = NULL;
   compositor_client_->DidDestroyCompositor(this);
 }
 
+void SynchronousCompositorImpl::SetTotalScrollOffset(gfx::Vector2dF new_value) {
+  DCHECK(CalledOnValidThread());
+  compositor_client_->SetTotalRootLayerScrollOffset(new_value);

[mkosiba (inactive), 2013/05/28 17:29:50]

actually it looks like this ends up causing a use-after-free problem if the
WebContents is being deleted (in which case the android_webview layer un-sets
the compositor_client_ *before* the SynchronousCompositorImpl gets destroyed).

Ideally we'd tie the existence of a RootLayerScrollOffsetDelegate with the
existence of the compositor_client_ (set the RLSCOD_ to NULL if the
compositor_client_ is NULL) but for that I'd need to change the code Jared is
currently adding as part of https://codereview.chromium.org/15920002/ to use the
SynchronousCompositorImpl.
Thoughts?

[joth, 2013/05/28 17:36:25]

Hmm.. I think the bug is in your prior refactor patch. The current ToT code
always does a null-check on compositor_client_ before calling through it. Can we
just use that same policy in the new synchronous_compositor_impl.cc arrangement?

https://code.google.com/p/chromium/codesearch#chromium/src/content/renderer/a...

[mkosiba (inactive), 2013/05/29 13:18:24]

ah, I see. Yes, I can do that. Do you know if the compositor_client_ will be
null in cases other than setup/teardown?

+}
+
+gfx::Vector2dF SynchronousCompositorImpl::GetTotalScrollOffset() {
+  DCHECK(CalledOnValidThread());
+  return compositor_client_->GetTotalRootLayerScrollOffset();
+}
+
 // Not using base::NonThreadSafe as we want to enforce a more exacting threading
 // requirement: SynchronousCompositorImpl() must only be used by
 // embedders that supply their own compositor loop via
 // OverrideCompositorMessageLoop().
 bool SynchronousCompositorImpl::CalledOnValidThread() const {
   return base::MessageLoop::current() && (base::MessageLoop::current() ==
       GetContentClient()->renderer()->OverrideCompositorMessageLoop());
 }
 
 }  // namespace content