Prefer ScopedNestableTaskAllower over manual save/restore

chrome/browser/ui/views/tab_contents/native_tab_contents_view_aura.cc

 // 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.
 
 #include "chrome/browser/ui/views/tab_contents/native_tab_contents_view_aura.h"
 
 // TODO(beng): USE_ASH
 #include "ash/wm/visibility_controller.h"
 #include "base/event_types.h"
 #include "base/message_loop.h"
@@ skipping 193 matching lines @@
   ui::OSExchangeDataProviderAura* provider = new ui::OSExchangeDataProviderAura;
   PrepareDragData(drop_data, provider);
   if (!image.isNull())
     provider->set_drag_image(image);
   ui::OSExchangeData data(provider);  // takes ownership of |provider|.
 
   scoped_ptr<WebDragSourceAura> drag_source(new WebDragSourceAura(this));
 
   // We need to enable recursive tasks on the message loop so we can get
   // updates while in the system DoDragDrop loop.
-  bool old_state = MessageLoop::current()->NestableTasksAllowed();
-  MessageLoop::current()->SetNestableTasksAllowed(true);
-  int result_op = aura::client::GetDragDropClient()->StartDragAndDrop(
-      data, ConvertFromWeb(ops));
-  MessageLoop::current()->SetNestableTasksAllowed(old_state);
+  int result_op = 0;
+  {
+    MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
+    result_op = aura::client::GetDragDropClient()->StartDragAndDrop(

[jar (doing other things), 2012/02/11 03:24:38]

nit: Does this actually induce a nested message loop?

If so, it might be good to make that much more apparent in the underlying code,
as the code needs to be re-entrant.

I tried to look at what I *thought* was the code... but it was not immediately
clear when/how it created a sub-message loop.

[dhollowa, 2012/02/13 17:44:26]

sky will be the best person to comment here.  But it looks like the message loop
is created at ash/drag_drop/drag_drop_controller.cc:54, where messages are
dispatched back to the host framework.

[jar (doing other things), 2012/02/14 01:51:43]

Sky@

Do you think this is as close as you can get to the running of the nested
message loops?

Thanks,

Jim

On 2012/02/13 17:44:26, dhollowa wrote:

[sky, 2012/02/14 04:45:05]

DragDropController::StartDragAndDrop is closer (in ash/drag_drop).

[dhollowa, 2012/02/14 05:42:20]

Shall I move the scoper there then?  Or would you like to handle it?

+        data, ConvertFromWeb(ops));
+  }
 
   EndDrag(ConvertToWeb(result_op));
   GetWebContents()->GetRenderViewHost()->DragSourceSystemDragEnded();
 }
 
 void NativeTabContentsViewAura::CancelDrag() {
   if (aura::client::GetDragDropClient())
     aura::client::GetDragDropClient()->DragCancel();
 }
 
@@ skipping 86 matching lines @@
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // NativeTabContentsView, public:
 
 // static
 NativeTabContentsView* NativeTabContentsView::CreateNativeTabContentsView(
     internal::NativeTabContentsViewDelegate* delegate) {
   return new NativeTabContentsViewAura(delegate);
 }