Prefer ScopedNestableTaskAllower over manual save/restore

base/message_loop.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 BASE_MESSAGE_LOOP_H_
 #define BASE_MESSAGE_LOOP_H_
 #pragma once
 
 #include <queue>
 #include <string>
@@ skipping 49 matching lines @@
 //
 // NOTE: MessageLoop has task reentrancy protection.  This means that if a
 // task is being processed, a second task cannot start until the first task is
 // finished.  Reentrancy can happen when processing a task, and an inner
 // message pump is created.  That inner pump then processes native messages
 // which could implicitly start an inner task.  Inner message pumps are created
 // with dialogs (DialogBox), common dialogs (GetOpenFileName), OLE functions
 // (DoDragDrop), printer functions (StartDoc) and *many* others.
 //
 // Sample workaround when inner task processing is needed:
-//   bool old_state = MessageLoop::current()->NestableTasksAllowed();
-//   MessageLoop::current()->SetNestableTasksAllowed(true);
-//   HRESULT hr = DoDragDrop(...); // Implicitly runs a modal message loop here.
-//   MessageLoop::current()->SetNestableTasksAllowed(old_state);
-//   // Process hr  (the result returned by DoDragDrop().
+//   {
+//     MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
+//     HRESULT hr = DoDragDrop(...); // Implicitly runs a modal message loop.
+//   }
+//   // Process |hr| (the result returned by DoDragDrop()).

[Ryan Sleevi, 2012/02/11 02:17:22]

Comment bug: |hr| is now out of scope once outside of the block

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

Done.

 //
 // Please be SURE your task is reentrant (nestable) and all global variables
 // are stable and accessible before calling SetNestableTasksAllowed(true).
 //
 class BASE_EXPORT MessageLoop : public base::MessagePump::Delegate {
  public:
 #if defined(OS_WIN)
   typedef base::MessagePumpWin::Dispatcher Dispatcher;
   typedef base::MessagePumpObserver Observer;
 #elif !defined(OS_MACOSX) && !defined(OS_ANDROID)
@@ skipping 574 matching lines @@
 #endif  // defined(OS_POSIX)
 };
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
 // MessageLoopForIO is often allocated via MessageLoop(TYPE_IO).  Any extra
 // data that you need should be stored on the MessageLoop's pump_ instance.
 COMPILE_ASSERT(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
                MessageLoopForIO_should_not_have_extra_member_variables);
 
 #endif  // BASE_MESSAGE_LOOP_H_