Monitor the IO message loop in the AudioDevice classes.

content/renderer/media/scoped_loop_observer.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_RENDERER_MEDIA_SCOPED_LOOP_OBSERVER_H_
+#define CONTENT_RENDERER_MEDIA_SCOPED_LOOP_OBSERVER_H_
+#pragma once
+
+#include "base/memory/ref_counted.h"
+#include "base/message_loop.h"
+#include "base/message_loop_proxy.h"
+
+namespace base {
+class WaitableEvent;
+}
+
+// A common base class for AudioDevice and AudioInputDevice that manages
+// a message loop pointer and monitors it for destruction. If the object goes
+// out of scope before the message loop, the object will automatically remove
+// itself from the message loop's list of destruction observers.
+// NOTE: The class that inherits from this class must implement the
+// WillDestroyCurrentMessageLoop virtual method from DestructionObserver.
+class ScopedLoopObserver
+    : public MessageLoop::DestructionObserver {
+ public:
+  explicit ScopedLoopObserver(MessageLoop* message_loop);
+  virtual ~ScopedLoopObserver();
+
+ protected:
+  // Accessor to the loop that's used by the derived class.
+  base::MessageLoopProxy* message_loop() { return loop_; }
+
+ private:
+  // Call to add or remove ourselves from the list of destruction observers for
+  // the message loop.
+  void ObserveLoopDestruction(bool enable, base::WaitableEvent* done);
+
+  // A pointer to the message loop's proxy. In case the loop gets destroyed
+  // before this object goes out of scope, PostTask etc will fail but not crash.
+  scoped_refptr<base::MessageLoopProxy> loop_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedLoopObserver);
+};
+
+#endif  // CONTENT_RENDERER_MEDIA_SCOPED_LOOP_OBSERVER_H_