Handle audio device changes on Mac (Take 2).

media/audio/mac/audio_device_listener_mac.h

Index: media/audio/mac/audio_device_listener_mac.h
diff --git a/media/audio/mac/audio_device_listener_mac.h b/media/audio/mac/audio_device_listener_mac.h
new file mode 100644
index 0000000000000000000000000000000000000000..21bf10511900ee99badf42987f6c670a2a903d7f
--- /dev/null
+++ b/media/audio/mac/audio_device_listener_mac.h
@@ -0,0 +1,37 @@
+// 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 MEDIA_AUDIO_MAC_AUDIO_DEVICE_LISTENER_MAC_H_
+#define MEDIA_AUDIO_MAC_AUDIO_DEVICE_LISTENER_MAC_H_
+
+#include "base/callback.h"
+#include "base/gtest_prod_util.h"
+#include "base/threading/thread_checker.h"
+#include "media/base/media_export.h"
+
+namespace media {
+
+// Listens for default device changes and forwards them to AudioManagerMac so it
+// can notify downstream clients.  Only output device changes are supported
+// currently.
+class MEDIA_EXPORT AudioDeviceListenerMac {
+ public:
+  // The listener callback may be called from a system level multimedia thread,
+  // thus the callee must be thread safe.  |listener_cb| is a permanent callback
+  // and must outlive AudioDeviceListenerWin.  A |listener_cb| call may be in
+  // progress while AudioDeviceListenerMac is destroyed, but no new calls will
+  // occur after destruction.
+  explicit AudioDeviceListenerMac(base::Closure* listener_cb);
+  ~AudioDeviceListenerMac();
+
+ private:
+  base::Closure* listener_cb_;

[scherkus (not reviewing), 2012/10/31 23:17:22]

what the!? just make a copy

[DaleCurtis, 2012/11/01 00:16:33]

Nope, we want the callback to outlive AudioDeviceListenerMac, if we reference a
copy and a callback is in progress during destruction it'll use-after-free.

[scherkus (not reviewing), 2012/11/01 00:47:48]

This concern still feels hinged on not knowing if it's possible to have the
callback execute post-removing-the-listener.

+  base::ThreadChecker thread_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(AudioDeviceListenerMac);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_AUDIO_MAC_AUDIO_DEVICE_LISTENER_MAC_H_