Add a method to tab_utils.h to find out whether a tab is playing audio.

content/browser/renderer_host/media/audio_renderer_host.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 "content/browser/renderer_host/media/audio_renderer_host.h"
 
 #include "base/bind.h"
 #include "base/metrics/histogram.h"
 #include "base/process.h"
 #include "base/shared_memory.h"
 #include "content/browser/browser_main_loop.h"
 #include "content/browser/media/media_internals.h"
 #include "content/browser/renderer_host/media/audio_mirroring_manager.h"
 #include "content/browser/renderer_host/media/audio_sync_reader.h"
 #include "content/common/media/audio_messages.h"
+#include "content/public/browser/content_browser_client.h"
+#include "content/public/browser/media_observer.h"
 #include "media/audio/shared_memory_util.h"
 #include "media/base/audio_bus.h"
 #include "media/base/limits.h"
 
 using media::AudioBus;
 
 namespace content {
 
 struct AudioRendererHost::AudioEntry {
   AudioEntry();
@@ skipping 298 matching lines @@
 
   AudioEntry* const entry = LookupById(stream_id);
   if (!entry) {
     SendErrorMessage(stream_id);
     return;
   }
 
   if (entry->render_view_id == render_view_id)
     return;
 
+  // TODO(miu): Merge "AssociateWithProducer" message into "CreateStream"
+  // message so AudioRendererHost can assume a simpler "render_view_id is set
+  // once" scheme. http://crbug.com/166779
   if (mirroring_manager_) {
     mirroring_manager_->RemoveDiverter(
         render_process_id_, entry->render_view_id, entry->controller);
   }
   entry->render_view_id = render_view_id;
   if (mirroring_manager_) {
     mirroring_manager_->AddDiverter(
         render_process_id_, entry->render_view_id, entry->controller);
   }
 }
 
 void AudioRendererHost::OnPlayStream(int stream_id) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
 
   AudioEntry* entry = LookupById(stream_id);
   if (!entry) {
     SendErrorMessage(stream_id);
     return;
   }
 
   entry->controller->Play();
   if (media_internals_)
     media_internals_->OnSetAudioStreamPlaying(this, stream_id, true);
+
+  MediaObserver* media_observer =
+      GetContentClient()->browser()->GetMediaObserver();
+  if (media_observer) {
+    media_observer->OnAudioStreamPlayingChanged(
+        render_process_id_, entry->render_view_id, true);
+  }
 }
 
 void AudioRendererHost::OnPauseStream(int stream_id) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
 
   AudioEntry* entry = LookupById(stream_id);
   if (!entry) {
     SendErrorMessage(stream_id);
     return;
   }
 
   entry->controller->Pause();
   if (media_internals_)
     media_internals_->OnSetAudioStreamPlaying(this, stream_id, false);
+
+  MediaObserver* media_observer =
+      GetContentClient()->browser()->GetMediaObserver();
+  if (media_observer) {
+    media_observer->OnAudioStreamPlayingChanged(
+        render_process_id_, entry->render_view_id, true);

[miu, 2013/02/05 23:18:25]

I think you meant s/true/false/ here.

[miu, 2013/02/05 23:18:25]

Also, note that streams are not necessarily paused before they are closed (or
error out).  You'll somehow need to call
MediaObserver::OnAudioStreamPlayingChanged(..., false) in those cases.  Probably
in AudioRendererHost::CloseAndDeleteStream().

...and the unfortunate news here is: How will you balance the number of
OnAudioStreamPlayingChanged(true) with OnAudioStreamPlayingChanged(false) calls?

[Bernhard Bauer, 2013/02/08 16:44:14]

Urr, yes. Done.

[Bernhard Bauer, 2013/02/08 16:44:14]

Hm. I changed it to a map from RenderView ID to a set of streams playing.

+  }
 }
 
 void AudioRendererHost::OnFlushStream(int stream_id) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
 
   AudioEntry* entry = LookupById(stream_id);
   if (!entry) {
     SendErrorMessage(stream_id);
     return;
   }
@@ skipping 23 matching lines @@
     SendErrorMessage(stream_id);
     return;
   }
 
   // Make sure the volume is valid.
   if (volume < 0 || volume > 1.0)
     return;
   entry->controller->SetVolume(volume);
   if (media_internals_)
     media_internals_->OnSetAudioStreamVolume(this, stream_id, volume);
 }

[miu, 2013/02/05 23:18:25]

If volume is set to zero (i.e., mute), or set back from non-zero, you will want
to call MediaObserver::OnAudioStreamPlayingChanged() here.

[Bernhard Bauer, 2013/02/08 16:44:14]

Done.

 
 void AudioRendererHost::SendErrorMessage(int32 stream_id) {
   Send(new AudioMsg_NotifyStreamStateChanged(
       stream_id, media::AudioOutputIPCDelegate::kError));
 }
 
 void AudioRendererHost::DeleteEntries() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
 
   for (AudioEntryMap::iterator i = audio_entries_.begin();
@@ skipping 65 matching lines @@
   return NULL;
 }
 
 media::AudioOutputController* AudioRendererHost::LookupControllerByIdForTesting(
     int stream_id) {
   AudioEntry* const entry = LookupById(stream_id);
   return entry ? entry->controller : NULL;
 }
 
 }  // namespace content