Listen for OutputNodeVolumeChanged signal

chromeos/audio/cras_audio_handler.cc

 // Copyright (c) 2013 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 "chromeos/audio/cras_audio_handler.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 544 matching lines @@
   // restarts after crashing.
   LogErrors();
   InitializeAudioState();
 }
 
 void CrasAudioHandler::NodesChanged() {
   if (cras_service_available_)
     GetNodes();
 }
 
+void CrasAudioHandler::OutputNodeVolumeChanged(uint64_t node_id, int volume) {
+  const AudioDevice* device = this->GetDeviceFromId(node_id);
+  int old_volume;
+
+  if (!device || device->is_input)
+    return;
+
+  // If this callback is triggered by a response to previous set volume command,
+  // do nothing.
+  old_volume =
+      static_cast<int>(audio_pref_handler_->GetOutputVolumeValue(device));
+  if (old_volume == volume)
+    return;
+
+  // Otherwise another app or the hardware itself just changed volume, update
+  // the new volume value.
+  SetOutputNodeVolumePercent(node_id, volume);

[jennyz, 2016/05/06 18:11:08]

This will have a glitch for the multiple active nodes case, since it will leave
other active nodes with a different volume node, which might not be the case you
want. You should call CrasAudioHandler::SetOutputVolumePercent to make all the
active nodes set to be same volume level, which is CrasAudioHandler and chromeOS
UI currently assumed.

[hychao, 2016/05/12 08:06:27]

Done.

+}
+
 void CrasAudioHandler::ActiveOutputNodeChanged(uint64_t node_id) {
   if (active_output_node_id_ == node_id)
     return;
 
   // Active audio output device should always be changed by chrome.
   // During system boot, cras may change active input to unknown device 0x1,
   // we don't need to log it, since it is not an valid device.
   if (GetDeviceFromId(node_id)) {
     LOG_IF(WARNING, log_errors_)
         << "Active output node changed unexpectedly by system node_id="
@@ skipping 703 matching lines @@
                      hdmi_rediscover_grace_period_duration_in_ms_),
       this, &CrasAudioHandler::UpdateAudioAfterHDMIRediscoverGracePeriod);
 }
 
 void CrasAudioHandler::SetHDMIRediscoverGracePeriodForTesting(
     int duration_in_ms) {
   hdmi_rediscover_grace_period_duration_in_ms_ = duration_in_ms;
 }
 
 }  // namespace chromeos