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 574 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 this is not an active output node, ignore this event. Because when this
+  // node set to active, it will be applied with the volume value stored in
+  // preference.
+  if (!device || !device->active || 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 to all active output nodes.
+  SetOutputVolumePercent(volume);
+}
+
 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