Add observer for AVAudioSession.outputVolume

webrtc/modules/audio_device/ios/objc/RTCAudioSession.h

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 56 matching lines @@
 /** Called on a WebRTC thread when the audio device is notified to begin
  *  playback or recording.
  */
 - (void)audioSessionDidStartPlayOrRecord:(RTCAudioSession *)session;
 
 /** Called on a WebRTC thread when the audio device is notified to stop
  *  playback or recording.
  */
 - (void)audioSessionDidStopPlayOrRecord:(RTCAudioSession *)session;
 
+/** Called when the AVAudioSession output volume value changes. */
+- (void)audioSession:(RTCAudioSession *)audioSession
+    didChangeOutputVolume:(float)outputVolume;
+
 @end
 
 /** This is a protocol used to inform RTCAudioSession when the audio session
  *  activation state has changed outside of RTCAudioSession. The current known use
  *  case of this is when CallKit activates the audio session for the application
  */
 RTC_EXPORT
 @protocol RTCAudioSessionActivationDelegate <NSObject>
 
 /** Called when the audio session is activated outside of the app by iOS. */
@@ skipping 142 matching lines @@
 /** Convenience method that calls both setConfiguration and setActive.
  *  |lockForConfiguration| must be called first.
  */
 - (BOOL)setConfiguration:(RTCAudioSessionConfiguration *)configuration
                   active:(BOOL)active
                    error:(NSError **)outError;
 
 @end
 
 NS_ASSUME_NONNULL_END