Adds support for AVAudioSessionSilenceSecondaryAudioHintNotification on iOS

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

Index: webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm
diff --git a/webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm b/webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm
index 87fa5b6cdd3dae8762822702f480c831353dcfc9..f6901844b5ae7a3b25cf0ee8c8b3aeed011cf673 100644
--- a/webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm
+++ b/webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm
@@ -66,7 +66,6 @@ NSInteger const kRTCAudioSessionErrorConfiguration = -2;
                selector:@selector(handleRouteChangeNotification:)
                    name:AVAudioSessionRouteChangeNotification
                  object:nil];
-    // TODO(tkchin): Maybe listen to SilenceSecondaryAudioHintNotification.
     [center addObserver:self
                selector:@selector(handleMediaServicesWereLost:)
                    name:AVAudioSessionMediaServicesWereLostNotification
@@ -75,6 +74,13 @@ NSInteger const kRTCAudioSessionErrorConfiguration = -2;
                selector:@selector(handleMediaServicesWereReset:)
                    name:AVAudioSessionMediaServicesWereResetNotification
                  object:nil];
+    // Posted on the main thread when the primary audio from other applications
+    // starts and stops. Foreground applications may use this notification as a
+    // hint to enable or disable audio that is secondary.
+    [center addObserver:self
+               selector:@selector(handleSilenceSecondaryAudioHintNotification:)
+                   name:AVAudioSessionSilenceSecondaryAudioHintNotification
+                 object:nil];
     // Also track foreground event in order to deal with interruption ended situation.
     [center addObserver:self
                selector:@selector(handleApplicationDidBecomeActive:)
@@ -516,6 +522,24 @@ NSInteger const kRTCAudioSessionErrorConfiguration = -2;
   [self notifyMediaServicesWereReset];
 }
 
+- (void)handleSilenceSecondaryAudioHintNotification:(NSNotification *)notification {
+  // TODO(henrika): just adding logs here for now until we know if we are ever
+  // see this notification and might be affected by it or if further actions
+  // are required.
+  NSNumber *typeNumber =
+      notification.userInfo[AVAudioSessionSilenceSecondaryAudioHintTypeKey];
+  AVAudioSessionSilenceSecondaryAudioHintType type =
+      (AVAudioSessionSilenceSecondaryAudioHintType)typeNumber.unsignedIntegerValue;
+  switch (type) {
+    case AVAudioSessionSilenceSecondaryAudioHintTypeBegin:
+      RTCLog(@"Another application's primary audio has started.");
+      break;
+    case AVAudioSessionSilenceSecondaryAudioHintTypeEnd:
+      RTCLog(@"Another application's primary audio has stopped.");
+      break;
+  }
+}
+
 - (void)handleApplicationDidBecomeActive:(NSNotification *)notification {
   if (self.isInterrupted) {
     RTCLog(@"Application became active after an interruption. Treating as interruption end.");