Added a callback function OnAddTrack to PeerConnectionObserver

webrtc/api/peerconnectioninterface.h

 /*
  *  Copyright 2012 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 575 matching lines @@
 
   // Ice candidates have been removed.
   // TODO(honghaiz): Make this a pure virtual method when all its subclasses
   // implement it.
   virtual void OnIceCandidatesRemoved(
       const std::vector<cricket::Candidate>& candidates) {}
 
   // Called when the ICE connection receiving status changes.
   virtual void OnIceConnectionReceivingChange(bool receiving) {}
 
+  // Called when a track is added to streams.
+  // TODO(zhihuang) Make this a pure virtual method when all its subclasses
+  // implement it.
+  virtual void OnAddTrack(

[honghaiz3, 2016/11/17 01:19:02]

Should we also Add OnRemoveTrack?
If users expect to be notified when tracks are added, they may also expect to be
notified when tracks are removed. 

[Zhi Huang, 2016/11/17 01:42:06]

According to spec https://www.w3.org/TR/webrtc/#event-track, the event is fired
when "A new incoming MediaStreamTrack has been created, and an associated
RTCRtpReceiver has been added to the set of receivers.". 

I agree with you that it would make sense to have another callback. But I think
that can comes later in a separate CL, we can land this first so that this won't
be a blocker for both iOS and android.

+      rtc::scoped_refptr<RtpReceiverInterface> receiver,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) {}
+
  protected:
   // Dtor protected as objects shouldn't be deleted via this interface.
   ~PeerConnectionObserver() {}
 };
 
 // PeerConnectionFactoryInterface is the factory interface use for creating
 // PeerConnection, MediaStream and media tracks.
 // PeerConnectionFactoryInterface will create required libjingle threads,
 // socket and network manager factory classes for networking.
 // If an application decides to provide its own threads and network
@@ skipping 151 matching lines @@
     cricket::WebRtcVideoEncoderFactory* encoder_factory,
     cricket::WebRtcVideoDecoderFactory* decoder_factory) {
   return CreatePeerConnectionFactory(
       worker_and_network_thread, worker_and_network_thread, signaling_thread,
       default_adm, encoder_factory, decoder_factory);
 }
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_PEERCONNECTIONINTERFACE_H_