Add the OnAddTrack callback for Objective-C wrapper.

webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+Private.h

 /*
  *  Copyright 2015 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 31 matching lines @@
       PeerConnectionInterface::IceConnectionState new_state) override;
 
   void OnIceGatheringChange(
       PeerConnectionInterface::IceGatheringState new_state) override;
 
   void OnIceCandidate(const IceCandidateInterface *candidate) override;
 
   void OnIceCandidatesRemoved(
       const std::vector<cricket::Candidate>& candidates) override;
 
+  void OnAddTrack(

[tkchin_webrtc, 2016/11/22 20:41:34]

what is the distinction between track and receiver? Strange that we're getting
callback that a track is being added, but instead get a receiver.

[Zhi Huang, 2016/11/22 23:38:42]

There is one track attached to one RtpReceiver.

According to the spec, we need to provide RtpRecceiver, Track, MediaStreams and
Transceivers.

We can get the Track from RtpReceiver and we haven't implemented the
Transceivers in native C++ yet. This is why we have these two parameters in the
call back.

+      rtc::scoped_refptr<RtpReceiverInterface> receiver,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) override;
+
  private:
-  __weak RTCPeerConnection *peer_connection_;
+  RTCMediaStream* GetOrCreateObjcStream(
+      rtc::scoped_refptr<MediaStreamInterface> stream);
+
+  __weak RTCPeerConnection* peer_connection_;
+  NSMutableDictionary* nativeToObjcMediaStream_ =
+      [NSMutableDictionary dictionary];

[tkchin_webrtc, 2016/11/22 20:41:34]

Shouldn't this be initialized in ctor?

[Zhi Huang, 2016/11/22 23:38:42]

Done.

 };
 
 } // namespace webrtc
 
 
 @interface RTCPeerConnection ()
 
 /** The native PeerConnectionInterface created during construction. */
 @property(nonatomic, readonly)
     rtc::scoped_refptr<webrtc::PeerConnectionInterface> nativePeerConnection;
@@ skipping 34 matching lines @@
     (webrtc::PeerConnectionInterface::IceGatheringState)nativeState;
 
 + (NSString *)stringForIceGatheringState:(RTCIceGatheringState)state;
 
 + (webrtc::PeerConnectionInterface::StatsOutputLevel)
     nativeStatsOutputLevelForLevel:(RTCStatsOutputLevel)level;
 
 @end
 
 NS_ASSUME_NONNULL_END