Add the OnAddTrack callback for Objective-C wrapper.

webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm

Index: webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
index b4a873847de5bda226a1c72d23a48b7eff97f1bb..708ffd32ea2671ebd663a0245803a534840b846e 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
@@ -127,9 +127,8 @@ void PeerConnectionDelegateAdapter::OnSignalingChange(
 
 void PeerConnectionDelegateAdapter::OnAddStream(
     rtc::scoped_refptr<MediaStreamInterface> stream) {
-  RTCMediaStream *mediaStream =
-      [[RTCMediaStream alloc] initWithNativeMediaStream:stream];
   RTCPeerConnection *peer_connection = peer_connection_;
+  RTCMediaStream *mediaStream = [peer_connection GetOrCreateObjcStream:stream];
   [peer_connection.delegate peerConnection:peer_connection
                               didAddStream:mediaStream];
 }
@@ -200,6 +199,25 @@ void PeerConnectionDelegateAdapter::OnIceCandidatesRemoved(
                     didRemoveIceCandidates:ice_candidates];
 }
 
+void PeerConnectionDelegateAdapter::OnAddTrack(
+    rtc::scoped_refptr<RtpReceiverInterface> receiver,
+    std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) {
+  RTCRtpReceiver *rtpReceiver =
+      [[RTCRtpReceiver alloc] initWithNativeRtpReceiver:receiver];
+  NSMutableArray *mediaStreams =
+      [NSMutableArray arrayWithCapacity:streams.size()];
+
+  RTCPeerConnection *peer_connection = peer_connection_;
+  for (const auto stream : streams) {
+    RTCMediaStream *mediaStream =
+        [peer_connection GetOrCreateObjcStream:stream];
+    [mediaStreams addObject:mediaStream];
+  }
+  [peer_connection.delegate peerConnection:peer_connection
+                               didAddTrack:rtpReceiver
+                           attachedStreams:mediaStreams];
+}
+
 }  // namespace webrtc
 
 
@@ -208,6 +226,7 @@ void PeerConnectionDelegateAdapter::OnIceCandidatesRemoved(
   std::unique_ptr<webrtc::PeerConnectionDelegateAdapter> _observer;
   rtc::scoped_refptr<webrtc::PeerConnectionInterface> _peerConnection;
   BOOL _hasStartedRtcEventLog;
+  NSMutableDictionary *_nativeToObjcMediaStream;
 }
 
 @synthesize delegate = _delegate;
@@ -237,6 +256,7 @@ void PeerConnectionDelegateAdapter::OnIceCandidatesRemoved(
     }
     _localStreams = [[NSMutableArray alloc] init];
     _delegate = delegate;
+    _nativeToObjcMediaStream = [NSMutableDictionary dictionary];
   }
   return self;
 }
@@ -585,6 +605,19 @@ void PeerConnectionDelegateAdapter::OnIceCandidatesRemoved(
   }
 }
 
+- (RTCMediaStream *)GetOrCreateObjcStream:
+    (rtc::scoped_refptr<webrtc::MediaStreamInterface>)stream {
+  RTCMediaStream *mediaStream =
+      _nativeToObjcMediaStream[[NSNumber numberWithLong:(long)stream.get()]];
+  if (!mediaStream) {
+    mediaStream =
+        [[RTCMediaStream alloc] initWithNativeMediaStream:stream.get()];
+    _nativeToObjcMediaStream[[NSNumber numberWithLong:(long)stream.get()]] =
+        mediaStream;
+  }
+  return mediaStream;
+}
+
 - (rtc::scoped_refptr<webrtc::PeerConnectionInterface>)nativePeerConnection {
   return _peerConnection;
 }