Implemented the GetSources() in native code.

webrtc/api/rtpreceiverinterface.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.
  */
 
 // This file contains interfaces for RtpReceivers
 // http://w3c.github.io/webrtc-pc/#rtcrtpreceiver-interface
 
 #ifndef WEBRTC_API_RTPRECEIVERINTERFACE_H_
 #define WEBRTC_API_RTPRECEIVERINTERFACE_H_
 
+#include <memory>
 #include <string>
+#include <vector>
 
 #include "webrtc/api/mediatypes.h"
 #include "webrtc/api/mediastreaminterface.h"
 #include "webrtc/api/proxy.h"
 #include "webrtc/api/rtpparameters.h"
 #include "webrtc/base/refcount.h"
 #include "webrtc/base/scoped_ref_ptr.h"
 
 namespace webrtc {
 
+struct RtpContributingSource {
+  RtpContributingSource() : timestamp(0), source(0) {}
+
+  RtpContributingSource(int64_t time, uint32_t src)
+      : timestamp(time), source(src) {}
+
+  int64_t timestamp;
+  // Can be either the SSRC or the CSRC.
+  uint32_t source;
+  // This isn't implemented yet and will always return an empty Optional.
+  // TODO(zhihuang): Implement this to return real audio level.
+  rtc::Optional<int8_t> audio_level;

[Taylor Brandstetter, 2017/03/31 22:10:50]

nit: I'd still be in favor of using getters/setters. For example: in the future
we could decide it would be helpful to return both the CSRC and the associated
SSRC in the same object. In which case it may change to:

// Returns the CSRC or SSRC depending on what the object represents.
uint32_t source() {
  return csrc_ ? *csrc_ : ssrc_;
}

uint32_t ssrc() { 
  return ssrc_;
}

private:

// Set if this object represents a CSRC, unset if it
// represents an SSRC.
rtc::Optional<uint32_t> csrc_;
uint32_t ssrc_;


If the data members were private, it would be easy to make this change.

[Zhi Huang, 2017/04/04 04:24:33]

Done.

+};
+
 class RtpReceiverObserverInterface {
  public:
   // Note: Currently if there are multiple RtpReceivers of the same media type,
   // they will all call OnFirstPacketReceived at once.
   //
   // In the future, it's likely that an RtpReceiver will only call
   // OnFirstPacketReceived when a packet is received specifically for its
   // SSRC/mid.
   virtual void OnFirstPacketReceived(cricket::MediaType media_type) = 0;
 
@@ skipping 16 matching lines @@
   // but this API also applies them to receivers, similar to ORTC:
   // http://ortc.org/wp-content/uploads/2016/03/ortc.html#rtcrtpparameters*.
   virtual RtpParameters GetParameters() const = 0;
   // Currently, doesn't support changing any parameters, but may in the future.
   virtual bool SetParameters(const RtpParameters& parameters) = 0;
 
   // Does not take ownership of observer.
   // Must call SetObserver(nullptr) before the observer is destroyed.
   virtual void SetObserver(RtpReceiverObserverInterface* observer) = 0;
 
+  // TODO(zhihuang): Remove the default implementation once the subclasses
+  // implement this. Currently, the only relevant subclass is the
+  // content::FakeRtpReceiver in Chromium.
+  virtual std::vector<RtpContributingSource> GetContributingSources() {
+    return std::vector<RtpContributingSource>();
+  }
+
  protected:
   virtual ~RtpReceiverInterface() {}
 };
 
 // Define proxy for RtpReceiverInterface.
 // TODO(deadbeef): Move this to .cc file and out of api/. What threads methods
 // are called on is an implementation detail.
 BEGIN_SIGNALING_PROXY_MAP(RtpReceiver)
   PROXY_SIGNALING_THREAD_DESTRUCTOR()
   PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)
   PROXY_CONSTMETHOD0(cricket::MediaType, media_type)
   PROXY_CONSTMETHOD0(std::string, id)
   PROXY_CONSTMETHOD0(RtpParameters, GetParameters);
   PROXY_METHOD1(bool, SetParameters, const RtpParameters&)
   PROXY_METHOD1(void, SetObserver, RtpReceiverObserverInterface*);
-END_PROXY_MAP()
+  PROXY_METHOD0(std::vector<RtpContributingSource>, GetContributingSources);
+  END_PROXY_MAP()
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_RTPRECEIVERINTERFACE_H_