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 {
 
+class RtpContributingSourceInterface {
+ public:
+  virtual ~RtpContributingSourceInterface() {}
+
+  virtual int64_t timestamp() const = 0;
+
+  // Can be either the SSRC or the CSRC.
+  virtual uint32_t source() const = 0;
+
+  virtual int8_t audio_level() const = 0;

[hbos, 2017/03/30 09:51:54]

This should be rtc::Optional<...>. 0 is a meaningful value. Otherwise, if a
negative value is supposed to mean undefined then document that, but I think
rtc::Optional is self-documenting.

Even if an implementation supports this field it would only be used if the RTP
header extension is used and might be undefined, if I understand this correctly.

Also: int8_t or uint8_t?

[Taylor Brandstetter, 2017/03/30 22:55:37]

It only ranges from 0 to 127. So might as well make it an int8_t, since we
prefer signed types by default.

I'd also suggest adding a comment making it clear that this isn't implemented
yet, and will always return an empty Optional.

+};
+
 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;
 
+  virtual std::vector<std::unique_ptr<RtpContributingSourceInterface>>

[Taylor Brandstetter, 2017/03/30 22:55:37]

Since this looks so similar to the JS API, but behaves differently, there should
be a comment that documents the differences. Namely:

* Each time the method is called, a vector of new objects is created. If the
application wants to identify them across calls, it should use the "source()"
value, not the value of the pointer.
* The returned RtpContributingSource objects don't get updated automatically
when more packets are received, as will happen with the JS
RTCRtpContributingSource objects. They're just structs, basically.

This confusion may be a good reason to just return a vector of structs, instead
of unique_ptrs wrapping an interface. In short, the issue is that returning a
pointer to an abstract class may make API users think that a "smart" object is
being returned, when really it isn't.

[Zhi Huang, 2017/03/31 06:44:04]

Yeah, I agree that returning plain structs would be more clear.

Done.

[the sun, 2017/03/31 07:03:49]

Perhaps that is the guideline we were looking for? :)

+  GetContributingSources() = 0;
+
  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<std::unique_ptr<RtpContributingSourceInterface>>,
+                GetContributingSources);
+  END_PROXY_MAP()
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_RTPRECEIVERINTERFACE_H_