Restoring behavior where PeerConnection tracks changes to MediaStreams.

talk/app/webrtc/mediastreamobserver.cc

Index: talk/app/webrtc/mediastreamobserver.cc
diff --git a/talk/app/webrtc/mediastreamobserver.cc b/talk/app/webrtc/mediastreamobserver.cc
index a8561647b2f59744f6bef8bb5dd3110854a85518..2650b9a6f79e3825bc1c7f68b341d346d0f9f416 100644
--- a/talk/app/webrtc/mediastreamobserver.cc
+++ b/talk/app/webrtc/mediastreamobserver.cc
@@ -27,4 +27,75 @@
 
 #include "talk/app/webrtc/mediastreamobserver.h"
 
-// This file is currently stubbed so that Chromium's build files can be updated.
+#include <algorithm>
+
+namespace webrtc {
+
+MediaStreamObserver::MediaStreamObserver(MediaStreamInterface* stream)
+    : stream_(stream),
+      cached_audio_tracks_(stream->GetAudioTracks()),
+      cached_video_tracks_(stream->GetVideoTracks()) {
+  stream_->RegisterObserver(this);
+}
+
+MediaStreamObserver::~MediaStreamObserver() {
+  stream_->UnregisterObserver(this);
+}
+
+void MediaStreamObserver::OnChanged() {
+  AudioTrackVector new_audio_tracks = stream_->GetAudioTracks();
+  VideoTrackVector new_video_tracks = stream_->GetVideoTracks();
+
+  // Find removed audio tracks.
+  for (const auto& cached_track : cached_audio_tracks_) {
+    auto it = std::find_if(
+        new_audio_tracks.begin(), new_audio_tracks.end(),
+        [cached_track](const AudioTrackVector::value_type& new_track) {
+          return new_track->id().compare(cached_track->id()) == 0;
+        });
+    if (it == new_audio_tracks.end()) {
+      SignalAudioTrackRemoved(cached_track.get(), stream_);
+    }
+  }
+
+  // Find added audio tracks.
+  for (const auto& new_track : new_audio_tracks) {
+    auto it = std::find_if(
+        cached_audio_tracks_.begin(), cached_audio_tracks_.end(),
+        [new_track](const AudioTrackVector::value_type& cached_track) {
+          return new_track->id().compare(cached_track->id()) == 0;
+        });
+    if (it == cached_audio_tracks_.end()) {
+      SignalAudioTrackAdded(new_track.get(), stream_);
+    }
+  }
+
+  // Find removed video tracks.
+  for (const auto& cached_track : cached_video_tracks_) {
+    auto it = std::find_if(
+        new_video_tracks.begin(), new_video_tracks.end(),
+        [cached_track](const VideoTrackVector::value_type& new_track) {
+          return new_track->id().compare(cached_track->id()) == 0;
+        });
+    if (it == new_video_tracks.end()) {
+      SignalVideoTrackRemoved(cached_track.get(), stream_);
+    }
+  }
+
+  // Find added video tracks.
+  for (const auto& new_track : new_video_tracks) {
+    auto it = std::find_if(
+        cached_video_tracks_.begin(), cached_video_tracks_.end(),
+        [new_track](const VideoTrackVector::value_type& cached_track) {
+          return new_track->id().compare(cached_track->id()) == 0;
+        });
+    if (it == cached_video_tracks_.end()) {
+      SignalVideoTrackAdded(new_track.get(), stream_);
+    }
+  }
+
+  cached_audio_tracks_ = new_audio_tracks;
+  cached_video_tracks_ = new_video_tracks;
+}
+
+}  // namespace webrtc