Add set_sender_codec

Author: mickel8

lib/ex_webrtc/peer_connection.ex

@@ -9,6 +9,7 @@ defmodule ExWebRTC.PeerConnection do
 
   require Logger
 
+  alias ExWebRTC.RTPCodecParameters
   alias __MODULE__.{Configuration, Demuxer, TWCCRecorder}
 
   alias ExWebRTC.{
@@ -152,9 +153,58 @@ defmodule ExWebRTC.PeerConnection do
     GenServer.call(peer_connection, :get_configuration)
   end
 
+  @doc """
+  Sets the codec that will be used for sending RTP packets.
+
+  `send_rtp/4` overrides some of the RTP packet fields.
+  In particular, when multiple codecs are negotiated, `send_rtp/4` will use
+  payload type of the most preffered by the remote side codec (i.e. the first
+  one from the list of codecs in the remote description).
+
+  Use this function if you want to select, which codec (hence payload type)
+  should be used for sending.
+
+  Although very unlikely, keep in mind that after renegotiation,
+  the selected codec may no longer be supported by the remote side and you might
+  need to call this function again, passing a new codec.
+
+  To check available codecs you can use `get_transceivers/1`:
+
+  ```
+  {:ok, pc} = PeerConnection.start_link()
+  {:ok, rtp_sender} = PeerConnection.add_track(MediaStreamTrack.new(:video))
+
+  tr =
+    pc
+    |> PeerConnection.get_transceiver()
+    |> Enum.find(fn tr -> tr.sender.id == rtp_sender.id end)
+
+  dbg(tr.codecs) # list of supported codecs both for sending and receiving
+
+  # e.g. always prefer h264 over vp8
+  h264 = Enum.find(tr.codecs, fn codec -> codec.mime_type == "video/H264" end)
+  vp8 = Enum.find(tr.codecs, fn codec -> codec.mime_type == "video/VP8" end)
+
+  :ok = PeerConnection.set_sender_codec(pc, rtp_sender.id, h264 || vp8)
+  ```
+  """
+  @spec set_sender_codec(peer_connection(), RTPSender.id(), RTPCodecParameters.t()) ::
+          :ok | {:error, term()}
+  def set_sender_codec(peer_connection, sender_id, codec) do
+    GenServer.call(peer_connection, {:set_sender_codec, sender_id, codec})
+  end
+
   @doc """
   Sends an RTP packet to the remote peer using the track specified by the `track_id`.
 
+  The following fields of the RTP packet will be overwritten by this function:
+  * payload type
+  * ssrc
+  * rtp header extensions
+
+  If you negotiated multiple codecs (hence payload types) and you want to choose,
+  which one should be used, see `set_sender_codec/3`.
+
   Options:
     * `rtx?` - send the packet as if it was retransmitted (use SSRC and payload type specific to RTX)
   """