Feat/use track method (#22)

* feat: setUnderlyingTrack method

* feat: useTrack method

* feat: track emits event to be handled by peer connection for new track

* fix: change method name to 'replaceUnderlyingTrack'

* fix: changed 'trackId' to 'oldTrackId'

* style: alphabetize local-track methods

src/local-track.ts

@@ -4,6 +4,19 @@ import { Track } from './track';
  * A wrapper around MediaStreamTrack.
  */
 export class LocalTrack extends Track {
+  /**
+   * Exchange the underlying track with a new track. Emit the `track-update` event to alert
+   * listeners to an update on a new track.
+   *
+   * @param track - New underlying track.
+   * @fires LocalTrack#track-update
+   */
+  replaceUnderlyingTrack(track: MediaStreamTrack): void {
+    this.emit('track-update', this.getUnderlyingTrack().id, track);
+    this.getUnderlyingTrack().stop();
+    this.setUnderlyingTrack(track);
+  }
+
   /**
    * Sets the track to be enabled or disabled.
    *

src/peer-connection.ts

@@ -14,6 +14,9 @@ class PeerConnection {
     log('PeerConnection init');
 
     this.pc = new RTCPeerConnection();
+
+    // Bind event handlers.
+    this.handleTrackUpdate = this.handleTrackUpdate.bind(this);
   }
 
   /**
@@ -23,8 +26,10 @@ class PeerConnection {
    * @param streams - (Optional) One or more local MediaStream objects to which the track should be
    *     added.
    * @returns The RTCRtpSender object which will be used to transmit the media data.
+   * @listens LocalTrack#track-update
    */
   addTrack(track: LocalTrack, ...streams: MediaStream[]): RTCRtpSender {
+    track.on('track-update', this.handleTrackUpdate);
     return this.pc.addTrack(track.getUnderlyingTrack(), ...streams);
   }
 
@@ -91,6 +96,17 @@ class PeerConnection {
   close(): void {
     this.pc.close();
   }
+
+  /**
+   * Handles `track-update` event and replaces track on sender.
+   *
+   * @param oldTrackId - Id of the existing track.
+   * @param newTrack - New LocalTrack.
+   */
+  handleTrackUpdate(oldTrackId: string, newTrack: MediaStreamTrack): void {
+    const sender = this.pc.getSenders().find((s: RTCRtpSender) => s.track?.id === oldTrackId);
+    sender?.replaceTrack(newTrack);
+  }
 }
 
 export { PeerConnection };

src/track.ts

@@ -28,4 +28,13 @@ export abstract class Track extends EventEmitter {
   getUnderlyingTrack(): MediaStreamTrack {
     return this.track;
   }
+
+  /**
+   * Sets the underlying track.
+   *
+   * @param track - The new underlying track.
+   */
+  setUnderlyingTrack(track: MediaStreamTrack): void {
+    this.track = track;
+  }
 }