MediaSwitcher class

A Javascript class that allows the switching of video and audio tracks in a MediaStream.

What does it do?

JavaScript Media and Streaming API's MediaRecorder has a problem. It can not switch streams or stream tracks while recording. Recording stops if this is attempted.

MediaSwitcher acts as an intermediary between your stream sources and the MediaRecorder. It connects to the MediaRecorder as a single, uninterrupted stream source, and transmits your various other stream sources (video, audio, etc.) to the recorder. As a result, you can switch between contents.

How does it work?

The class is the implementation of a long known workaround for the above problem. It creates an RTCPeerConnection with itself and sends your stream through it. The output is actually the receiving peer's stream.

Live demo

Demo: https://dhk6x2.csb.app/

Sandbox: https://codesandbox.io/s/dhk6x2

Installation

You would've never guessed:

npm i media-switcher

and then:

const MediaSwitcher = require('media-switcher');
const mediaSwitcher = new MediaSwitcher();

Alternatively you can include MediaSwitcher.class.js the traditional way:

<script src="MediaSwitcher.class.js" />

Methods

initialize(inputStream: MediaStream): Promise(outputStream: MediaStream)

Initializes a MediaSwitcher. The stream can be an empty MediaStream and you can add tracks later. Returns a Promise, which resolves to the MediaStream coming from the MediaSwitcher. Use this stream for your MediaRecorder.

changeTrack(track: MediaStreamTrack | CanvasCaptureMediaStreamTrack)

Changes the current audio or video track to the supplied one. The kind of the track will be autodetected.

changeStream(stream: MediaStream)

Changes both the audio and the video tracks to the one in the supplied stream.

Examples

Let's say you have two <video> elements in your HTML DOM:

<video id="inputVideo" loop controls src="nyancat.mp4" />
<video id="outputVideo" muted />

The latter is muted so we won't hear two audio streams.

Let's connect the stream from inputVideo to outputVideo:

//  Get videos
const inputVideo = document.getElementById('inputVideo');
const outputVideo = document.getElementById('outputVideo');

//  Create a stream source
var myStream = inputVideo.captureStream();

//  Create instance
var mediaSwitcher = new MediaSwitcher();

//  Initialize MediaSwitcher
mediaSwitcher.initialize(myStream)
    .then(switcherStream => {
        outputVideo.srcObject = switcherStream;
        outputVideo.play();
    })
    .catch(err => console.error(err.message));

Now, if you play inputVideo, its content will appear in outputVideo, but it passes through the MediaSwitcher.

Switching streams

Let's change the audio track to a different one, coming from an MP3 file.

<video id="inputVideo" loop controls src="nyancat.mp4" />
<video id="outputVideo" muted />
<audio id="inputAudio" loop controls src="erika.mp3" />

To achieve this, add this to the code:

const inputAudio = document.getElementById('inputAudio');

inputAudio.onplay = () => {
    inputVideo.muted = true;
    const track = inputAudio.srcObject.getAudioStreams()[0];
    mediaSwitcher.changeTrack(track);
}

Now when you click the Play button on the audio element, the MP3 will replace the video's original soundtrack, while the video keeps playing.