new MediaRecorder(stream[, options]) stream can living modify? - mediarecorder

Question

new MediaRecorder(stream[, options]);
I want record the user camera and audio
I need mixing the song.mp3 to the audio track in recording.
and result export a video file to download by link.
But the MediaRecorder first params stream can living modify ?
But When I use recoder.stop()
It tips error: Failed to execute 'stop' on 'MediaRecorder': The MediaRecorder's state is 'inactive'.
My code:
function getFileBuffer(filepath) {
return fetch(filepath, {method: 'GET'}).then(response => response.arrayBuffer())
}
function mp3play() {
getFileBuffer('song.mp3')
.then(buffer => context.decodeAudioData(buffer))
.then(buffer => {
console.log(buffer)
const source = context.createBufferSource()
source.buffer = buffer
let volume = context.createGain()
volume.gain.value = 1
source.connect(volume)
dest = context.createMediaStreamDestination()
volume.connect(dest)
// volume.connect(context.destination)
source.start(0)
const _audioTrack = stream.getAudioTracks();
if (_audioTrack.length > 0) {
_audioTrack[0].stop();
stream.removeTrack(_audioTrack[0]);
}
//
// console.log(dest.stream)
// console.log(dest.stream.getAudioTracks()[0])
// stream.addTrack(dest.stream.getAudioTracks()[0])
})
}
function startRecording() {
recorder = new MediaRecorder(stream, {
mimeType: 'video/webm'
})
recorder.start()
stopBtn.removeAttribute('disabled')
startBtn.disabled = true
}

Answer 1

No we still can't record a MediaStream whose tracks are changed after the recording began, doing so will stop() the MediaRecorder. Here is a very related Q/A which was more about recording video.
What can be done though is to create a kind of merger MediaStream.
It's way easier with audio, moreover since you are already using the WebAudio API: all you need to do is to create an other MediaStreamDestination node, and connect / disconnect the different sources.
const base = "https://upload.wikimedia.org/wikipedia/en/d/";
const urls = [
"d3/Beach_Boys_-_Good_Vibrations.ogg",
"dc/Strawberry_Fields_Forever_%28Beatles_song_-_sample%29.ogg"
].map( url => base + url );
const context = new AudioContext();
const button = document.querySelector( 'button' );
button.onclick = async () => {
button.disabled = true;
context.resume();
const audiobuffers = await Promise.all( urls.map( fetchAsAudioBuffer ) );
button.remove();
const streamNode = context.createMediaStreamDestination();
const stream = streamNode.stream;
const recorder = new MediaRecorder( stream );
const chunks = [];
recorder.ondataavailable = evt => chunks.push( evt.data );
recorder.onstop = evt => exportAudio( new Blob( chunks ) );
document.getElementById( 'record-stopper' ).onclick = evt => {
recorder.stop();
current_source.stop( 0 );
};
let current_index = 0;
let current_source = null;
document.getElementById( 'switcher' ).onclick = switchAudioSource;
switchAudioSource();
recorder.start();
function switchAudioSource() {
if( current_source ) {
current_source.stop( 0 );
}
current_index = (current_index + 1) % audiobuffers.length;
current_source = context.createBufferSource();
current_source.buffer = audiobuffers[ current_index ];
current_source.loop = true;
current_source.connect( streamNode );
current_source.connect( context.destination );
current_source.start( 0 );
}
};
function exportAudio( blob ) {
const aud = new Audio( URL.createObjectURL( blob ) );
aud.controls = true;
document.body.prepend( aud );
}
async function fetchAsAudioBuffer( url ) {
const buf = await fetchAsBuffer( url );
return context.decodeAudioData( buf );
}
async function fetchAsBuffer( url ) {
const resp = await fetch( url );
return resp.arrayBuffer();
}
button+.recording-controls,
audio+.recording-controls {
display: none;
}
<button>begin</button>
<div class="recording-controls">
<label>Recording...</label>
<button id="switcher">Switch Audio Sources</button>
<button id="record-stopper">Stop Recording</button>
</div>
For video that would imply recording a CanvasMediaStreamTrack and drawing the different video streams on the source <canvas>, but we generally loose a lot of quality doing so...