Saving desktopCapturer to video file in Electron

Question

The desktopCapturer api example shows how to write a screen capture stream to a <video> element.

// In the renderer process.
var desktopCapturer = require('electron').desktopCapturer;

desktopCapturer.getSources({types: ['window', 'screen']}, function(error, sources) {
  if (error) throw error;
  for (var i = 0; i < sources.length; ++i) {
    if (sources[i].name == "Electron") {
      navigator.webkitGetUserMedia({
        audio: false,
        video: {
          mandatory: {
            chromeMediaSource: 'desktop',
            chromeMediaSourceId: sources[i].id,
            minWidth: 1280,
            maxWidth: 1280,
            minHeight: 720,
            maxHeight: 720
          }
        }
      }, gotStream, getUserMediaError);
      return;
    }
  }
});

function gotStream(stream) {
  document.querySelector('video').src = URL.createObjectURL(stream);
}

function getUserMediaError(e) {
  console.log('getUserMediaError');
}

I tried to replace the gotStream function with the following:

function gotStream(stream) {
  var fs = require('fs');
  fs.writeFileSync('vid.mp4', stream);
}

This creates a text file with [object MediaStream] as the contents.

How can I record this stream and save to a file on disk?

Answer 1

I answered my own question with the help of Demian's link to MediaRecorder as well as other related questions.

Below is an excerpt from magnemite with some minor simplifications and converted from TypeScript to JavaScript ES5 for better understanding to most readers.

var fs = require('fs');
var electron = require('electron');

var SECRET_KEY = 'Magnemite';

var recorder;
var blobs = [];

function startRecording() {
    var title = document.title;
    document.title = SECRET_KEY;

    electron.desktopCapturer.getSources({ types: ['window', 'screen'] }, function(error, sources) {
        if (error) throw error;
        for (let i = 0; i < sources.length; i++) {
            let src = sources[i];
            if (src.name === SECRET_KEY) {
                document.title = title;

                navigator.webkitGetUserMedia({
                    audio: false,
                    video: {
                        mandatory: {
                            chromeMediaSource: 'desktop',
                            chromeMediaSourceId: src.id,
                            minWidth: 800,
                            maxWidth: 1280,
                            minHeight: 600,
                            maxHeight: 720
                        }
                    }
                }, handleStream, handleUserMediaError);
                return;
            }
        }
    });
}

function handleStream(stream) {
    recorder = new MediaRecorder(stream);
    blobs = [];
    recorder.ondataavailable = function(event) {
        blobs.push(event.data);
    };
    recorder.start();
}

function stopRecording() {
    recorder.stop();
    toArrayBuffer(new Blob(blobs, {type: 'video/webm'}), function(ab) {
        var buffer = toBuffer(ab);
        var file = `./videos/example.webm`;
        fs.writeFile(file, buffer, function(err) {
            if (err) {
                console.error('Failed to save video ' + err);
            } else {
                console.log('Saved video: ' + file);
            }
        });
    });
}

function handleUserMediaError(e) {
    console.error('handleUserMediaError', e);
}

function toArrayBuffer(blob, cb) {
    let fileReader = new FileReader();
    fileReader.onload = function() {
        let arrayBuffer = this.result;
        cb(arrayBuffer);
    };
    fileReader.readAsArrayBuffer(blob);
}

function toBuffer(ab) {
    let buffer = new Buffer(ab.byteLength);
    let arr = new Uint8Array(ab);
    for (let i = 0; i < arr.byteLength; i++) {
        buffer[i] = arr[i];
    }
    return buffer;
}

// Record for 7 seconds and save to disk
startRecording();
setTimeout(function() { stopRecording() }, 7000);

This will record the current electron window for 7 seconds and save to disk.

Answer 2

Take a look at the MediaRecorder API

You should convert that stream to binary chunks.

Answer 3

Thanks for the example, saved me tons of time.

The only thing when you do: recorder.stop();

you should put toArrayBuffer inside recorder.onstop = function(event) {

}

    function stopRecording() {
        recorder.stop();
        recorder.onstop = function (event) {
            toArrayBuffer(new Blob(blobs, { type: 'video/webm' }), function (ab) {
                var buffer = toBuffer(ab);
                var file = `./videos/example.webm`;
                fs.writeFile(file, buffer, function (err) {
                    if (err) {
                        console.error('Failed to save video ' + err);
                    } else {
                        console.log('Saved video: ' + file);
                    }
                });
            });
        }
    }