separation of concerns

renderer/lib/capture-video-frame.js

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+module.exports = captureVideoFrame
+
+function captureVideoFrame (video, format) {
+  if (typeof video === 'string') video = document.querySelector(video)
+  if (!video || video.nodeName !== 'VIDEO') {
+    throw new Error('First argument must be a <video> element or selector')
+  }
+  if (format == null) format = 'png'
+  if (format !== 'png' && format !== 'jpg' && format !== 'webp') {
+    throw new Error('Second argument must be one of "png", "jpg", or "webp"')
+  }
+
+  var canvas = document.createElement('canvas')
+  canvas.width = video.videoWidth
+  canvas.height = video.videoHeight
+
+  canvas.getContext('2d').drawImage(video, 0, 0)
+
+  var dataUri = canvas.toDataURL('image/' + format)
+  var data = dataUri.split(',')[1]
+
+  return new Buffer(data, 'base64')
+}

renderer/lib/torrent-poster.js

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+module.exports = torrentPoster
+
+var captureVideoFrame = require('./capture-video-frame')
+var path = require('path')
+
+function torrentPoster (torrent, cb) {
+  // use largest file
+  var file = torrent.files
+    .filter(function (file) {
+      var extname = path.extname(file.name)
+      return ['.mp4', '.webm', '.mov', '.mkv'].indexOf(extname) !== -1
+    })
+    .reduce(function (a, b) {
+      return a.length > b.length ? a : b
+    })
+  var index = torrent.files.indexOf(file)
+
+  var server = torrent.createServer(0)
+  server.listen(0, onListening)
+
+  function onListening () {
+    var port = server.address().port
+    var url = 'http://localhost:' + port + '/' + index
+    var video = document.createElement('video')
+    video.addEventListener('canplay', onCanPlay)
+
+    video.volume = 0
+    video.src = url
+    video.play()
+
+    function onCanPlay () {
+      video.removeEventListener('canplay', onCanPlay)
+      video.addEventListener('seeked', onSeeked)
+
+      video.currentTime = Math.min((video.duration || 600) * 0.03, 60)
+    }
+
+    function onSeeked () {
+      video.removeEventListener('seeked', onSeeked)
+
+      var buf = captureVideoFrame(video)
+
+      // unload video element
+      video.pause()
+      video.src = ''
+      video.load()
+
+      server.destroy()
+
+      cb(null, buf)
+    }
+  }
+}