Stupid Simple Poll Renderer

index.js

const {app, BrowserWindow, ipcMain} = require('electron')
const http = require('http')
const url = require('url')

app.disableHardwareAcceleration()

let window
app.once('ready', () => {
  window = new BrowserWindow({
    show: false,
    frame: false,
    webPreferences: {
      nodeIntegration: false,
      offscreen: true,
      preload: `${__dirname}/readyCheck.js`,
      webSecurity: false
    }
  })

  http.createServer((req, res) => {
    let params = url.parse(req.url, true).query
    if (params.screenshot_url) {
      res.statusCode = 200
      res.setHeader('Content-type', 'image/png; charset=binary')
      res.setHeader('Transfer-Encoding', 'chunked')
      readyPage(params.screenshot_url, parseInt(params.width), parseInt(params.height), res)
    } else {
      res.statusCode = 404
      res.end()
    }
  }).listen(3000, '127.0.0.1', () => {
    console.log('server listening')
  })
})

const readyPage = (url, width, height, res) => {
  console.log(`loading ${width}x${height} ${url}`)
  window.setBounds({x: 0, y: 0, width: width, height: height})
  window.loadURL(url, {userAgent: 'PollEverywhere/Poll Renderer'})

  window.webContents.once('did-finish-load', () => {
    if (url.includes("https://viz.polleverywhere.com")) {
      window.webContents.executeJavaScript('__electron__checkVizReady()')
    } else {
      takeScreenshot(res)
    }
  })

  ipcMain.once('viz-loaded', (event, arg) => {
    takeScreenshot(res)
  })
}

const takeScreenshot = (res) => {
  window.capturePage((data) => {
    res.end(data.toPng(), 'binary')
  })
}

package.json

{
  "name": "electron-offscreen",
  "version": "0.0.1",
  "main": "index.js",
  "dependencies": {
    "electron": "^1.3.4"
  },
  "devDependencies": {},
  "scripts": {
    "start": "electron index.js",
    "start-linux": "xvfb-run --server-args=\"-screen 0 1600x900x24\" node_modules/electron/dist/electron index.js"
  }
}

readyCheck.js

const {ipcRenderer} = require('electron')

process.once('loaded', () => {
  global.__electron__ipc = ipcRenderer

  global.__electron__checkVizReady = () => {
    console.log('[Worker is loaded?: ' + PollEv.app.loaded())
    if (PollEv.app.loaded()) {
      window.requestAnimationFrame(function () {
        console.log('[Worker ##{process.pid}] sending event viz-loaded')
        global.__electron__ipc.send('viz-loaded')
      })
    } else {
      setTimeout(global.__electron__checkVizReady, 500)
    }
  }
})