nightmare.js如何实现window.callPhantom（）函数的功能？

I have the following code:

我有以下代码:

var nightmare = Nightmare(option)
nightmare
  .goto('some url')
  .evaluate(() => {
    window.callback = function(cid) {
      // do some stuff
      // should screenshot at this point.
    }
  })
  .screenshot('./png.png') //Doesn't get accurate png here
  .end()
var nigh