metamask-extension/app/scripts/contentscript.js

const LocalMessageDuplexStream = require('./lib/local-message-stream.js')
const PortStream = require('./lib/port-stream.js')
const ObjectMultiplex = require('./lib/obj-multiplex')

// inject in-page script
var scriptTag = document.createElement('script')
scriptTag.src = chrome.extension.getURL('scripts/inpage.js')
scriptTag.onload = function () { this.parentNode.removeChild(this) }
var container = document.head || document.documentElement
// append as first child
container.insertBefore(scriptTag, container.children[0])

// setup communication to page and plugin
var pageStream = new LocalMessageDuplexStream({
  name: 'contentscript',
  target: 'inpage',
})
pageStream.on('error', console.error.bind(console))
var pluginPort = chrome.runtime.connect({name: 'contentscript'})
var pluginStream = new PortStream(pluginPort)
pluginStream.on('error', console.error.bind(console))

// forward communication plugin->inpage
pageStream.pipe(pluginStream).pipe(pageStream)

// connect contentscript->inpage reload stream
var mx = ObjectMultiplex()
mx.on('error', console.error.bind(console))
mx.pipe(pageStream)
var reloadStream = mx.createStream('reload')
reloadStream.on('error', console.error.bind(console))

// if we lose connection with the plugin, trigger tab refresh
pluginStream.on('close', function () {
  reloadStream.write({ method: 'reset' })
})