code: function setup (iframe){
      var id = Math.random() * (1 << 24) | 0
      iframe.contentWindow.postMessage('slackin:' + id, '*')
      window.addEventListener('message', function (e){
        if (typeof e.data !== 'string') return
  
        // show dialog upon click
        if ('slackin-click:' + id  === e.data) {
          showDialog(iframe)
        }
  
        // update width
        var wp = 'slackin-width:' + id + ':'
        if (wp === e.data.substr(0, wp.length)) {
          var width = e.data.substr(wp.length)
          iframe.style.width = width + 'px'
  
          // ensure it's shown (since first time hidden)
          iframe.style.visibility = 'visible'
        }
  
        // redirect to URL
        var redir = 'slackin-redirect:' + id + ':'
        if (redir === e.data.substr(0, redir.length)) {
          location.href = e.data.substr(redir.length)
        }
      })
    }
docs: setup an "RPC" channel between iframe and us