PostMessage with multiple functions or custom callbacks

Question

So far I've only seen tutorials for postmessage where one window sends a single kind of message, and the other window interprets the message in only a single way.

What if I want to have many different kinds of interactions between windows, can postmessage handle that?

Is that going against the grain of what postmessage is supposed to do?

For example, what if I wanted to be able to send custom callbacks back and forth, etc?

Answer 1

There are a couple of ways to pass a multi-part message on to a postMessage handler. The first (and less "clean" way) is to use a delimiter character, then pass your data through a string.

Let's say we wanted to pass a user ID, an action, and the users name. The string would look like this:

54|do_logout|chris

Within the postMessage handler, the passed data can be split (docs) on the | character, then each segment of the message can be used as needed.

Another route, instead of manually creating/splitting a string, is to use JSON (docs) to convert an object into a string on one side, and use JSON to convert back to an object in the handler.

var pass_data = {
    'name':'Chris',
    'id':54,
    'action':'do_logout'
};
target_window.postMessage(JSON.stringify(pass_data), "http://www.example.net");

... then in the handler:

function (event) {
    var pass_data = JSON.parse(event.data);
}

Be sure to test, though, as the JSON object is not provided on all user agents, especially older ones. There are many (many, many) third-party libraries out there to shim JSON support, so don't let the lack of complete adoption scare you away - JSON is definitely a safe "moving forward" standard.

Wouldn't it be nicer if we could just pass that object straightaway? Well, staring in Firefox 6 (source), the data you pass to a postmessage handler may be an object. The object will be serialized, so there are some concerns on that front, but:

var pass_data = {
    'name':'Chris',
    'id':54,
    'action':'do_logout'
};
target_window.postMessage(pass_data, "http://www.example.net");

A little nicer, eh? Unfortunately, current versions of IE will only deal with strings. I was not able to find any discussion on future plans regarding postMessage for IE 10. Further, there is a known bug in IE 8/9 which breaks postMessage for anything other than frames. (source).

Getting in to a specific aspect of your question - callbacks. Unless you're able to pass the callback by function name, there isn't a way to pass a function; no anonymous functions for you. This is related to the way the data is actually passed on to the handler. In practice, there "is not" support for objects as data, behind the scenes the browser is turning your passed object into a string (serialization).

All that said, then, you should understand that passing an object is exactly the same as using JSON to stringify an object before passing, only in the former case the browser is doing its own serialization (and subsequent unserialization), whereas with the latter route it is up to you to serialize/unserialize.

The take-away points here:

• postMessage still has limited cross-browser support
• The trend for newer versions of standards-compliant browsers is to allow passage of objects in addition to strings
• The passed object will be serialized, so no function references allowed
• The widest support "in the wild" is for string-only data, which means you'll have to stick with strings and "pack" your data as demonstrated above if you want to support a wide variety of user agents
• Internet Explorer will ruin every plan you ever make (including family holidays)

Documentation and References

• window.postMessage: https://developer.mozilla.org/en/DOM/window.postMessage
• String.split: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String/split
• IE 8/9 HTML5 support notes: http://blogs.msdn.com/b/ieinternals/archive/2009/09/16/bugs-in-ie8-support-for-html5-postmessage-sessionstorage-and-localstorage.aspx
• JSON.stringify: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/JSON/stringify
• JSON.parse: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/JSON/parse

Answer 2

Callbacks with postMessage: very possible and very useful

There is a nice plugin I've found on npm called "silver-bullet". It does postMessage with callbacks and uses eventEmitter to get specific events as well. Its very nice.

But to implement this I would do something like...

phostMessage(iframe, someObj, callback);

You have to do this:

1. You need a common callback ID passed between frames communicating.
2. The sender creates a unique callback ID on each message and stores it in a callback lookup hash to find the callback after sending.
3. The receiver of the message only ensures the callback ID is sent back.
4. All frames communicating use the same JS library for this.

Here's a very basic demonstration of that:

var callbacks = {};

// when receiving messages
window.addEventListener('message', function(ev) {
  // todo: add origin check
  if (!ev.data)
    return;

  var message;
  try {
    message = JSON.parse(ev.data);
  } catch (ex) {
    console.error(ex);
  }

  // ignore messages not having a callback ID
  if (!message || !message.callbackId)
    return;

  // we are the sender getting the callback
  if (callbacks[message.callbackId]) {
    callbacks[message.callbackId](message);
    delete callbacks[message.callbackId];
    return;
  }

  // we are the receiver so we respond with the callback ID
  // todo: restrict who can receive message (last param)
  iframe.contentWindow.postMessage(JSON.stringify(message), '*');
});

// when sending messages
function phostMessage(iframe, obj, callback) {
  obj.eventId = Math.random();
  callbacks[obj.eventId] = callback;
  // todo: restrict who can receive message (last param)
  iframe.contentWindow.postMessage(JSON.stringify(obj), '*');
}

I take this concept a bit further and use a message handler lookup where the message has the desired handler function name to evoke and pass a message to. The message handler takes a callback as well that when completed fires the callback. The callback just has the simple logic of calling the native post message again sending back its received callback id.

So the last line of code for the message event handling would be:

if (messageHandler[message.handler])
  messageHandler[message.handler](message, function() {
    iframe.contentWindow.postMessage(JSON.stringify(message), '*');
  });
else
  iframe.contentWindow.postMessage(JSON.stringify(message), '*');

which allows asynchronous stuff to happen.