How to implement callback after sending message to NaCl (Chrome Native Client)?

Simple question from NaCl newbie ...

In my javascript, I post a message to the NaCl module.
How do I execute a callback in the javascript after this message is handled by the NaCl module?

In the getting-started-tutorial, the following example is given.

 function moduleDidLoad() {
      HelloTutorialModule = document.getElementById('hello_tutorial');
      updateStatus('SUCCESS');
      // Send a message to the Native Client module
      HelloTutorialModule.postMessage('hello');
    }

How do I execute a callback function in HelloTutorialModule.postMessage('hello'); ?

Thanks.

There is no direct way to get a callback that a particular message was received by the NaCl module. You can do it yourself manually, however by passing along an id, and mapping ids to callbacks.

Something like this (untested):

var idCallbackHash = {};
var nextId = 0;

function postMessageWithCallback(msg, callback) {
  var id = nextId++;
  idCallbackHash[id] = callback;
  HelloTutorialModule.postMessage({id: id, msg: msg});
}

// Listen for messages from the NaCl module.
embedElement.addEventListener('message', function(event) {
  var id = event.data.id;
  var msg = event.data.msg;
  var callback = idCallbackHash[id];
  callback(msg);
  delete idCallbackHash[id];
}, true);

Then in the NaCl module:

  virtual void HandleMessage(const pp::Var& var) {
    pp::VarDictionary dict_var(var);
    pp::Var id = dict_var.Get("id");
    pp::Var msg = dict_var.Get("msg");

    // Do something with the message...

    pp::VarDictionary response;
    response.Set("id", id);
    response.Set("msg", ...);
    PostMessage(response);
  }