[feature] change how meta events work

[janhancic]

There is a new pattern that is being used across different projects now, and we realised that it could be made simpler.

It's a form of sticky events, but not really. An example:

// this will send a new listener for "status-changed" event the current status
this.on(emitr.meta.AddListenerEvent, function(addEvent) {
    if (addEvent.event === 'status-changed') {
        addEvent.listener.call(addEvent.context, this._status);
    }
}.bind(this));

So after discussing with @dchambers and @bit-shifter we decided to add a new method to emitr: .onMetaEvent(metaEvent, eventName, callback, ctx). And deprecate the current .on(metaEvent, ... (which will internally invoke the new method for backwards compatibility).

If eventName is null emitr instance will be told of every new listener, otherwise it will only be told of the event it is interested in.

callback will have the following signature: callback(eventName, sendToListenerFunc).

sendToListenerFunc will be a function that will take care of invoking the listener with the correct context and so on.

From a user's perspective it will look like this:

function MyClass() {
  this._status = 42;
  this.onMetaEvent(emitr.meta.AddListenerEvent, 'status-changed', function(sendToListenerFunc) {
    sendToListenerFunc(this._status);
  }, this);
}
Emitr.mixInto(MyClass);

I'm just capturing what we discussed here, I'll make a PR as soon as I have some time :)

[dchambers]

Hey @janhancic, was there ever any progress on this?

[janhancic]

No :( I haven't had the time ...

[dchambers]

Okay, no probs.

[improbable-adam]

this seems like a specific instance of a more general concept - filtering event listeners based on some of their parameters. This functionality would be useful in normal events too, not just metaevents.