Defines the :once pseudo event for classes that have implemented the Events class as well as the definePseudos method for adding your own.

Like Element.Event.Pseudos for Elements, you can use pseudos for Class Events. An event pseudo looks the same as a CSS pseudo: event:pseudo(value). event is the eventname you used to use, and pseudo is the name of the pseudo.

The event will only fire once.


var database = new Class({

    Implements: Events,

    connect: function(){


var db = new database();
db.addEvent('connect:once', function(){
    alert('i am connected');

db.connect(); // will alert 'i am connected'
db.connect(); // nothing will happen

Makes sure the event is not fired more than once in a certain timespan. This is especially useful for events that might fire a lot, but it isn't really necessary to execute a heavy function, like sending requests.

The default timespan is 250 milliseconds.


myClass.addEvent('scroll:throttle', function(){
    // Will only fire once every 250 ms

myClass.fireEvent('scroll'); // is fired
myClass.fireEvent('scroll'); // throttled
myClass.fireEvent.delay(300, myClass, 'scroll'); // after 250 ms, so it's fired

myClass.addEvent('resize:throttle(400)', function(){
    // Will only fire once every 400 ms

The event is only fired when the original event is not fired again in the given time. So when the first event is fired, and a second after 100 ms, the first event is cancelled and only the second is fired.

The default pausetime is 250 milliseconds.


myClass.addEvent('keydown:pause', function(){
    // Default time is 250 ms
myClass.fireEvent('keydown'); // The first event is cancelled
myClass.fireEvent('keydown'); // this one will fired
myClass.fireEvent.delay(1000, myClass, 'keydown'); // This one is after 250 ms, so the previous is not cancelled

myClass.addEvent('keydown:pause(100)', function(){
    // The pause time is now 100 ms.

This function defines a new pseudo.


Events.definePseudo(name, fn);


  1. name - (string) The pseudo name, for example once will become click:once
  2. fn - (function) The function that will get fired when the event is fired. This function should decide what will happen with the event, for example execute the event and remove the event


fn(split, fn, args)
  1. split - (object) A parsed object of the string passed in addEvent: event:pseudo(value)
    • event - (string) the part before the :
    • value - (string) between ( and )
    • pseudo - (string) between the : and (
    • original - (string) the original event name, thus event:pseudo(value)
  2. fn - (function) This is the function that has been passed in the addEvent method. So it is the 'fn' in myClass.addEvent('event:pseudo', fn)
  3. args - (array) The arguments that are passed into the 'fireEvent' method.


This is how the :once pseudo is implemented

Events.definePseudo('once', function(split, fn, args){
    fn.apply(this, args)
    this.removeEvent(split.original, fn);