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dojo/on

Authors:Kris Zyp
Project owner:Kris Zyp
since:1.7.0

The dojo/Evented module is a class that can be used as a base class or mixin for JavaScript classes that have their own events.

dojo/Evented API

The Evented class provides two methods, on(eventType, listener) and emit(eventType, eventObject) which correspond to on() and on.emit() with the target being the instance object. For example, we could create a class:

define(["dojo/Evented"], function(Evented){
  var MyComponent = dojo.declare([Evented], {
    startup: function(){
      // once we are done with startup, fire the "ready" event
      this.emit("ready", {});
    }
  });

  component = new MyComponent();
  component.on("ready", function(){
    // this will be called when the "ready" event is emitted
    // ...
  });
  component.startup();
});

Event Handler Properties

The Evented class stores events in a property/method slot named after the event with an “on” prefix, following the DOM0 convention. For example, listening to the “foo” event will put the listeners in the “onfoo” method. If a class has an “onMouseDown” method, this corresponds to a “MouseDown” event that you can listen to with:

component.on("MouseDown");

While it is possible to trigger the event by directly calling the onfoo() or onMouseDown() method, it is recommend that you use emit(“foo”) or emit(“MouseDown”) for consistency.

One can upgrade most classes with onEvent methods to be usable with dojo/on by simply adding the Evented class as a base class.

Note that some event emitting objects, like Dijit widgets, have their own event emitting implementation, and define their own event names.

Transitioning to Dojo 2.0

There are a number of objects in Dojo (like Dnd and Animation) that are derived from Evented. However, these event emitting objects are likely to change their events (both names and emitted event objects) in 2.0, so one should not assume that code using dojo/Evented to listen to Dojo objects will work unchanged in 2.0.