javascript - Implementing events in custom object

Question

What I want to have is a custom object which provides some events. For example:

var CustomObjectTextChangedEventName = 'textChanged';
var CustomObject = function () {
    var _this = this;
    var _text = "";

    _this.OnTextChanged = document.createEvent("Event");
    _this.OnTextChanged.initEvent(CustomObjectTextChangedEventName, true, false);

    _this.ChangeText = function (newText) {
        _text = newText;
        fireTextChanged();
    };

    function fireTextChanged() {
        _this.dispatchEvent(_this.OnTextChanged);
    }
}

The code to use the event would look like:

myCustomObject = new CustomObject();
myCustomObject.addEventListener(CustomObjectTextChangedEventName, handleTextChanged, false);

As you can see... the default way of using events in JS, but I can not make it work...

Currently my problem is that my object does not implement addEventListener and dispatchEvent, but this functions are normally implemented from "element"...

Can I make them available somehow or do I have to implement them for my own? How I have to implement them?

Do I have to implement my own event handling? (having a internal list of handlers, an "add"- and "remove"-handler function, and fire each handler when I want to fire the event)?

Answer 1

The addEventListener function is a method of Element class. One way is to make CustomObject inherit from Element like this:

CustomObject.prototype = Element.prototype;

The problem is that Element class may have different implementations among different browsers. So for example firing events may not be easy (see this post).

So I advice doing this by yourself. It is not difficult, try something like this:

var CustomObject = function () {
    var _this = this;
    _this.events = {};

    _this.addEventListener = function(name, handler) {
        if (_this.events.hasOwnProperty(name))
            _this.events[name].push(handler);
        else
            _this.events[name] = [handler];
    };

    _this.removeEventListener = function(name, handler) {
        /* This is a bit tricky, because how would you identify functions?
           This simple solution should work if you pass THE SAME handler. */
        if (!_this.events.hasOwnProperty(name))
            return;

        var index = _this.events[name].indexOf(handler);
        if (index != -1)
            _this.events[name].splice(index, 1);
    };

    _this.fireEvent = function(name, args) {
        if (!_this.events.hasOwnProperty(name))
            return;

        if (!args || !args.length)
            args = [];

        var evs = _this.events[name], l = evs.length;
        for (var i = 0; i < l; i++) {
            evs[i].apply(null, args);
        }
    };
}

Now using it is as simple as:

var co = new CustomObject();
co.addEventListener('textChange', function(name) {
    console.log(name); 
});
co.fireEvent('textChange', ['test']);

This is a basic solution. You may want to alter it, but I think you should grasp the idea.