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I'm creating an event hub object in for a large project I am building in JavaScript. The project will have modules in charge of the UI, commands, and rendering. Each module will use this hub for broadcasting events to each other.

This is my JavaScript library (I cut out RequireJS for conciseness):

Hub = Object.create ({});
// initialize this hub
Hub.init = function () {
    this.events = {};
    this.contexts = {};
    return this;
};
// register a function and context
Hub.on = function (name, callback, contexts) {
    // create event type if it isn't already and push our callback
    (this.events [name] || (this.events [name] = [])).push (callback);
    (this.contexts [name] || (this.contexts [name] = [])).push (contexts);
};
// un-register a function
Hub.off = function (name, callback) {
    // if this event type exists, splice out our callback
    this.events [name] && this.events [name].splice (this.events [name].indexOf (callback), 1);
    this.contexts [name] && this.contexts [name].splice (this.events [name].indexOf (callback), 1);
};
// fire all of a type of functions
Hub.trigger = function (name) {
    if (!this.events [name] || this.events [name].length === 0) return;
    var args = Array.prototype.slice.call (arguments, 1),
        i = 0, event = this.events [name], context = this.contexts [name], l = event.length;
    // if this event type exists, run all the callbacks
    for (; i < l; event [i].apply (context [i++], args));
};

And this about how it is used:

// MAIN.JS: create the main hub
Main.hub = Object.create (Hub);

// RENDER.JS: listen for entity addition
Main.hub.on ('entity_add', function (entity) {
    this.draw (entity);
}, this);

// DRAW_CMD.JS: listen for canvas click
DrawCMD.init = function () {
    Main.hub.on ('canvas_click', this.clicked, this);
};
DrawCMD.clicked = function (ev) {
    // tell everyone that an entity is added
    Main.hub.trigger ('entity_add', this.entity);
    Main.hub.off ('canvas_click', clicked);
};

// UI.JS: fire canvas click
canvas.addEventListener ('click', function (ev) {
    Main.hub.trigger ('canvas_click', ev);
});

So, I have a few questions about this:

  1. Efficiency: There are a few scenarios where this seems resource-wasteful, for example: when there are no commands active, click and mouse-move events are still being broadcast. Are there ways I make my .trigger function faster for this?
  2. Organization: Does this seem like it will make my project more or less elegant compared to just directly calling functions on each relevant module?
  3. OOP: This is my first attempt at using the new Object.create. Could I be using it better, or does this look good?
  4. Context: I am using two arrays (events and contexts) so I can have registered functions keep the same context they would have normally. I would use Function.bind, except that doesn't allow me to use .off to un-register events. Is there a more elegant way to do this?
  5. Messes: Does anyone see any other potential pitfalls with this approach?
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  • \$\begingroup\$ Backbone has this built-in so I'm using that now: BackboneJS#Events \$\endgroup\$
    – Robin
    Commented Aug 7, 2015 at 20:24

1 Answer 1

Reset to default
3
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It looks good to me, but your questions are hard to evaluate with such a small snippet.

  1. Efficiency

    Basically, what else can you do? You have to iterate over all registered events.

  2. Organisation

    It seems like a good idea to me. This way you would have central point for binding events to DOM elements - if I'm understanding your code correctly.

  3. OOP

    Go for it!

  4. Context

    This is problematic, but not dramatic! Basically, your .off function does not keep the to arrays in sync, which is really hard because events are asynchronous.

    Consider this example with simple inheritance:

    <html>
<body>
<div id="clickMe">Click Me!</div>
<script>
(function(){

    var Hub = Object.create ({});
    //initialize this hub
    Hub.init = function () {
        this.events = {};
        this.contexts = {};
        return this;
    };
    //register a function and context
    Hub.on = function (name, callback, contexts) {
        // create event type if it isn't already and push our callback
        (this.events [name] || (this.events [name] = [])).push (callback);
        (this.contexts [name] || (this.contexts [name] = [])).push (contexts);
    };
    //un-register a function
    Hub.off = function (name, callback) {
        // if this event type exists, splice out our callback
        console.log(this.events [name].indexOf (callback));
        console.log((this.events));
        console.log(JSON.stringify(this.contexts));
        this.events [name] && this.events [name].splice (this.events [name].indexOf (callback), 1);
        this.contexts [name] && this.contexts [name].splice (this.events [name].indexOf (callback), 1);
        console.log((this.events));
        console.log(JSON.stringify(this.contexts));
    };
    //fire all of a type of functions
    Hub.trigger = function (name) {

        if (!this.events [name] || this.events [name].length === 0) return;
        var args = Array.prototype.slice.call (arguments, 1),
     i = 0, event = this.events [name], context = this.contexts [name], l = event.length;
        // if this event type exists, run all the callbacks
        for (; i < l; event [i].apply (context [i++], args));
    };


    var Main = {};

    Main.hub = Object.create(Hub);
    Main.hub.init();


    var Foo = {};
    Foo.init = function (name) {

        this.name = name;
        Main.hub.on("lala", this.onClick, this);

    };
    Foo.onClick = function Foo_onClick (){

        alert(this.name);

    };

    var Bar = Object.create(Foo);;
    Bar.init = function (name) {

        Foo.init.call(this, name);


    };
    Bar.onClick = function Bar_onClick (){

        Foo.onClick.call(this);
        Main.hub.off("lala", this.onClick);

    };

    var f = Object.create(Foo);
    var b = Object.create(Bar);
    var c = Object.create(Foo);

    f.init("foo");
    b.init("bar");
    c.init("c");

    console.log(Object.is(f,c));

    var clickMe = document.getElementById("clickMe");
    clickMe.addEventListener("click",function(e){

        Main.hub.trigger("lala",e);

    });


}());
</script>
</body>
</html>

    Here, calling Hub.off inside an event callback forces Hub.events.length to decrement while i in Hub.trigger is still incrementing, so event[i] will be undefined. Also, this.events[name].indexOf(callback) will give a different result in the second line:

    this.events [name] && this.events [name].splice (this.events [name].indexOf (callback), 1);
this.contexts [name] && this.contexts [name].splice (this.events [name].indexOf (callback), 1);

  5. Messes

    Besides (4), I'd recommend another formatting style. Yours is really hard to read. This is important, if you consider your product to be maintained in the long run.

  6. Solution for asynchronous calls for .on and .off

    Here is a solution for a Hub, which can deal with asynchrounous calls of .on and .off. I also made some minor API changes, which strictly (un-)registers only pairs of (callback, context). I'm not certain if this is necessary for unregister, but it makes the API more symmetric.

    var Hub = Object.create ({});
// initializes Hub
Hub.init = function () {

    this._handlers = {};     // centra registry for custom events
    this._running = false;   // determines if custom evetns ar triggered

};

// delays the exectuion of fn while Hub is triggering custom events (_running === true)
Hub._delay = function Hub_delay (fn) {

    var hub, interval, id;

    hub = this;
    interval = 0;

    // setInterval(fn,0) is the JS equivalent for while(true){}
    // the actual while(true) will certainly kill the process
    id = setInterval(function(){

        if (!this._running) {

            fn.call(hub);
            clearInterval(id);

        }

    },interval);

};

// registers the pair (callback, context) for the custom event name
Hub.on = function Hub_on (name, callback, context) {

    this._delay(function Hub_on_delayed (){

        var handler;

        if (!Array.isArray(this._handlers[name])) {

            this._handlers[name] = [];

        }

        handler = {};
        handler.callback = callback;
        handler.context  = context;

        this._handlers[name].push(handler);

    });

};

// unregisters the pair (callback, context) for the custom event name
Hub.off = function Hub_off (name, callback, context) {

    this._delay(function Hub_off_delayed (){

        if (!Array.isArray(this._handlers[name])) {

            this._handlers[name] = [];

        }

        console.log(JSON.stringify(this._handlers[name]));

        this._handlers[name] = this._handlers[name].filter(function(handler){

            return !(handler.callback === callback && handler.context === context);

        });

        console.log(JSON.stringify(this._handlers[name]));

    });

};

// triggers all handlers for the custom event name
Hub.trigger = function Hub_trigger (name) {

    var args, i, handlers, callback, context, invoke;

    // delay asynchronous registering and unregistering
    this._running = true;

    args = Array.prototype.slice.call (arguments, 1);
    handlers = Array.isArray(this._handlers[name]) ? this._handlers[name] : [];

    for (i = 0; i < handlers.length; i++) {

        callback = handlers[i].callback;
        context  = handlers[i].context; 

        // allow invokation only fo valid callbacks and contexts
        invoke = (
            typeof callback === "function" 
            && typeof context !== "undefined"
            && context !== null
        );

        if (invoke === true) {

            callback.apply(context, args);

        }

    }

    // allow registering and unregistering
    this._running = false;

};
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