Observer

A way of notifying change to a number of classes.

• The Observer pattern offers a subscription model in which objects subscribe to an event and get notified when the event occurs.

• This pattern is the cornerstone of event driven programming, including JavaScript.

• The Observer pattern facilitates good object-oriented design and promotes loose coupling.

• When building web apps you end up writing many event handlers. Event handlers are functions that will be notified when a certain event fires.

• These notifications optionally receive an event argument with details about the event.

EX:
function Click() {
    this.handlers = [];  // observers
}
 
Click.prototype = {
 
    subscribe: function(fn) {
        this.handlers.push(fn);
    },
 
    unsubscribe: function(fn) {
        this.handlers = this.handlers.filter(
            function(item) {
                if (item !== fn) {
                    return item;
                }
            }
        );
    },
 
    fire: function(o, thisObj) {
        var scope = thisObj || window;
        this.handlers.forEach(function(item) {
            item.call(scope, o);
        });
    }
}

function run() { 
    var clickHandler = function(item) { 
        console.log("fired: " + item); 
    }; 
    var click = new Click(); 
    click.subscribe(clickHandler);
    click.fire('event #1');
    click.unsubscribe(clickHandler);
    click.fire('event #2');
    click.subscribe(clickHandler);
    click.fire('event #3');

}

run();