;(function(window) {
    'use strict';

    function Observer() {
        this._listeners = {};

        if (arguments.length) {
            this.on.apply(this, arguments);
        }
    }

    Observer.prototype = {
        on: function(name, handler, isOnce) {
            var handlers = this._listeners[name],
                that = this,
                isAlreadyOn = false,
                handlerItem = {
                    handler: handler,
                    isOnce: !!isOnce
                };

            if (handlers) {
                isAlreadyOn = handlers.some(function(item) {
                    return item.handler === handlerItem.handle;
                });

                !isAlreadyOn && handlers.push(handlerItem);
            } else{
                this._listeners[name] = [handlerItem];
            }
        },

        one: function(name, handler) {
            var args = [].slice.call(arguments);
            args.push(true);
            this.on.apply(this, args);
        },

        off: function(name, handler) {
            var handlers = this._listeners[name],
                that = this,
                index;

            if (!handlers) return;
            if (handler) {
                index = handlers.findIndex(function(item) {
                    return item.handler === handler;
                });

                if (index > -1) {
                    handlers.splice(index, 1);
                    handlers.length === 0 && deleteListener();
                }
            } else {
                deleteListener();
            }

            function deleteListener() {
                delete that._listeners[name];
            }
        },

        fire: function(name, data) {
            var handlers = this._listeners[name],
                that = this;

            if (handlers) {
                handlers.forEach(function(item) {
                    item.handler.call(this, {
                        type: name,
                        data: data
                    });

                    if (item.isOnce) {
                        that.off(name, item.handler);
                    }
                });
            }
        }
    };

    Observer._listeners = {};
    for (var key in Observer.prototype) {
        Observer[key] = Observer.prototype[key];
    }

    window.Observer = Observer;
})(window);