mraid页面事件监听避免这样写

var EventListeners = function (event) {
        this.event = event;
        this.count = 0;
        var listeners = {};
        this.add = function (func) {
            var id = String(func);
            if (!listeners[id]) {
                listeners[id] = func;
                this.count++
            }
        };
        this.remove = function (func) {
            var id = String(func);
            if (listeners[id]) {
                listeners[id] = null;
                delete listeners[id];
                this.count--;
                return true
            } else {
                return false
            }
        };
        this.removeAll = function () {
            for (var id in listeners) {
                if (listeners.hasOwnProperty(id)) {
                    this.remove(listeners[id])
                }
            }
        };
        this.broadcast = function (args) {
            for (var id in listeners) {
                if (listeners.hasOwnProperty(id)) {
                    listeners[id].apply(mraid, args)
                }
            }
        };
        this.toString = function () {
            var out = [event, ":"];
            for (var id in listeners) {
                if (listeners.hasOwnProperty(id)) {
                    out.push("|", id, "|")
                }
            }
            return out.join("")
        }
    };

// mraid 监听页面是否展示
function onSdkReady(fn) {
    if (mraid.isViewable()) {
        fn && fn();
    } else {
        mraid.addEventListener('viewableChange', function videoViewableChange(viewable) {
            if (viewable) {
                mraid.removeEventListener('viewableChange', videoViewableChange);
                fn && fn();
            }
        });
    }
}

// mraid show 判断，执行对应方法
function mraidShow(fn) {
    "use strict";
    if (typeof mraid !== 'undefined') {
        if (mraid.getState() === 'loading') {
            mraid.addEventListener('ready', function(){
                fn && onSdkReady(fn);
            });
        } else {
            fn && onSdkReady(fn);
        }
    } else {
        fn && fn();
    }
}

function onSdkReady(fn) {
    if (mraid.isViewable()) {
        fn && fn();
    } else {
        typeof fn === 'function' && mraid.addEventListener('viewableChange', fn);
    }
}