简单事件模型，JS防止单个函数异步重复调用

function Emitter(){
	this._listener = [];
}

Emitter.prototype.bind = function(eventName, callback){
	let listener = this._listener[eventName] || [];

	listener.push(callback);
	this._listener[eventName] = listener;
}

Emitter.prototype.trigger = function(eventName){
	//args为了获取eventName后面的参数（注册事件的参数）
	let args = Array.prototype.slice.apply(arguments).slice(1);
	let listener = this._listener[eventName];

	if (!Array.isArray(listener)) {
		return;
	}

	listener.forEach(function(callback){
		try{
			callback.apply(this, args);
		}catch(e){
			console.error(e);
		}
	});
}

let emitter = new Emitter();
emitter.bind("myEvent", function(arg1, arg2){
	console.log(arg1, arg2);
});

emitter.bind("myevent", function(arg1, arg2){
	console.log(arg1, arg2);
});

emitter.trigger("myevent", "a", "b");

var status = "ready";
var doSomething = function (callback) {
    emitter.bind("selected", callback);
    console.log("add a listener!");
    if (status === "ready") {
        status = "pending";
        setTimeout(function () {
            emitter.trigger("selected");
            status = "ready";
            console.log("resolved");
        }, 1000);
    };
};

//模拟并发请求
for (var i = 0; i < 20; i++) {
    setTimeout(function (i) {
        return (function (i) {
            doSomething(function () {
                console.log(i)
            });
        })(i)
    }, 100 * i, i);
};