JavaScript异步方法队列链

(function() {
  function _$(els) {
    this.elements = [];
    for (var i = 0, len = els.length; i < len; ++i) {
      var element = els[i];
      if (typeof element == 'string') {
        element = document.getElementById(element);
      }
      this.elements.push(element);
}
  };
  _$.prototype = {
    each: function(fn) {
      for ( var i = 0, len = this.elements.length; i < len; ++i ) {
        fn.call(this, this.elements[i]);
      }
      return this;
    },
    setStyle: function(prop, val) {
      this.each(function(el) {
        el.style[prop] = val;
      });
      return this;
    },
    show: function() {
      var that = this;
      this.each(function(el) {
        that.setStyle('display', 'block');
      });
      return this;
    },
    addEvent: function(type, fn) {
      var add = function(el) {
        if (window.addEventListener) {
          el.addEventListener(type, fn, false);
        }
        else if (window.attachEvent) {
          el.attachEvent('on'+type, fn);
        }
      };
      this.each(function(el) {
        add(el);
      });
      return this;
    }
  };
  window.$ = function() {
    return new _$(arguments);
  };
})();

function Queue() {
  // store your callbacks
  this._methods = [];
  // keep a reference to your response
  this._response = null;
  // all queues start off unflushed
  this._flushed = false;
}

Queue.prototype = {
  // adds callbacks to your queue
  add: function(fn) {
    // if the queue had been flushed, return immediately
    if (this._flushed) {
      fn(this._response);

    // otherwise push it on the queue
    } else {
      this._methods.push(fn);
    }
  },

  flush: function(resp) {
    // note: flush only ever happens once
    if (this._flushed) {
      return;
    }
    // store your response for subsequent calls after flush()
    this._response = resp;
    // mark that it's been flushed
    this._flushed = true;
    // shift 'em out and call 'em back
    while (this._methods[0]) {
      this._methods.shift()(resp);
    }
  }
};