code: function Drag(elem, event, GPU) {
  
      if (elem) {
          return new Drag.create(elem, event, GPU);
      }
  
      this.start = $.proxy(this.start, this);
      this.move = $.proxy(this.move, this);
      this.end = $.proxy(this.end, this);
  }
docs: 拖拽事件类，解决浏览器兼容问题
  @constructor