docs: function __init(opts) {
      //扩展参数
      extend(this.options, opts || (opts = {}));
      //获取目标元素
      this.appearWatchElements = this.appearWatchElements || __getElements.call(this, '.' + this.options.cls);
      //初始化位置信息
      __initBoundingRect.call(this, this.appearWatchElements);
      //绑定事件
      __bindEvent.call(this);
    }