code: function doBindEvent() {
    if (this.el[CONTEXT].binded) {
      return;
    }
    this.el[CONTEXT].binded = true;
  
    this.scrollEventListener = handleScrollEvent.bind(this);
    this.scrollEventTarget = Utils.getScrollEventTarget(this.el);
  
    const disabledExpr = this.el.getAttribute('waterfall-disabled');
    let disabled = false;
    if (disabledExpr) {
      this.vm.$watch(disabledExpr, (value) => {
        this.disabled = value;
        this.scrollEventListener();
      });
      disabled = Boolean(this.vm[disabledExpr]);
    }
    this.disabled = disabled;
  
    const offset = this.el.getAttribute('waterfall-offset');
    this.offset = Number(offset) || OFFSET;
  
    on(this.scrollEventTarget, 'scroll', this.scrollEventListener, true);
  
    this.scrollEventListener();
  }
docs: 绑定事件到元素上 读取基本的控制变量