上滑隐藏，下滑显示

  lastPos:0,
  onPageScroll(e) {
    let currentPos = e.scrollTop;
    if (currentPos - this.lastPos > 50) {
      this.setData({
        menuShow: false
      })
    } else if (this.lastPos - currentPos > 50) {
      this.setData({
        menuShow: true
      })
    }
    this.lastPos = currentPos;
  },

监听300次后不再增长
301VM2068 WAService.js:2 Invoke event onPageScroll in page: pages/post/index