小程序 - - - - - 实现渐隐渐显（监听滚动距离版）

代码如下：

 <!-- fixed-left -->
<view class="fixed-box" animation="{{animationData}}">
   这里是渐隐渐显的标签
</view>

.fixed-box {
  position: fixed;
  left: 0;
  top: 0;
  z-index: 999;
  background-color: #ccc;
  /* background-color: #f9f9f9; */
  width: 100%;
  height: 90rpx;
  line-height: 90rpx;
  padding: 0rpx 30rpx;
  padding-top: 8rpx;
  box-sizing: border-box;
  display: flex;
  align-items: center;
  opacity: 0;
}

data:{
    animationData: {},
    fixedShow: false,
},

 /**
  * Monitor the scrolling distance and display animation
  * @param {*} event 
  */
  onPageScroll(event) {
    const scrollTop = event.scrollTop;
    if (scrollTop < 440 && this.data.fixedShow) {
      this.fadeOut();
    } else if (scrollTop >= 440 && !this.data.fixedShow) {
      this.fadeIn();
    }
  },
  fadeIn: function () {
    const animation = this.animation;
    animation.opacity(1).step();
    this.setData({
      animationData: animation.export(),
      fixedShow: true
    });
  },

  fadeOut: function () {
    const animation = this.animation;
    animation.opacity(0).step();
    this.setData({
      animationData: animation.export(),
      fixedShow: false
    });
  }