当music-list向上滑动的时候，设置layer层，随其滚动，覆盖图片，往下滚动时候，图片随着展现出来

1  <div class="bg-layer" ref="layer"></div>

1  mounted(){//因为上面的背景图的高度是和宽度保持着10：7的比例，因此，不同浏览器下高度不一样，下面的song-list高度也不一样，需要计算得出；
2         this.imageHeight = this.$refs.bgImage.clientHeight//得到的是上面图片的高度
3         this.minTranslateY = -this.imageHeight+RESERVED_HEIGHT
4         this.$refs.songScrollList.$el.style.top = `${this.imageHeight}px`
5     
6       },

 1  scrollY(newY){
 2           let translateY = Math.max(this.minTranslateY,newY)
 3           let zIndex = 0;
 4           this.$refs.layer.style[‘transform‘] = `translate3d(0,${translateY}px,0)`
 5           this.$refs.layer.style[‘webkittransform‘]  = `translate3d (0,${translateY}px,0)`
 6           if(newY <this.minTranslateY){//还没有移动到顶部的时候
 7               zIndex = 10
 8              this.$refs.bgImage.style.paddingTop = 0;
 9               this.$refs.bgImage.style.height = `${RESERVED_HEIGHT}px`
10           }else{
11             this.$refs.bgImage.style.paddingTop = ‘70%‘
12             this.$refs.bgImage.style.height = 0;
13           }
14           this.$refs.bgImage.style.zIndex = zIndex
15         }