Page({

  data: {

    realScrollTop: 0,//页面滚动距离

　　 driveHeight　　　//屏幕高度可初始化设置

},

scroll(e){

  if(e.detail.scrollTop > this.data.realScrollTop){

      this.setData({

          realScrollTop: e.detail.scrollTop

      });

  }

 }

});

<scroll-view scroll-y="true" scroll-top="{{resetScrollTop}}" bindscroll="scroll" >

<image wx:if="{{realScrollTop+driveHeight*2 > (index*70)}}" src="{{item['img']}}"></image>　　

<image wx:else src="{{item['bgm']}}"></image>

</scroll-view>