回答

wxml

<view class='item-{{index}}' wx:for="{{lazyData}}" wx:key="*this.src">
  <image src='{{item.show? item.src : loadIcon}}' class='{{item.show? "active" : ""}}'></image>
  <view>a-{{index}}</view>
</view>

js

Page({
  data: {
    lazyData: [{
        src: "http://repo.bfw.wiki/bfwrepo/image/5fb34b3c1322f.png?x-oss-process=image/auto-orient,1/resize,m_fill,w_300,h_200,/quality,q_90",
 
      },
      {
        src: "http://repo.bfw.wiki/bfwrepo/image/5e62efa9e6df8.png?x-oss-process=image/auto-orient,1/resize,m_fill,w_300,h_200,/quality,q_90",
 
      },
      {
        src: "http://repo.bfw.wiki/bfwrepo/image/5fc8aa82a7330.png?x-oss-process=image/auto-orient,1/resize,m_fill,w_300,h_200,/quality,q_90",
 
      },
      {
        src: "http://repo.bfw.wiki/bfwrepo/image/5e62ef60656fd.png?x-oss-process=image/auto-orient,1/resize,m_fill,w_300,h_200,/quality,q_90",
 
      },
 
     
    ]
  },
  onLoad: function() {
    /**
     * this - 当前对象
     * this.data.lazyData - 数据源
     * lazy_name - 数据名
     * loading_icon 加载图标 -
     */
    lazyImg(this, this.data.lazyData, 'lazyData', 'http://repo.bfw.wiki/bfwrepo/icon/5f7abfbff367a.gif')
  }
})
 
 
 
 
/**
 * 数据名必须为:lazyData或其它名(与数据以及页面相同)
 *
 * 模拟数据：lazyData
 * 传输数据：_that,data(this,lazyData,lazy_name,loading_icon)
 */
const lazyImg = (_that, data, lazy_name, loading_icon) => {
  for (let i = 0, len = data.length; i < len; i++) {
    wx.createIntersectionObserver().relativeToViewport({
      bottom: 2
    }).observe('.item-' + i, (ret) => {
      ret.intersectionRatio > 0 ? data[i].show = true : '';
     
      _that.setData({
        [lazy_name]: data,
        loadIcon: loading_icon
      })
    })
  }
}