在微信小程序中如何实现图片懒加载

<block wx:for="{{list}}" wx:key="">
  <image class=&#39;relative width-100 mgb-20 fade_in&#39; src=&#39;{{item.cover_url}}&#39; mode=&#39;widthFix&#39; style=&#39;display:none&#39; bindload="_imgOnLoad" id=&#39;{{item.cover_url}}&#39;></image>
   <view class=&#39;tag-bg {{item.checked?"tag-bg1":""}}&#39;></view>
   <view class=&#39;tag-text fz-30 fwb&#39;>{{item.type_name}}</view>
   <image class=&#39;relative width-100 mgb-20 fade_in {{item.loaded?"":"loading-img"}}&#39; src=&#39;{{item.loaded?item.cover_url:item.url}}&#39; mode=&#39;widthFix&#39;></image>
 </block>

//ajax请求数据
onLoad: function () {
  var that = this
  var page = that.data.page
  wx.request({
   url: request_url,
   data: {
    'signature': signature,
    'page':1,
    'pageSize': 2
   },
   success: function (res) {
    let list = res.data.content
    for (var i = 0; i < list.length; i++) {
     list[i].url = "../../img/771.gif" //用json的格式创建url，作为加载过度图片
    }
    that.setData({
     list: list,
    })
   }
  })
 },

//监听图片加载页面
 _imgOnLoad: function (e) {
  // console.log(e)
  var loadedUrl = e.target.id
  let that = this
  let list = that.data.list
  for (var i = 0; i < list.length; i++) {
   if (list[i].cover_url == loadedUrl) {
    list[i].loaded = true
   }
   that.setData({
    list
   })
  }
 }