js图片加载效果实例代码（延迟加载+瀑布流加载）

.picList{
  img{
    width: 100px;
    height: 100px;
    position: relative;

    /*加载失败时显示灰底文字*/
    &:after{
      content: "( ⊙ o ⊙ )加载失败";
      font-size: 6px;
      font-family: FontAwesome;
      color: rgb(100, 100, 100);
      display: flex;
      justify-content: center;
      align-items: center;
      position: absolute;
      top: 0;
      left: 0;
      width: 100px;
      height: 100px;
      background-color: #ddd;
    }
  }
}

.lazy{
  background: url(../pic/loading.gif) center no-repeat;
  border: 1px solid black;
}

#loadBar{
  width: 200px;
  height: 15px;
  background: linear-gradient(90deg,#187103,#81b50b,#187103);
  border: 10px solid white;

  position: absolute;
  top: 150px;
  left: 50%;
  margin-left: -100px;

  #loadBarMask{
    width: 70%;//这个数值显示没有加载成功的图片
    height: 100%;
    background-color: beige;
    position: absolute;
    right: 0;
  }
}

var lazyPic = $('img.lazy');
var loadBarMask = $('#loadBarMask');
var picList = $('.picList');

var activePic = 0;
var totalPic = lazyPic.length;

/*每秒加载一张图片*/

function loadPicPerSecond(){

  lazyPic.each(function(index){

    var self = $(this);

    //console.log(self[0].complete);
    /*img标签已经事先写在html中，所以此时的complete状态全部都是true*/

    setTimeout(function(){

      src[index] = self.attr('data-src');
      self.attr('src',src[index]);
      self.removeClass('lazy');

      //图片获得正确src地址之后，可以执行下面的onload操作
      self[0].Onload= function(){

        //加载读条loadBar动画
        countPic();
      }

      //图片获得的src地址不正确时，执行下面的onerror操作
      self[0].Onerror= function(){
        /*this.style.background = 'url(pic/compressed/picList18.jpg) center no-repeat';*/
        countPic();
      }

    },1000*index);

  })

}

/*根据onload的执行情况来计算当前的图片加载进度.每onload一次，activePic就增加1*/

function countPic(){

  activePic++;

  var leftPic = totalPic - activePic;
  var percentPic = Math.ceil(leftPic/totalPic*100);//没有加载的图片百分比，和loadBarMask的宽度占比配合

  console.log("已加载"+(100-percentPic)+"%");

  loadBarMask.css("width",percentPic+"%");

  if(percentPic==0){
    $('#loadBar').hide();
  }
}

var lazyPic = $('img.lazy');
var loadBarMask = $('#loadBarMask');
var picList = $('.picList');

var scrollTop,
  clientHeight,
  scrollHeight;

var threshold = 200; //最后一张图片距离窗口200px的时候开始加载图片

var src = [];

var activePic = 0;
var totalPic = lazyPic.length;

//待加载的图片数据
var dirtSrc = "pic/compressed/picList";
var picData = {imgSrc:[
  dirtSrc + "20.jpg",
  dirtSrc + "21.jpg",
  dirtSrc + "22.jpg",
  dirtSrc + "23.jpg",
  dirtSrc + "24.jpg",
  dirtSrc + "25.jpg",
  dirtSrc + "26.jpg",
  dirtSrc + "27.jpg",
  dirtSrc + "28.jpg",
  dirtSrc + "29.jpg",
  dirtSrc + "30.jpg",
  dirtSrc + "31.jpg",
  dirtSrc + "32.jpg",
  dirtSrc + "33.jpg",
  dirtSrc + "34.jpg",
  dirtSrc + "35.jpg",
  dirtSrc + "36.jpg",
  dirtSrc + "37.jpg",
  dirtSrc + "38.jpg",
  dirtSrc + "39.jpg",
  dirtSrc + "40.jpg",
  dirtSrc + "41.jpg",
  dirtSrc + "42.jpg",
  dirtSrc + "43.jpg",
  dirtSrc + "44.jpg",
  dirtSrc + "45.jpg",
  dirtSrc + "46.jpg",
  dirtSrc + "47.jpg",
  dirtSrc + "48.jpg",
  dirtSrc + "49.jpg",
]};

//加载次数计数器
var scrollIndex = 0;

$(function(){

  /*监听窗口滚动情况*/
  $(window).on('scroll',function(){

    scrollTop = $(window).scrollTop();//$(window).scrollTop()==document.body.scrollTop
    clientHeight = $(window).height();
    scrollHeight = picList.last().height();//picList.last()[0].clientHeight

    /*目标与窗口的距离达到阈值时开始加载*/
    if(scrollHeight-clientHeight-scrollTop picData.imgSrc.length){
    totalAmount = picData.imgSrc.length;
  }
  for(scrollIndex;scrollIndex