图片懒加载

interface LoadImages {
  [propName: string]: string
}
export function loadImages(sources:LoadImages, callback?:Function){
  let count = 0,
    images:any = {},
    imgNum = 0;
  for(let src in sources){
    imgNum++
  }
  for (let src in sources){
    images[src] = new Image()
    images[src].onload = ()=>{
    if(++count >= imgNum){
        callback && callback(images)
      }
    }
    images[src].src = sources[src]
  }
}