mounted () {
    window.addEventListener('scroll', this.handleScroll)
},
destroyed () {
    window.removeEventListener('scroll', this.handleScroll)
},

handleScroll () {
       //变量scrollTop是滚动条滚动时，距离顶部的距离
       var scrollTop = document.documentElement.scrollTop||document.body.scrollTop;
       //变量windowHeight是可视区的高度
       var windowHeight = document.documentElement.clientHeight || document.body.clientHeight;
       //变量scrollHeight是滚动条的总高度
       var scrollHeight = document.documentElement.scrollHeight||document.body.scrollHeight;
       //滚动条到底部的条件
       if(scrollTop+windowHeight==scrollHeight &&this.list.length !==0){
           this.loadMore() // 加载的列表数据
       }
   }