<template>
 <div>
  <div class="scroll" :class="{show:isActive}">
   <div id="toTop" @click="toTop(step)">&lt;</div>
   <div id="toBottom" @click="toBottom(step)">&gt;</div>
  </div>
 </div>
</template>
<script>
 export default{
  props:{
   step:{ //此數據是控制動畫快慢的
    type:Number,
    default:50 
   }
  },
  data(){
   return {
    isActive:false,
   }
  },
  methods:{
   toTop(i){
    //參數i表示間隔的幅度大小，以此來控制速度
    document.documentElement.scrollTop-=i;
    if (document.documentElement.scrollTop>0) {
     var c=setTimeout(()=>this.toTop(i),16);
    }else {
     clearTimeout(c);
    }
   },
   toBottom(i){
    var clientHeight=document.documentElement.clientHeight||document.body.clientHeight;
    var scrollHeight=document.documentElement.scrollHeight;
    var height=scrollHeight-clientHeight; //超出窗口上界的值就是底部的scrolTop的值
    document.documentElement.scrollTop+=i;
    if (document.documentElement.scrollTop<height) {
     var c=setTimeout(()=>this.toBottom(i),16);
    }else {
     clearTimeout(c);
    }
   }
  },
  created(){
   var vm=this;
   window.οnscrοll=function(){
    if (document.documentElement.scrollTop>60) {
     vm.isActive=true;
    }else {
     vm.isActive=false;
    }
   }
  }
 }
</script>
<style scoped>
 .scroll{
   position: fixed;
   right: 10px;
   bottom: 60px;
   width: 45px;
   height: 90px;
   cursor: pointer;
   display: none;
  }
  .scroll>div{
   width: 45px;
   height: 45px;
   transform: rotate(90deg);
   line-height: 45px;
   text-align: center;
   font-size: 35px;
   font-family: "黑體";
   background-color: rgba(0,0,0,.2);
   color: #fff;
  }
  .scroll>div:hover{
   background-color: rgba(0,0,0,.5);
  }
  .show{
   display: block;
  }
</style>