鼠标滚动判断dom元素是否在可视区

1、利用IntersectionObserver监听dom元素

<div class="box_over">
      <ul>
        <li v-for="(item,index) in 10" :style="`background:rgb(${255 * Math.random() * 2},${255 * Math.random() * 2},${255*Math.random() * 4})`">{{index}}</li>
      </ul>
    </div>

let observer = new IntersectionObserver((entries, observer) => {  
        entries.forEach(entry => {  
           // 当元素进入视口时  
          if (entry.isIntersecting) {  
            console.log("元素进入",entry.target.textContent)
            entry.target.style.transform = "translateX(0px)";  
            entry.target.style.opacity = 1;
          } else {  
             // 当元素离开视口时 
            let i = Math.floor(Math.random() * 2);
            console.log(i)
            console.log("元素消失",entry.target.textContent)
            entry.target.style.transform = `translateX(${i?100:-100}px)`;  
            entry.target.style.opacity = 0;
            
          }  
        });  
      }, {  
        rootMargin: '0px',  
        threshold: 0.1 // 你可以调整这个值来控制元素与视口的交叉程度  
      });  
      document.querySelectorAll("li").forEach((item) => {
        observer.observe(item);
      });

2、传统方式监听滚动条

      let list = document.querySelectorAll("li");

      document.querySelector(".box_over").onscroll = function (event){//记得给盒子box_over添加position:relative;样式，因为offsetTop是计算距离顶部父元素已定位的距离
          list.forEach((item,index)=>{//对每个li标签都做了判断所以用了forEach循环
            if(item.offsetTop - event.target.scrollTop < event.target.clientHeight && item.offsetTop + item.clientHeight > event.target.scrollTop){
              console.log("元素进入",item.textContent)
              item.style.transform = "translateY(0px)";  
              item.style.opacity = 1;
            }else{
              let i = Math.floor(Math.random() * 2);
              item.style.transform = `translateY(${ 200}px)`;  
              item.style.opacity = 0;
            }
          })
      }