原生JS实现图片放大镜效果 - 干货分享

准备工做

1. 须要先了解JS的三个鼠标事件

   onmouseenter 鼠标进入

   onmouseleave 鼠标移出

   onmousemove 鼠标移动

   注意：鼠标移入的时候须要阻止事件的冒泡传播（ev.stopPropagation），因此不用onmouseover和onmouseout

2. 理解盒子模型

   offsetParent：当前盒子的父级参照物

   offsetLeft / offsetTop：当前盒子距离其父级参照物的偏移量

   offsetWidth / offsetHeight：当前元素可视区域的宽高（内容的宽高+padding+border）

   scrollLeft / scrollTop：滚动条卷去的宽度/高度

3. 区分鼠标事件对象

   ev.clientX / ev.clientY： 当前鼠标触发点距离当前窗口左上角的X/Y轴坐标

   ev.pageX / ev.pageY：当前鼠标触发点距离body（第一屏幕）左上角的X/Y轴坐标

   注意：低版本浏览器事件对象中不存在pageX/pageY

   ev.pageX = ev.clientX + (document.documentElement.scrollLeft || document.body.scrollLeft);
   ev.pageY = ev.clientY + (document.documentElement.scrollTop || document.body.scrollTop)
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4. 若是图片是在某些盒子里，而且盒子距离左上角的距离根据浏览器窗口的大小改变，那么计算偏移量的时候就不能用ev.clientX/ev.clientY，而是须要用ev.pageX/ev.pageY

思路

1. 布局

<div id="smallBox" class="smallBox">
    <img src="smallImg.jpg"/>
    <div class="mask" id="mask"></div>
</div>

<div class="big" id="bigBox">
    <img id="bigImg" src="bigImg.jpg" alt=""/>
</div>
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2. 样式

.small{width:400px;height:400px}
.small img{width:100%;height:100%}
.small .mask{width: 200px; height: 200px; background: rgba(255, 255, 0, 0.4); position: absolute; left: 0; top: 0; display: none; cursor: move; }
.big{ width: 480px; height: 480px; border: 1px solid #999; position: absolute; top: 0; left: 400px; overflow: hidden; z-index:999; display: none;}
.big img{width:800px; height: 800px;}
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3. JS逻辑

   3.1 页面加载完成后触发

   window.onload = function(){}
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   3.2 获取操做的元素

   let small = document.getElementById("smallBox"),
       big = document.getElementById("bigBox"),
       bigImg = document.getElementById("bigImg"),
       mask = document.getElementById("mask");
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   3.3 计算小图距离body的偏移量

   let smallLeft = small.offsetLeft,//小图距body的左偏移量
       smallTop = small.offsetTop,//小图距body的上偏移量
       smallBox = small;//临时值，用于计算小图距body的偏移量
   
   //计算小图距body的偏移量
   while(smallBox.offsetParent.nodeName !== 'BODY'){
       smallBox = smallBox.offsetParent;
       smallLeft += smallBox.offsetLeft;
       smallTop += smallBox.offsetTop;
   }
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   3.4 设置鼠标移入/移出小图时，大图和遮罩层的隐藏和显示

   small.onmouseenter = function(){
       big.style.display = "block";
       mask.style.display = "block";
   };
   small.onmouseleave = function(){
       big.style.display = "none";
       mask.style.display = "none";
   };
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   3.5 遮罩层随着鼠标移动

   small.onmousemove = function(event){
       event = event || window.event;
   
       //兼容低版本浏览器 - 低版本浏览器中不存在pageX和pageY
       event.pageX = event.clientX + (document.documentElement.scrollLeft || document.body.scrollLeft);
       event.pageY = event.clientY + (document.documentElement.scrollTop || document.body.scrollTop);
   
       var x = event.pageX - smallLeft - mask.offsetWidth/2;
       var y = event.pageY - smallTop - mask.offsetHeight/2;
       
       mask.style.top = y + "px";
       mask.style.left = x + "px";
   }
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   3.6 鼠标移动时不超出小图时拉回

   //水平方向的最大值
   var maxX = small.clientWidth-mask.clientWidth;
   //竖直方向的最大值
   var maxY = small.clientHeight-mask.clientHeight;
   
   //超出左侧时,拉回
   if(x<0) x=0;
   
   //超出右侧时,拉回
   if(x>maxX) x=maxX;
   
   //顶部超出
   if(y<0) y=0;
   
   //底部超出
   if(y>maxY) y=maxY;
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   3.7 大图根据比例显示

   //比例 = 大图移动的距离/mask移动的距离 = 大图/小图
   var scale = bigImg.offsetWidth / small.offsetWidth;
   bigImg.style.marginLeft = -scale * x +"px";
   bigImg.style.marginTop = -scale * y +"px";
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主要代码

//-------------------- HTML start --------------------
<div id="smallBox" class="smallBox">
    <img src="smallImg.jpg"/>
    <div class="mask" id="mask"></div>
</div>
<div class="big" id="bigBox">
    <img id="bigImg" src="bigImg.jpg" alt=""/>
</div>
//--------------------- HTML end ---------------------

//--------------------- CSS start --------------------
.small{width:400px;height:400px}
.small img{width:100%;height:100%}
.small .mask{width: 200px; height: 200px; background: rgba(255, 255, 0, 0.4); position: absolute; left: 0; top: 0; display: none; cursor: move; }
.big{ width: 480px; height: 480px; border: 1px solid #999; position: absolute; top: 0; left: 400px; overflow: hidden; z-index:999; display: none;}
.big img{width:800px; height: 800px;}
//--------------------- CSS end ---------------------

//--------------------- JS start --------------------
window.onload = function(){
    var small = document.getElementById("smallImg"),
        big = document.getElementById("bigBox"),
        bigImg = document.getElementById("bigImg"),
        mask = document.getElementById("imgMask"),
        smallLeft = small.offsetLeft,//小图距body的左偏移量
        smallTop = small.offsetTop,//小图距body的上偏移量
        smallBox = small;//临时值，用于计算小图距body的偏移量

    //计算小图距body的偏移量
    while(smallBox.offsetParent.nodeName !== 'BODY'){
        smallBox = smallBox.offsetParent;
        smallLeft += smallBox.offsetLeft;
        smallTop += smallBox.offsetTop;
    }

    //big和mask在鼠标移入small时显示，移出时隐藏
    small.onmouseenter = function(){
        big.style.display = "block";
        mask.style.display = "block";
    };
    small.onmouseleave = function(){
        big.style.display = "none";
        mask.style.display = "none";
    };
    small.onmousemove = function(event){
        event = event || window.event;

        //兼容低版本浏览器 - 低版本浏览器中不存在pageX和pageY
        event.pageX = event.clientX + (document.documentElement.scrollLeft || document.body.scrollLeft);
        event.pageY = event.clientY + (document.documentElement.scrollTop || document.body.scrollTop);

        var x = event.pageX - smallLeft - mask.offsetWidth/2;
        var y = event.pageY - smallTop - mask.offsetHeight/2;

        //水平方向的最大值
        var maxX = small.clientWidth-mask.clientWidth;
        //竖直方向的最大值
        var maxY = small.clientHeight-mask.clientHeight;
        if(x<0){
            //至关于超出左侧,超出左侧时,拉回
            x=0;
        }
        //超出右侧时拉回
        if(x>maxX){
            x=maxX;
        }
        //顶部超出
        if(y<0){
            y=0;
        }
        //底部超出
        if(y>maxY){
            y=maxY;
        }
        mask.style.top = y + "px";
        mask.style.left = x + "px";

        //比例 = 大图移动的距离/mask移动的距离 = 大图/小图
        var scale = bigImg.offsetWidth / small.offsetWidth;
        bigImg.style.marginLeft = -scale * x +"px";
        bigImg.style.marginTop = -scale * y +"px";

    }
};
//--------------------- JS end ----------------------
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