JavaScript-220：计算鼠标在盒子中的坐标

效果图

结构

    <div class="login-header"><a href="javascript:;" id="link">点击，弹出登陆框</a></div>

   <div class="login">
       <div class="login-title">登陆会员
           <span><a href="javascript:;" id="closeBtn">关闭</a></span>
       </div>

       <div class="login-input-content">
           <div class="login-input">
               <label for="">用户名</label>
               <input type="text" placeholder="请输入用户名" name="info[username]" id="username">
           </div>
           <div class="login-input">
               <label for="">登陆密码</label>
               <input type="text" placeholder="请输入登陆密码" name="info[password]" id="password">
           </div>
       </div>

       <div class="login-button"><a href="javascript:;" id="login-submit">登陆会员</a></div>
   </div>
   <div class="login-bg"></div>
   <!--遮盖层-->

CSS

        .login-header {
    
    
            width: 100%;
            text-align: center;
            height: 30px;
            font-size: 24px;
            line-height: 30px;
        }

        .login {
    
    
            display: none;
            width: 512px;
            height: 280px;
            position: fixed;
            border: #ebebeb solid 1px;
            left: 50%;
            top: 50%;
            background: #ffffff;
            box-shadow: 0px 0px 20px #ddd;
            z-index: 9999;
            transform: translate(-50%, -50%);
            /* 水平居中 垂直居中 */
        }

        .login-title {
    
    
            width: 100%;
            margin: 10px 0px 0px 0px;
            text-align: center;
            line-height: 40px;
            height: 40px;
            font-size: 18px;
            position: relative;
            cursor: move;
        }

        .login-input-content {
    
    
            margin-top: 20px;
        }

        .login-button {
    
    
            width: 50%;
            margin: 30px auto 0px auto;
            line-height: 40px;
            font-size: 14px;
            border: #ebebeb 1px solid;
            text-align: center;
        }

        a {
    
    
            text-decoration: none;
            color: #000000;
        }

        .login-button a {
    
    
            display: block;
        }

        .login-input input {
    
    
            float: left;
            line-height: 35px;
            height: 35px;
            width: 350px;
            border: #ebebeb 1px solid;
            text-indent: 5px;
        }

        .login-input {
    
    
            overflow: hidden;
            margin: 0px 0px 20px 0px;
        }

        .login-input label {
    
    
            float: left;
            width: 90px;
            padding-right: 10px;
            text-align: right;
            line-height: 35px;
            height: 35px;
            font-size: 14px;
        }

        .login-title span {
    
    
            position: absolute;
            font-size: 12px;
            right: -20px;
            top: -30px;
            background: #ffffff;
            border: #ebebeb solid 1px;
            width: 40px;
            height: 40px;
            border-radius: 20px;
        }

        .login-bg {
    
    
            display: none;
            width: 100%;
            height: 100%;
            position: fixed;
            top: 0px;
            left: 0px;
            background: rgba(0, 0, 0, .3);
        }

js

    // 1 获取元素
    var login = document.querySelector(".login");
    var mask = document.querySelector(".login-bg");
    var link = document.querySelector("#link");
    var closeBtn = document.querySelector("#closeBtn");
    var title = document.querySelector(".login-title");
    // 2 点击弹出层这个链接 link 让mask 和login 显示出来
    link.addEventListener("click", function ()
    {
    
    
        mask.style.display = "block";
        login.style.display = "block";
    })
    // 3 点击 closeBtn 就隐藏 mask 和login
    closeBtn.addEventListener("click", function ()
    {
    
    
        mask.style.display = "none";
        login.style.display = "none";
    })

    // 4 开始拖拽
    title.addEventListener("mousedown", function (e)
    {
    
      //鼠标按下的时候，得到鼠标在盒子里面的坐标
        var x = e.pageX - login.offsetLeft;
        var y = e.pageY - login.offsetTop;
        document.addEventListener("mousemove", move);  //鼠标移动的时候，吧鼠标在页面中的坐标 减去 鼠标在盒子内的坐标就是模态框的lift和top值
        function move (e)
        {
    
    
            login.style.left = e.pageX - x + "px";
            login.style.top = e.pageY - y + "px";
        }
        document.addEventListener("mouseup", function ()
        {
    
      //鼠标弹起的时候，解除鼠标移动事件
            document.removeEventListener("mousemove", move);
        })
    })