弹出层拖拽案例

<style>
    * {
      margin: 0;
      padding: 0;
    }

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

    ul,
    li,
    ol,
    dl,
    dt,
    dd,
    div,
    p,
    span,
    h1,
    h2,
    h3,
    h4,
    h5,
    h6,
    a {
      padding: 0px;
      margin: 0px;
    }

    .login {
      width: 512px;
      height: 280px;
      position: absolute;
      border: #ebebeb solid 1px;
      left: 50%;
      right: 50%;
      background: #ffffff;
      box-shadow: 0px 0px 20px #ddd;
      z-index: 9999;
      margin-left: -256px;
      margin-top: 140px;
      display: none;
    }

    .login-title {
      width: 100%;
      margin: 10px 0px 0px 0px;
      text-align: center;
      line-height: 40px;
      height: 40px;
      font-size: 18px;
      position: relative;
      cursor: move;
      -moz-user-select: none;
      /*火狐*/
      -webkit-user-select: none;
      /*webkit浏览器*/
      -ms-user-select: none;
      /*IE10*/
      -khtml-user-select: none;
      /*早期浏览器*/
      user-select: none;
    }

    .login-input-content {
      margin-top: 20px;
      -moz-user-select: none;
      /*火狐*/
      -webkit-user-select: none;
      /*webkit浏览器*/
      -ms-user-select: none;
      /*IE10*/
      -khtml-user-select: none;
      /*早期浏览器*/
      user-select: none;
    }

    .login-button {
      width: 50%;
      margin: 30px auto 0px auto;
      line-height: 40px;
      font-size: 14px;
      border: #ebebeb 1px solid;
      text-align: center;
      -moz-user-select: none;
      /*火狐*/
      -webkit-user-select: none;
      /*webkit浏览器*/
      -ms-user-select: none;
      /*IE10*/
      -khtml-user-select: none;
      /*早期浏览器*/
      user-select: none;
    }

    .login-bg {
      width: 100%;
      height: 100%;
      position: fixed;
      top: 0px;
      left: 0px;
      background: #000000;
      filter: alpha(opacity=30);
      -moz-opacity: 0.3;
      -khtml-opacity: 0.3;
      opacity: 0.3;
      display: none;
    }

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

    .login-button a {
      display: block;
    }

    .login-input input.list-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;
    }
  </style>

<div class="login-header">
    <!--如果a的href属性值中.协议名是javascript. 那么点击a标签的时候,就会执行:后面的代码-->
    <a id="link" href="javascript:;">点击，弹出登录框</a>
  </div>
  <div id="login" class="login">
    <div id="title" class="login-title">登录会员
      <span>
        <a id="closeBtn" href="javascript:void(0);" class="close-login">关闭</a>
      </span>
    </div>
    <div class="login-input-content">
      <div class="login-input">
        <label>用户名：</label>
        <input type="text" placeholder="请输入用户名" name="info[username]" id="username" class="list-input">
      </div>
      <div class="login-input">
        <label>登录密码：</label>
        <input type="password" placeholder="请输入登录密码" name="info[password]" id="password" class="list-input">
      </div>
    </div>
    <div id="loginBtn" class="login-button">
      <a href="javascript:void(0);" id="login-button-submit">登录会员</a>
    </div>
  </div>
  <!-- 遮盖层 -->
  <div id="bg" class="login-bg"></div>

<script>
  
  // 获取元素
  var link = document.getElementById(‘link‘); //点击的文字
  var bg = document.getElementById(‘bg‘); //遮盖层
  var login = document.getElementById(‘login‘); //登录框
  var title = document.getElementById(‘title‘); //登录框的标题
  var closeBtn = document.getElementById(‘closeBtn‘);
//  需求:
//      1.点击文字,展示遮盖层和登录框
//        1.1获取元素
//        1.2 给link注册点击事件
        link.onclick = function(){
          //        1.3 在事件处理函数中,让login和bg展示出来
          login.style.display = ‘block‘;
          bg.style.display = ‘block‘;
        }

  
//      2. 鼠标在登录框的头部按下时,可以拖动
//      2.1 给title注册mousedown事件
        title.onmousedown = function(e){
//          2.4 获取鼠标在title上按下的坐标
          var downX = e.clientX;
          var downY = e.clientY;
          
//          2.6 获取鼠标按下时,login的初始位置
          var posX = login.offsetLeft;
          var posY = login.offsetTop;
          
          // 2.2 在mousedown中给document注册mousemove事件
          document.onmousemove = function(e){
            //2.3 获取鼠标移动的坐标
            var x = e.clientX;
            var y = e.clientY;
            
//            2.5 计算鼠标移动了多少  移动的坐标- 按下的坐标
            var instanceX = x - downX;
            var instanceY = y - downY;
            
            //2.7 计算login最终处于的位置   鼠标移动的距离 + login初始的位置
            var targetX = instanceX + posX;
            var targetY = instanceY + posY;
  
  
            // 3. 限制登录框的位置
//              3.1 获取最小距离和最大距离
                var maxX = window.innerWidth - login.offsetWidth -21; //可视区的宽度 - login的宽度
                var maxY = window.innerHeight - login.offsetHeight; //可视区的宽度 - login的宽度
//              3.2 判断target是否小于最小距离或者大于最大距离
                if(targetX < 0){
                  targetX = 0;
                }
                if(targetY < 21){
                  targetY = 21;
                }
  
                if(targetX > maxX){
                  targetX = maxX;
                }
  
                if(targetY > maxY){
                  targetY = maxY;
                }
            //由于target算出来的就是login应该处于的位置,但是在最后渲染的一瞬间,css中的margin又作用到了login的身上,导致位置发生了变化,为了抵消掉css中margin的值,所以应该在最后赋值的那一刻,把margin减掉
            login.style.left = targetX + 256 + ‘px‘;
            login.style.top = targetY - 140 + ‘px‘;
          }
        }
        
        //2.8 松手,login不在移动
    document.onmouseup = function(){
      document.onmousemove = null;
    }
</script>