js实现登录框鼠标拖拽效果

效果图：
代码如下：
<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>登录框鼠标拖拽效果</title>
 <style type="text/css">
 body {
  background: url("https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1488778794834&di=e97c96dfe7860297d1968c30adc862a2&imgtype=0&src=http%3A%2F%2Fpic1.5442.com%3A82%2F2015%2F0409%2F01%2F15.jpg%2521960.jpg") no-repeat top center #ffffff;
  background-size: 100%;
  padding: 0;
  margin: 0;
  font-size: 12px;
  font-family: "微软雅黑", sans-serif;
 }
 .ui-dialog {
  width: 380px;
  position: absolute;
  z-index: 9000;
  top: 100px;
  left: 100px;
  border: 1px solid #d5d5d5;
  background-color: #ffffff;
  /*display: none;*/
 }
 .ui-dialog-title {
  height: 48px;
  line-height: 48px;
  padding-left: 20px;
  color: #535353;
  font-size: 16px;
  background-color: #f5f5f5;
  border-bottom: 1px solid #efefef;
  cursor: move;
 }
 .ui-dialog-title-closebutton {
  width: 16px;
  height: 16px;
  display: inline-block;
  position: absolute;
  right: 20px;
  color: #000;
  text-decoration: unset;
 }
 .ui-dialog-title-closebutton:hover {
  color: #4ca8ff;
 }
 .ui-dialog-content {
  padding: 15px 20px;
 }
 .ui-dialog-pt15 {
  padding-top: 15px;
 }
 .ui-dialog-l40 {
  height: 40px;
  line-height: 40px;
  text-align: right;
 }
 .ui-dialog-input {
  width: 100%;
  height: 40px;
  margin: 0;
  padding: 0;
  border:1px solid #d5d5d5;
  font-size: 16px;
  color: #c1c1c1;
  text-indent: 25px;
  outline: none;
 }
 .ui-dialog-input-username {
  background: url("images/input_username.png") no-repeat 2px;
 }
 .ui-dialog-input-password {
  background: url("images/input_password.png") no-repeat 2px;
 }
 .ui-dialog-submit {
  width: 100%;
  height: 50px;
  background: #3b7ae3;
  border: none;
  font-size: 16px;
  color: #ffffff;
  outline: none;
  text-decoration: none;
  display: block;
  text-align: center;
  line-height: 50px;
 }
 .ui-dialog-submit:hover {
  background: #3f81b0;
 }
 .ui-mask {
  width: 100%;
  height: 100%;
  background: #000;
  opacity: 0.4;
  position: absolute;
  top: 0;
  left: 0;
  z-index: 8000;
  display: none;
 }
 .link {
  text-align: right;
  line-height: 20px;
  padding-right: 40px;
 }
 </style>
</head>
<body>
 <div class="ui-dialog" id="dialog">
 <div class="ui-dialog-title" id="dialogTitle">
  登录
  <!-- 右上角的关闭按钮 -->
  <a href="javascript:hideDialog();" rel="external nofollow" class="ui-dialog-title-closebutton">X</a>
 </div>
 <div class="ui-dialog-content">
  <div class="ui-dialog-l40 ui-dialog-pt15">
  <input class="ui-dialog-input ui-dialog-input-username" type="input" placeholder="手机/邮箱/用户名" />
  </div>
  <div class="ui-dialog-l40 ui-dialog-pt15">
  <input class="ui-dialog-input ui-dialog-input-password" type="input" placeholder="密码" />
  </div>
  <div class="ui-dialog-l40">
  <a href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" >忘记密码</a>
  </div>
  <div>
  <a class="ui-dialog-submit" href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" >登录</a>
  </div>
  <div class="ui-dialog-l40">
  <a href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" >立即注册</a>
  </div>
 </div>
 </div>
 <div class="ui-mask" id="mask"></div>
 <div class="link">
 <a href="javascript:showDialog();" rel="external nofollow" style=" text-decoration: unset;color: #4ca8ff">登录</a>
 </div>
 <script type="text/javascript">
 // 获取元素对象
 function g(id) {
  return document.getElementById(id);
 }
 // 自动居中函数 -- 登录浮层
 // el {Element}
 function autoCenter(el) {
  // 获得可视区域的宽和高
  var bodyW = document.documentElement.clientWidth;
  var bodyH = document.documentElement.clientHeight;
  // 获得元素 el 的宽和高
  var elW = el.offsetWidth;
  var elH = el.offsetHeight;
  // 设置元素的 style 样式
  el.style.left = (bodyW - elW) / 2 + 'px';
  el.style.top = (bodyH - elH) / 2 + 'px';
 }
 // 扩展元素到整个可视区域 -- 遮罩层
 // el {Element}
 function fillToBody(el) {
  // 将元素的宽和高设置的和可视区域一样
  el.style.width = document.documentElement.clientWidth + 'px';
  el.style.height = document.documentElement.clientHeight + 'px';
 }
 // 定义全局变量
 var mouseOffsetX = 0;
 var mouseOffsetY = 0;
 var isDragging = false;
 // 鼠标事件1 -- 在标题栏上按下
 // 计算鼠标相对拖拽元素的的左上角的坐标， 并且标记元素为可拖动
 g('dialogTitle').addEventListener('mousedown', function(e) {
  var e = e || window.event;
  // 用鼠标按下时的坐标减去 dialog 的左上角坐标
  mouseOffsetX = e.pageX - g('dialog').offsetLeft;
  mouseOffsetY = e.pageY - g('dialog').offsetTop;
  isDragging = true;
 });
 // 鼠标事件2 -- 鼠标移动
 document.onmousemove = function(e) {
  var e = e || window.event;
  // 鼠标当前位置
  var mouseX = e.pageX;
  var mouseY = e.pageY;
  // 鼠标从单击时至当前时刻移动的距离
  var moveX = 0;
  var moveY = 0;
  if (isDragging === true) {
  moveX = mouseX - mouseOffsetX;
  moveY = mouseY - mouseOffsetY;
  // 范围限定
  // moveX > 0 且 moveX < (页面最大宽度 - 浮层宽度)
  // moveY > 0 且 moveY < (页面最大宽度 - 浮层高度)
  var pageWidth = document.documentElement.clientWidth;
  var pageHeight = document.documentElement.clientHeight;
  // 登录浮层的宽、高
  var dialogWidth = g('dialog').offsetWidth;
  var dialogHeight = g('dialog').offsetHeight;
  var maxX = pageWidth - dialogWidth;
  var maxY = pageHeight - dialogHeight;
  moveX = Math.min(maxX, Math.max(0, moveX));
  moveY = Math.min(maxY, Math.max(0, moveY));
  g('dialog').style.left = moveX + 'px';
  g('dialog').style.top = moveY + 'px';
  }
 };
 // 鼠标事件3 -- 鼠标松开
 document.onmouseup = function() {
  isDragging = false;
 };
 // 展现登录浮层
 function showDialog() {
  g('dialog').style.display = 'block';
  g('mask').style.display = 'block';
  autoCenter(g('dialog'));
  fillToBody(g('mask'));
 }
 // 隐藏登录浮层
 function hideDialog() {
  g('dialog').style.display = 'none';
  g('mask').style.display = 'none';
 }
 window.onresize = function() {
  autoCenter(g('dialog'));
  fillToBody(g('mask'));
 };
 showDialog();
 autoCenter(g('dialog'));
 </script>
</body>
</html>
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