原生js实现可拖动的登录框效果

实现原理

1.onmousemove事件触发时不断更新鼠标的pageXY改变位置，

登陆框的偏移量=鼠标当前位置-鼠标到登录框边框的距离

2.onmousedown鼠标摁下时触发事件获取鼠标到登陆框的距离，再设置true允许拖拽

3.onmouseup 鼠标弹起设置false停止拖拽

4.登录框居中显示公式：(可视区域宽高-登录框宽高)/2

5.当浏览器窗口大小变化时触发事件window.onresize 再更新登陆框居中显示

代码中有详细的注释

完整代码

body,h1,h2,h3,h4,h5,h6,hr,p,blockquote,dl,dt,dd,ul,ol,li,pre,form,fieldset,legend,button,input,textarea,th,td{margin:0;padding:0;}
body,button,input,select,textarea{font:12px/1.5 tahoma,arial,5b8b4f53;}
h1,h2,h3,h4,h5,h6{font-size:100%;}
address,cite,dfn,em,var{font-style:normal;}
code,kbd,pre,samp{font-family:courier new,courier,monospace;}
small{font-size:12px;}
ul,ol{list-style:none;}
a{text-decoration:none;}
a:hover{text-decoration:underline;}
sup{vertical-align:text-top;}
sub{vertical-align:text-bottom;}
legend{color:#000;}
fieldset,img{border:0;}
button,input,select,textarea{font-size:100%;}
table{border-collapse:collapse;border-spacing:0;}
.clear{clear: both;float: none;height: 0;overflow: hidden;}

#btn{width: 80px;
 height: 40px;
 background: #3b7ae3;
 margin:0 auto;
 display: block;
 cursor: pointer;
 border-style: none;
 color: #fff;
 font-size: 16px;}
#mask{
 background: #000;
 opacity: 0.75;
 filter: alpha(opacity=75);
 height: 1000px;
 width: 100%;
 position: absolute;
 left: 0;
 top: 0;
 z-index: 1000;
}
#login{position: absolute; top: 100px; left: 100px; width: 400px; height: auto; border:1px solid #d5d5d5; z-index: 1001; }
.title{position: relative;background-color: #f7f7f7; cursor: move; height: 50px; line-height: 50px; font-size: 16px; color: #333; padding-left:30px;}
.close{position: absolute; top:0; right: 10px; color: #ccc;}
.content{background: #fff; padding: 15px 20px;}
.user{margin-bottom: 15px;}
.password{margin-bottom: 15px;}
.pt{display: block;
 height: 38px;
 padding-left: 15px;
 border: 1px solid #ddd;
 transition: .3s;
 font-size: 14px;
 color: #666;
 width: 343px;
 }
.sm{display: block;
 height: 48px;
 border: 1px solid #ddd;
 transition: .3s;
 font-size: 16px;
 color: #666;
 width: 360px;
 background: #3b7ae3;
 color: #fff;}
 
 

 
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