//创建遮罩层
function AlertMsg(){
/**第一步：创建DIV遮罩层。*/
var sWidth,sHeight;
sWidth = window.screen.availWidth-21;//进修提醒：宽度还应该减去滚动条21px
//屏幕可用工作区高度： window.screen.availHeight;
//屏幕可用工作区宽度： window.screen.availWidth;
//网页正文全文宽：     document.body.scrollWidth;
//网页正文全文高：     document.body.scrollHeight;
if(window.screen.availHeight > document.body.scrollHeight){ //当高度少于一屏
sHeight = window.screen.availHeight-127//进修提醒：高度还应该减去127px;
}else{//当高度大于一屏
sHeight = document.body.scrollHeight;
}
//创建遮罩背景
var maskObj = document.createElement("div");
maskObj.setAttribute('id','BigDiv');
maskObj.style.position = "absolute";
maskObj.style.top = "0";
maskObj.style.left = "0";
maskObj.style.background = "#010101";
maskObj.style.filter = "Alpha(opacity=50);";
maskObj.style.opacity = "0.5";
maskObj.style.width = sWidth + "px";
maskObj.style.height = sHeight + "px";
maskObj.style.zIndex = "10000";
document.body.appendChild(maskObj);


/**第二步：动态设置div的上边距和左边距，使弹出div居中打开。*/
var MyDiv =document.getElementById("MyDiv");
var MyDiv_w = getStyle(MyDiv,"width");
var MyDiv_h = getStyle(MyDiv,"height");

MyDiv_w = parseInt(MyDiv_w); //去掉 单位 "px"
MyDiv_h = parseInt(MyDiv_h);

var width = pageWidth(); 
var height = pageHeight();
var left = leftPosition();
var top = topPosition();

var Div_topposition = top + (height / 2) - (MyDiv_h / 2) - 78; //计算上边距
var Div_leftposition = left + (width / 2) - (MyDiv_w / 2); //计算左边距

MyDiv.style.left = Div_leftposition + "px"; //拼接上 单位"px"
MyDiv.style.top = Div_topposition + "px";
MyDiv.style.display = "block"; //设置弹出div显示
}

/**第三步：关闭窗口和遮罩层。*/
function CloseDiv(){
var Bigdiv = document.getElementById("BigDiv");
var Mydiv = document.getElementById("MyDiv");
document.body.removeChild(Bigdiv); 
Mydiv.style.display="none";
}
/** 第四步：弹出层拖动。*/ 
window.onload = function(){
if(document.getElementById("MyDiv")){
var MyDiv = document.getElementById("MyDiv");
var dragMe   = document.getElementById("dragMe");
//Drag.init(dragMe, MyDiv);
}
}

/**第五步：弹出层跟随滚动条滚动。*/
window.onscroll = window_onscroll;
function window_onscroll(){
var MyDiv =document.getElementById("MyDiv");
var MyDiv_h = getStyle(MyDiv,"height");
MyDiv_h = parseInt(MyDiv_h);
var height = pageHeight();
var top = topPosition();
var Div_topposition = top + (height / 2) - (MyDiv_h / 2); //计算上边距
MyDiv.style.top = Div_topposition + "px";
}
// 计算当前窗口的宽度 //
function pageWidth(){
return window.innerWidth != null ? window.innerWidth : document.documentElement && document.documentElement.clientWidth ? document.documentElement.clientWidth : document.body != null ? document.body.clientWidth : null;
}

// 计算当前窗口的高度 //
function pageHeight(){
return window.innerHeight != null? window.innerHeight : document.documentElement && document.documentElement.clientHeight ? document.documentElement.clientHeight : document.body != null? document.body.clientHeight : null;
}

// 计算当前窗口的上边滚动条//
function topPosition(){
return typeof window.pageYOffset != 'undefined' ? window.pageYOffset : document.documentElement && document.documentElement.scrollTop ? document.documentElement.scrollTop : document.body.scrollTop ? document.body.scrollTop : 0;
}

// 计算当前窗口的左边滚动条//
function leftPosition(){
return typeof window.pageXOffset != 'undefined' ? window.pageXOffset : document.documentElement && document.documentElement.scrollLeft ? document.documentElement.scrollLeft : document.body.scrollLeft ? document.body.scrollLeft : 0;
}

//解决外嵌样式style , 用js获取不到的问题。
function getStyle(elem, name){
if(elem.style[name])
return elem.style[name];
else if(elem.currentStyle) //ie
return elem.currentStyle[name];
else if(document.defaultView && document.defaultView.getComputedStyle){ //w3c
name = name.replace(/([A-Z])/g,"-$1");
name = name.toLowerCase();

var s = document.defaultView.getComputedStyle(elem,"");
return s && s.getPropertyValue(name);
} else
return null
}