Get window size and position

var $ = function(id) {
	return document.getElementById(id);
},
 
// 触点
html = document.documentElement,
box = $('cnt'),
posX = $('left'),
posY = $('top'),
width = $('width'),
height = $('height'),
 
// 浮动框大小
boxWidth = box.offsetWidth,
boxHeight = box.offsetHeight,
 
// 设置位置
setPos = function() {
 
        // 可视窗口
    var htmlWidth = html.clientWidth,
        htmlHeight = html.clientHeight,
 
        // margin 值
        marginX = htmlWidth > boxWidth ? -(boxWidth/2) : 0,
        marginY = htmlHeight > boxHeight ? -(boxHeight/2) : 0,
 
        // 可视宽度如果太小，要注意让内容能完整显示出来
        left = marginX == 0 ? 0 : '50%',
        top = marginY == 0 ? 0 : '50%';
 
        box.style.left = left;
        box.style.top = top;
        box.style.marginLeft = marginX + 'px';
        box.style.marginTop = marginY + 'px';
},
 
// 找到位置
getPos = function() {
    var curLeft =0, curTop = 0, obj = box;
    if (obj.offsetParent) {
        while (obj.offsetParent) {
            curLeft += box.offsetLeft;
            curTop += box.offsetTop;
            obj = obj.offsetParent;
        }
    }
    posX.innerHTML = curLeft;
    posY.innerHTML = curTop;
};
 
width.innerHTML = boxWidth;
height.innerHTML = boxHeight;
 
setPos();
getPos();
 
// 用户放大/缩小窗口，浮动应做相应处理
window.onresize = function(){
    setPos();
    getPos();
}