// 插件来源于互联网，henry@celay做了适当优化
// 2009年8月
$(document).ready(function() {
	var menuBar = "ul#topnav";
	var menuStr = "ul#topnav li";

	//居中主菜单
	var menuWidth = ($(menuStr).length) * ($(menuStr).width());
	var totalWidth = $(menuBar).parent().width();
	var leftPos = (totalWidth - menuWidth)/2 ;
	$(menuBar).css("left", leftPos-10);

	//调整每个子菜单的位置
	$(menuStr).each(function(){
		var navMenu = $(menuBar)
		var navOffset = navMenu.offset();

		var upOffset = $(this).offset();
		var subSpan = $(this).find("span");
		var spanOffset = subSpan.offset();
		var leftPos = upOffset.left - (subSpan.width() - $(this).width())/2;
		if (leftPos < navOffset.left){
			leftPos = navOffset.left;
		}
		if ((leftPos + subSpan.width()) > (navOffset.left + navMenu.width())){
			leftPos = navOffset.left + navMenu.width() - subSpan.width();
		}
		subSpan.css("left", leftPos - navOffset.left);
	}),
	$(menuStr).hover(function() { //Hover over event on list item
		$(this).find("span").show(); //Show the subnav
	},
	function() { //on hover out...
		$(this).find("span").hide(); //Hide the subnav
	});	
});