二级导航制作实现不是很理解？
这一段js不是很明白，为什么要判断长度是否大于零啊，stop(true,true)为什么用这个呢，不是很理解
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</nav>// 导航移上去显示二级导航的效果
$(".nav_list_all").find(".nav_list").hover(function(){
if($(this).find(".nav_child").length>0){
$(".nav_child_bg").stop(true,true).slideDown(200);
$(this).find(".nav_child").stop(true,true).slideDown(300);
}
},function(){
$(".nav_child_bg").stop(true,true).slideUp(300);
$(this).find(".nav_child").stop(true,true).slideUp(200);
})
