//右下角  
$("#id").floatdiv("rightbottom");  
//左下角  
$("#id").floatdiv("leftbottom");  
//右下角  
$("#id").floatdiv("rightbottom");  
//左上角  
$("#id").floatdiv("lefttop");  
//右上角  
$("#id").floatdiv("righttop");  
//居中  
$("#id").floatdiv("middle");  

jQuery.fn.floatdiv=function(location){  
  //判斷瀏覽器版本  
  var isIE6=false;  
  var Sys = {};  
  var ua = navigator.userAgent.toLowerCase();  
  var s;  
  (s = ua.match(/msie ([\d.]+)/)) ? Sys.ie = s[1] : 0;  
  if(Sys.ie && Sys.ie=="6.0"){  
    isIE6=true;  
  }  
  var windowWidth,windowHeight;//窗口的高和寬  
  //取得窗口的高和寬  
  if (self.innerHeight) {  
    windowWidth=self.innerWidth;  
    windowHeight=self.innerHeight;  
  }else if (document.documentElement&&document.documentElement.clientHeight) {
    windowWidth=document.documentElement.clientWidth;  
    windowHeight=document.documentElement.clientHeight;  
  } else if (document.body) {  
    windowWidth=document.body.clientWidth;  
    windowHeight=document.body.clientHeight;  
  }  
  return this.each(function(){  
    var loc;//層的絕對(duì)定位位置  
    var wrap=$("<div></div>");  
    var top=-1;  
    if(location==undefined || location.constructor == String){  
      switch(location){  
        case("rightbottom")://右下角  
          loc={right:"0px",bottom:"0px"};  
          break;  
        case("leftbottom")://左下角  
          loc={left:"0px",bottom:"0px"};  
          break;   
        case("lefttop")://左上角  
          loc={left:"0px",top:"0px"};  
          top=0;  
          break;  
        case("righttop")://右上角  
          loc={right:"0px",top:"0px"};  
          top=0;  
          break;  
        case("middletop")://居中置頂  
          loc={left:windowWidth/2-$(this).width()/2+"px",top:"0px"};  
          top=0;  
          break;  
        case("middlebottom")://居中置低  
          loc={left:windowWidth/2-$(this).width()/2+"px",bottom:"0px"};  
          break;  
        case("leftmiddle")://左邊居中  
          loc={left:"0px",top:windowHeight/2-$(this).height()/2+"px"};  
          top=windowHeight/2-$(this).height()/2;  
          break;  
        case("rightmiddle")://右邊居中  
          loc={right:"0px",top:windowHeight/2-$(this).height()/2+"px"};  
          top=windowHeight/2-$(this).height()/2;  
          break;  
        case("middle")://居中  
          var l=0;//居左  
          var t=0;//居上  
          l=windowWidth/2-$(this).width()/2;  
          t=windowHeight/2-$(this).height()/2;  
          top=t;  
          loc={left:l+"px",top:t+"px"};  
          break;  
        default://默認(rèn)為右下角  
          location="rightbottom";  
          loc={right:"0px",bottom:"0px"};  
          break;  
      }  
    }else{  
      loc=location;  
      alert(loc.bottom);  
      var str=loc.top;  
      //09-11-5修改：加上top為空值時(shí)的判斷  
      if (typeof(str)!= 'undefined'){  
        str=str.replace("px","");  
        top=str;  
      }  
    }  
    /*fied ie6 css hack*/  
    if(isIE6){  
      if (top>=0)  
      {  
        wrap=$("<div style=\"top:e­xpression(documentElement.scrollTop+"+top+");\"></div>");  
      }else{  
        wrap=$("<div style=\"top:e­xpression(documentElement.scrollTop+documentElement.clientHeight-this.offsetHeight);\"></div>");
      }  
    }  
    $("body").append(wrap);  
    wrap.css(loc).css({position:"fixed",z_index:"999"});  
    if (isIE6)  
    {  
      wrap.css("position","absolute");  
      //沒(méi)有加這個(gè)的話，ie6使用表達(dá)式時(shí)就會(huì)發(fā)現(xiàn)跳動(dòng)現(xiàn)象  
      //至于為什么要加這個(gè)，還有為什么要加nothing.txt這個(gè)，偶也不知道，希望知道的同學(xué)可以告訴我  
      $("body").css("background-attachment","fixed").css("background-image","url(n1othing.txt)");  
    }  
    //將要固定的層添加到固定層里  
    $(this).appendTo(wrap);  
  });  
};