;(function($) {
 jQuery.fn.scrollFix = function(height, dir) {
  height = height || 0;
  height = height == "top" "bottom") {
    height = document.documentElement.clientHeight - this.scrollHeight;
   } else if (height < 0) {
    height = document.documentElement.clientHeight - this.scrollHeight + height;
   }
   var that = $(this),
    oldHeight = false,
    p, r, l = that.offset().left;
   dir = dir == "bottom" "top"; //默认滚动方向向下
   if (window.XMLHttpRequest) { //非ie6用fixed


    function getHeight() { //>=0表示上面的滚动高度大于等于目标高度
     return (document.documentElement.scrollTop || document.body.scrollTop) + height - that.offset().top;
    }
    $(window).scroll(function() {
     if (oldHeight === false) {
      if ((getHeight() >= 0 && dir == "top") || (getHeight() <= 0 && dir == "bottom")) {
       oldHeight = that.offset().top - height;
       that.css({
        position: "fixed",
        top: height,
        left: l
       });
      }
     } else {
      if (dir == "top" && (document.documentElement.scrollTop || document.body.scrollTop) < oldHeight) {
       that.css({
        position: "static"
       });
       oldHeight = false;
      } else if (dir == "bottom" && (document.documentElement.scrollTop || document.body.scrollTop) > oldHeight) {
       that.css({
        position: "static"
       });
       oldHeight = false;
      }
     }
    });
   } else { //for ie6
    $(window).scroll(function() {
     if (oldHeight === false) { //恢复前只执行一次，减少reflow
      if ((getHeight() >= 0 && dir == "top") || (getHeight() <= 0 && dir == "bottom")) {
       oldHeight = that.offset().top - height;
       r = document.createElement("span");
       p = that[0].parentNode;
       p.replaceChild(r, that[0]);
       document.body.appendChild(that[0]);
       that[0].style.position = "absolute";
      }
     } else if ((dir == "top" && (document.documentElement.scrollTop || document.body.scrollTop) < oldHeight) || (dir == "bottom" && (document.documentElement.scrollTop || document.body.scrollTop) > oldHeight)) { //结束
      that[0].style.position = "static";
      p.replaceChild(that[0], r);
      r = null;
      oldHeight = false;
     } else { //滚动
      that.css({
       left: l,
       top: height + document.documentElement.scrollTop
      })
     }
    });
   }
  });
 };
})(jQuery);