jQuery scrollFix滚动定位插件

http://www.cnblogs.com/Hodor/archive/2012/07/25/2607831.html
【插件功能】

当用户向上或向下滚动页面到一定位置时,目标元素开始固定定位(position:fixed),当回滚到原位置时目标元素恢复到原状态,可以定制触发滚动相对屏幕位置和触发滚动方向,兼容IE6

【插件参数】

$(".target_element").scrollFix( [ "top" | "bottom" | length(可以为负,表示相对bottom), [ "top" | "bottom" ] ]);

第一个参数: 可选,默认为"top",当目标元素到了屏幕相对的位置时开始触发固定,可以填一个数值,如100,-200 ,负值表示相对于屏幕下方

第一个参数: 可选,默认为"top",表示触发固定的滚动方向,"top"表示从上向下滚动时触发,"bottom"表示从下向上滚动时触发

下载插件(download)

;(function($) {
  jQuery.fn.scrollFix = function(height, dir) {
    height = height || 0;
    height = height == "top" ? 0 : height;
    return this.each(function() {
      if (height == "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" ? dir : "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);
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