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】scrollFix(jb51.net).rar

复制代码 代码如下:
<script type="text/javascript" src="jquery-1.7.2.min.js"></script>
<script type="text/javascript" src="scrollFix.js"></script>
<p><span style="color: #808000;">【代码示例】</span></p>
<div class="d">
<div class="demo" style="background: #ff6000;">$("#a").scrollFix(-200);
<div>滚动到距离下面200px时开始固定，默认从上到下触发</div>
</div>
&nbsp;</div>
<div class="d">
<div class="demo" style="background: #82BF00;">$("#b").scrollFix(200,"bottom");
<div>滚动到距离上面200px时开始固定，指定"bottom"从下到上触发</div>
</div>
&nbsp;</div>
<div class="d">
<div class="demo" style="background: #0C9CAE;">$("#c").scrollFix("top","top");
<div>滚动到距离上面0时开始固定，指定"top"从上到下触发</div>
</div>
&nbsp;</div>
<div class="d">
<div class="demo" style="background: #478FCE;">$("#d").scrollFix("bottom","top");
<div>滚动到距离下面0时开始固定，指定"bottom"从下到上触发</div>
</div>
</div>

实现代码：
复制代码 代码如下:
<script type="text/javascript">// <![CDATA[
window.onload=function(){
$(".demo:eq(0)").scrollFix(-200);
$(".demo:eq(1)").scrollFix(200,"bottom");
$(".demo:eq(2)").scrollFix("top","top");
$(".demo:eq(3)").scrollFix("bottom","bottom");
}
// ]]></script>

核心代码：

;(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);