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问题:

I'm trying to create a div that stays fixed on top when a user scrolls down and when he scrolls back up, goes back to the original position.

I need this exact same behaviour that 9gag offers -> http://9gag.com/gag/293756

Thanks!

回答1:

Do the following:

Create a scroll event handler on the $(window).
In this event handler, check if the user has scrolled lower than the top of the element you always want in view. You can use offest and scrollTop methods to do this.
If yes, set the element to position: fixed with top: 0. You may also need to adjust the left attribute, depending on your layout.
If no, set the element to position: static.

回答2:

Use the Jquery Waypoints plugin: http://imakewebthings.github.com/jquery-waypoints/

Extremely easy to implement.

回答3:

The code that creates exactly the same behavior as 9gag.com:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<!-- tags html, header, body, divs, anything -->

<div id="post-control-bar" class="spread-bar-wrap">
    <div class="spread-bar" style="width:600px">
        Facebook button, Twitter button, anything...
    </div>
</div>

<script type="text/javascript">
window.onscroll = function()
{
    if( window.XMLHttpRequest ) {
        if (document.documentElement.scrollTop > 173 || self.pageYOffset > 173) {
            $('post-control-bar').style.position = 'fixed';
            $('post-control-bar').style.top = '0';
        } else if (document.documentElement.scrollTop < 173 || self.pageYOffset < 173) {
            $('post-control-bar').style.position = 'absolute';
            $('post-control-bar').style.top = '';
        }
    }
}
</script>

<!-- content, footer, anything -->