Running the Effect

1. Insert a new item at the top that is: opacity: 0 & height: 0.

1. Fade the last item out.

1. Increase first item’s height to real height.

1. …at the same time, decrease the height of the last item.

1. Once height changes have finished, remove the last item.

1. Repeat.

HTML

The HTML is very simple for the effect - since the non-JS version of the page the HTML appears the same, except with a longer list.

As such, rather than the complete listing, which you can see in the live demo, we’re just using a simple list element:

<ul class="spy">
    <li>
      <!-- contents of list item -->
    </li>
</ul>

jQuery

For this example of code, we’re creating a reusable plugin. So I’ve wrapped our code in the follow pattern:

(function ($) {
  
// our plugin goes here
  
})(jQuery)

This allows me to reference the $ variable knowing that it won’t conflict with other libraries such as Prototype - because we’ve passed the jQuery variable in to the function in the following line:

})(jQuery)

Version 1: simultaneously height animate

This is the first version of the plugin. It resizes the first and last item simultaneously:

(function ($) {    
$.fn.simpleSpy = function (limit, interval) {
  // set some defaults
  limit = limit || 4;
  interval = interval || 4000;
  
  return this.each(function () {
    // 1. setup
      // capture a cache of all the list items
    var $list = $(this),
      items = [], // uninitialised
      currentItem = limit,
      total = 0, // initialise later on
      height = $list.find('> li:first').height();
          
    // capture the cache
    $list.find('> li').each(function () {
      items.push('<li>' + $(this).html() + '</li>');
    });
    
    total = items.length;
    
    // chomp the list down to limit li elements    
    $list.find('> li').filter(':gt(' + (limit - 1) + ')').remove();

    // 2. effect        
    function spy() {
      // insert a new item with opacity and height of zero
      var $insert = $(items[currentItem]).css({
        height : 0,
        opacity : 0,
        display : 'none'
      }).prependTo($list);
                    
      // fade the LAST item out
      $list.find('> li:last').animate({ opacity : 0}, 1000, function () {
        // increase the height of the NEW first item
        $insert.animate({ height : height }, 1000).animate({ opacity : 1 }, 1000);

        // AND at the same time - decrease the height of the LAST item
        $(this).animate({ height : 0 }, 1000, function () {
            // finally fade the first item in (and we can remove the last)
            $(this).remove();
        });
      });
        
      currentItem++;
      if (currentItem >= total) {
        currentItem = 0;
      }
        
      // trigger the effect again in 4 seconds
      setTimeout(spy, interval);
    }
    
    spy();
  });
};    
})(jQuery);

Version 2: fixed height

The second version has the following changes, and allows us to remove one of the animations, reducing the work the browser has to do.

We do this by created a fixed height wrapper around the ul.spy, and it works because the styling is on an outer div.

We change:

// chomp the list down to limit li elements
$list.find('> li').filter(':gt(' + (limit - 1) + ')').remove();

To add a line before that wraps our spy:

$list.wrap('<div class="spyWrapper" />').parent().css({ height : height * limit });

// chomp the list down to limit li elements
$list.find('> li').filter(':gt(' + (limit - 1) + ')').remove();

Then we need to comment out the animate height to zero.

We change:

// AND at the same time - decrease the height of the LAST item
$(this).animate({ height : 0 }, 1000, function () {
    // finally fade the first item in (and we can remove the last)
    $(this).remove();
});

To only remove the element:

// finally fade the first item in (and we can remove the last)
$(this).remove();

Version 3: loop once

Though I didn’t cover this in the screencast, it’s simple to change the spy to run once, by changing the following:

if (currentItem >= total) {
  currentItem = 0;
}
  
// trigger the effect again in 4 seconds
setTimeout(spy, interval)

To, if we’ve hit the limit, then don’t set a new timeout:

if (currentItem >= total) {
  // let the spy finish
} else {
  // trigger the effect again in 4 seconds
  setTimeout(spy, interval);
}