Lazy Load Images with Prototype
Download lazyload.js.
Requires prototype.js version 1.6.0_rc0 or above.
Element.addMethods({
lazyload: function(element, options)
{
/**
What does it do?
It delays loading of images in (long) pages. Images below the fold (far down in the
page) won’t be loaded before the user scrolls down. This is exact opposite of image
preloading. With long pages containing heavy image content end user result is the
same. Page feels snappier. Browser is in ready state after loading visible images.
No need to wait for n pictures to load.
From Wikipedia:
Lazy loading is a design pattern commonly used in computer programming to defer
initialization of an object until the point at which it is needed. It can contribute
to efficiency in the program’s operation if properly and appropriately used.
Inspired by:
http://www.appelsiini.net/2007/9/lazy-load-images-jquery-plugin
Requires:
Prototype.js version 1.6.0_rc0 or later
A page with lots of big images below the fold (optional)
*/
function $restore()
{
// this function restores the original image source; called when above the fold
if ( true === $(element).hasAttribute('_src') )
{
$(element).writeAttribute({ src: $(element).readAttribute('_src') });
}
}
function $scroll()
{
// this function returns the amount the page is scrolled vertically
var scroll_y = self.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop;
return parseInt(scroll_y);
}
function $height()
{
// this function returns the height of the viewport
var window_height = window.innerHeight || document.documentElement.clientHeight;
return parseInt(window_height);
}
var element = $(element);
var options = Object.extend({
threshold : 0,
placeholder : '/images/grey.gif',
event : 'scroll',
frequency : 0.1
}, options || {});
var offset = $(element).cumulativeOffset()[1];
var activate_on = (offset - options.threshold) - $height();
var old_source = $(element).readAttribute(’src’);
var new_source = options.placeholder;
$(element)
.writeAttribute({ src : new_source })
.writeAttribute({ ‘_src’ : old_source });
if ( ’scroll’ === options.event )
{
new PeriodicalExecuter(function($executor)
{
if ( activate_on <= $scroll() )
{
$restore(); $executor.stop();
}
}, options.frequency);
}
else
{
$(element).observe(options.event, function(event)
{
$restore(); $(element).stopObserving();
});
}
return $(element);
}
});
document.observe('contentloaded', function()
{
$$('img').invoke('lazyload');
});
