Lazy loading is a commonly used design pattern in 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. The opposite of lazy loading is Eager Loading.
In web it can be used in very effective way in photo galleries to speed up the interaction.
Lazy Loading in JQuery
Lazy loader is a jQuery plugin written in JavaScript. It delays loading of images in (long) web pages. Images outside of viewport (visible part of web page) wont be loaded before user scrolls to them. This is opposite of image preloading.
Using lazy load on long web pages containing many large images makes the page load faster. Browser will be in ready state after loading visible images. In some cases it can also help to reduce server load.
Lazyloader is inspired by YUI ImageLoader Utility by Matt Mlinac. Demo page is available.
How to use?
Lazy Load depends on jQuery (doh!) .Include them in your header:
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<script src="jquery.js" type="text/javascript"></script>
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<script src="jquery.lazyload.js" type="text/javascript"></script>
and in your code do:
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$("img").lazyload();
This causes all images below the fold to be lazy loaded.
Lazy Load Images with Prototype
Inspired by:
http://www.appelsiini.net/2007/9/lazy-load-images-jquery-plugin
Requires:
Download lazyload.js.
Requires prototype.js version 1.6.0_rc0 or above.
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-
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function $restore()
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{
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// this function restores the original image source; called when above the fold
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if ( true === $(element).hasAttribute(‘_src’) )
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{
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$(element).writeAttribute({ src: $(element).readAttribute(‘_src’) });
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}
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}
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function $scroll()
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{
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// this function returns the amount the page is scrolled vertically
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var scroll_y = self.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop;
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return parseInt(scroll_y);
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}
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function $height()
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{
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// this function returns the height of the viewport
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var window_height = window.innerHeight || document.documentElement.clientHeight;
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return parseInt(window_height);
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}
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var element = $(element);
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var options = Object.extend({
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threshold : 0,
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placeholder : ‘/images/grey.gif’,
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event : ‘scroll’,
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frequency : 0.1
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}, options || {});
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var offset = $(element).cumulativeOffset()[1];
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var activate_on = (offset – options.threshold) – $height();
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var old_source = $(element).readAttribute(‘src’);
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var new_source = options.placeholder;
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$(element)
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.writeAttribute({ src : new_source })
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.writeAttribute({ ‘_src’ : old_source });
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if ( ‘scroll’ === options.event )
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{
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new PeriodicalExecuter(function($executor)
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{
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if ( activate_on <= $scroll() )
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{
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$restore(); $executor.stop();
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}
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}, options.frequency);
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}
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else
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{
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$(element).observe(options.event, function(event)
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{
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$restore(); $(element).stopObserving();
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});
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}
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return $(element);
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}
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});
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document.observe(‘contentloaded’, function()
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{
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$$(‘img’).invoke(‘lazyload’);
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});
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