Make your page lighter – defer images to load
June 21st, 2009 Filed Under Javascript, Tips & Tricks, comparisons, jQuery, optimization, snippet, utilities
As web professionals we are generally concerned with the page load time, especially when we have a lot of images loading from third party servers which adds up to the page performance.
Portal developers are mostly worried and concerned with this problem.
I and my friend Shon Thomas have tried putting some thoughts and developed a jQuery plugin which will solve this problem.
The technique used here goes as follows:
We do not assign the “src” value to the images we want to load dynamically. Instead assign that URI in a different validator-friendly attribute. For example, you can use “class” or “longdesc”.
Upon scrolling the page, the plug-in detects the images in the viewport area of the browser window and swaps longdesc(or class) with src.
In both the attributes(class and longdesc) I don’t see any problem and they can be used as per your convenience and belief.There could be some argument regarding usage of both of these attributes; But as far as I think I have my reasonable words for both.
1) class: 99% of the time you don’t assign a class name to any image, instead you control them with their parent elements. Even then, if you are so specific, use longdesc.
2) longdesc: longdesc is a very rarely used attribute. I don’t often see people using this attribute on the content images. Even then, if you are specific then either use class attribute or don’t use this functionality for that particular image(as simple as that).
The image html that your server spits should be somewhat like this:
<img src="http://o.aolcdn.com/shopping.aol.com/img/notavailable.gif" longdesc="http://ai.pricegrabber.com/pi/7/19/48/71948063_160.jpg" alt="Dansko Reese Dress Pumps - Calfskin Leather - For Women" title="Dansko Reese Dress Pumps - Calfskin Leather - For Women"/>
Source Code: The source code below is the plug-in which can go in your jquery plugin file(s) or can get included as a standalone include.
/**
* jQuery plugin: Lazy Image Loader
* version: 3.0
* Author: Vivek Pohre,Shon Thomas
* Website: (http://www.vivekpohre.com)
* Example: call lzload() at or after document ready
*/
function lzload(){
var w=window,adv=30,vph,tmpLD,nowView,collImg,imgTop,scrTop,ci;
function getVPH(){
//DETERMINE WINDOW VIEWPORT HEIGHT ALWAYS TO AVOID RESIZING BROWSER ERROR
if(typeof (vph = w.innerHeight?w.innerHeight:$(w).innerHeight()) != "Number")
vph = document.documentElement.clientHeight;
};
//SAVE THE Y AXIS IN TOP ATTRIB
getVPH();//GET THE VIEWPORT HEIGHT
imgTop = 0;//INIT IMGTOP TO ZERO--TEMP VAR
collImg = $("#content img").filter(':[longdesc]');//COLLECT ALL IMAGES HAVING longdesc ATTR
collImg.each(function(){
imgTop = $(this).offset().top;
$(this).attr("top",imgTop);
});
$(w).bind("scroll",function(){loadImgs()}).bind("resize",function(){getVPH();loadImgs()});//ATTACH SCROLL & RESIZE EVENT TO WINDOW
$.loadImgs = loadImgs = function(imgSet){//MAKE loadImgs FUNCTION PUBLIC
ci=(imgSet)?imgSet.show():collImg;//DECIDE COLLECTION FROM PASSED OR ALL DOM IMAGES
scrTop = $(w).scrollTop();
//DETERMINE THE Y-AXIS IN THE VIEWABLE AREA
nowView = (scrTop+vph+adv);
ci.filter(':[longdesc]').filter(':visible').each(function(i){
if($(this).attr("top")<nowView){
tmpLD = $(this).attr("longdesc");
if(typeof tmpLD!='undefined')
$(this).attr("src",tmpLD)[0].removeAttribute("longdesc",0);
}
});
ci=collImg;
};
loadImgs();
$("#content img").error(function(){
$(this).attr("src","http://o.aolcdn.com/shopping.aol.com/img/notavailable.gif");
});
}
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