I wanted to improve my slideshow with image preloading. What I wanted was something that resembles a queue for preloading.

On a blog I found a piece by Rebecca Murphey, she seems a very smart programmer. I picked up on the idea of triggering events, then read some remarks on binding data to an element, and on event delegation.

Being a noob, I concocted thus :

Each element I use to run the ‘slideshow’ has three basic functions,

  • AjaxLoadImages : loads the xml data ( image urls) into $(elem).data()
  • displayImage : replaces the css background-image url
  • preLoad : creates an img element in the document with the image url as source, to force the browser to download the picture so once I put it in the css background the image file is already downloaded, that stops the flickering

The latter two run on a timer, more about that later on in the post.

With bind(), I tie the event handler function to an element, and can use my own events. I can delegate the handling of events up the dom tree by leaving the handler of the lower elements void and binding a handler with the same name to a higher level element, like body.

$("div").bind('preLoad', function(e) {});
$("div").bind('displayImage', function(e){});
$("div").bind('AjaxLoadImages', function(e){});

In ‘bubbling up’ the element data of the element where the event was triggered is sent along. I can access that elements .data() (with my image list etc.) through the generic $(event.target) object.

//AjaxLoadImages is a http-get that retrieves
//an xml file, and adds the links in it to the .data()
//attribute of the div

//I trigger the event on the div, which is handed upward
//to this function, with the div as 'target' 

$("body").bind('AjaxLoadImages', function(e, fname) {
	var target = $(e.target);

	$.get(fname, {}, function(xml)  {
		var counter = 0;
		$('entry', xml).each(function(i) {
                     //key = (x + counter)
$(e.target).data(('x'+counter).toString(), $(this).find("link").text());

//some general data
		$(e.target).data("preload", "0");
		$(e.target).data("currentimage", "1");
		$(e.target).data("countimage", counter.toString());

//displayImage retrieves the 'currentimage' value 
//which, with an added X, 
//is the key of the imageurl in the .data() element
//(which is key=>value array) 

//Once I have it, I can replace the background image url
//in the css

$("body").bind('displayImage', function(e) {
	var target = $(e.target);
        $(e.target).css('background-image', 'url(' + $(e.target).data(('x'+ target.data("currentimage") ).toString()) +  ')');

//preload does the same trick, with the source attribute
//of an image I add to force the browser to preload
$("body").bind('preLoad', function(e) {
    src: $(e.target).data(('x' + $(e.target).data("currentimage")).toString()) });

the xml file is plain :
<?xml version=”1.0″ encoding=”UTF-8″?>


That sets up the basic data and functions for the div elements.

Now to make it run.

Timing, even with the SetInterval method, was gonna be a pain as it seems I can not tie SetInterval  to an element. Luckily (blair mitchelmore?) was so kind as to write a jQuery plugin to handle multiple timers.

With that, I can write one event handler on body level that assigns each element that comes bubbling up a function on a timer interval to handle the display and preloading, as if it were a loop.

//binding the event with void handler
$("div").bind('ScheduleSlides', function(e){});

///...then binding the actual event handler to body,
//param holds the interval
$('body').bind("ScheduleSlides", function(e, param) {

	var target = $(e.target);

//runs a function on set intervals for the target element
	target.everyTime(param, function(i) {

//display the current image by triggering displayImage,
//which bubbles up to the body event handler
		var imgcount = target.data("currentimage");

//increase the functions internal counter

//set currentimage to the increased counter
                target.data("currentimage", (imgcount).toString());

//if at the end of the image list,
//reset currentimage
//and end preloading
		if( target.data("currentimage") == target.data("countimage") ) {
			target.data("preload", "1");
			target.data("currentimage", "1");

//preload the new image if necessary
		if(target.data("preload") == "0") {

All that remains is to load an xml file and trigger the scheduling…

$("#dia_show").trigger('ScheduleSlides', 1000);

and another one…

$("#dia_show_2").trigger('ScheduleSlides', 2000);

I really wonder why I wrote it that way, but it seems to work without the flickering bit and without having to code the links in css or in the source.

I put the test page and the source of it on the server.

I read some remarks on other blogs that after adding 14 img elements IE throws a stack overflow, I haven’t looked into that.

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