Re: Image preloading

steve ross
2010-04-12T06:35:54Z

It all depends, as you say, on why you''re preloading the images. Most
of the image preloading code looks exactly like the OP''s and works fine
if your UI relies on just having the images cached by the browser so they can
work in smooth rollovers or other page effects that rely on image swapping. Your
code is more capable, but possibly overkill if all that''s being handled
is making sure mouseovers look good.

Note also that the preloading code has been blindly included in Web pages for
years, even as bandwidth has increased to render the benefits insignificant.

Just my $.02

On Apr 11, 2010, at 10:42 AM, Christophe Decaux wrote:
> I''m just a rails amateur, but I''ve been dealing with
Prototype stuff a bit longer and I bumped into this image preloading stuff
already.
> I don''t clearing understand what you want to do while your images
are preloading and also what you want to do once they are preloaded and this is
pretty crucial as this may last some time. You''ll never know.
> And in addition your code is too simplistic.
>
> So take a look at this that I posted to the ProtoScripty group a while ago:
> ====================================================> I think the
following code should help you, I found it sometime ago it
> is based on this page :
http://www.webreference.com/programming/javascript/gr/column3/
> Basically, you need to have
>         - an array with url''s for your big images such as
picSrc[0] = ''images/
> big1.jpg''; picSrc[1] = ''images/big3.jpg''
> - a hidden in your page which
displays your
> waiting message
>
> When you want to load your big images you call these two lines:
>
> $(''waitingPic'').show()
> initPics()
>
> Here''s the code
>
> function initPics(){
>         var ImagePreLoader = Class.create({
>                 callback: null,
>                 imageCache: new Array,
>                 loaded: 0,
>                 processed: 0,
>                 noOfImages: 0,
>                 initialize: function(images, options) {
>                         if (options) {
>                                 if (options.callback) this.callback =
options.callback;
>                         }
>
>                         this.noOfImages = images.length;
>                         for ( var i = 0; i < images.length; i++ )
>                       this.preload(images[i]);
>                 },
>                 preload: function(imgSrc) {
>                         var image = new Image;
>                         this.imageCache.push(image);
>                         Event.observe(image, ''load'',
this.onload.bindAsEventListener(this),
> false);
>                         Event.observe(image, ''error'',
this.onerror.bindAsEventListener
> (this), false);
>                         Event.observe(image, ''abort'',
this.onabort.bindAsEventListener
> (this), false);
>                         image.preloader = this;
>                         image.loaded = false;
>                         image.src = imgSrc;
>                 },
>                 onComplete: function() {
>                         this.processed++;
>                         if (this.processed==this.noOfImages) {
>                                 this.callback(this.imageCache,
this.loaded);
>                         }
>                 },
>                 onload: function(e) {
>                         this.loaded++;
>                         this.onComplete();
>                 },
>                 onerror: function(e) {
>                         this.onComplete();
>                 },
>                 onabort: function(e) {
>                         this.onComplete();
>                 }
>         });
>
>         var imgPreloadCallback = function(imageCache, loaded) {
>             // where:
>             //     imageCache is an array of the loaded images
>             //     loaded is an int of the number of images that loaded.
>             //doSomethingAfterImagesAreLoaded();
>                 $(''waitingPic'').hide();
>                 picsPreLoaded = true;
>         }
>
>         var imgLoader = new ImagePreLoader(picSrc,
> {callback:imgPreloadCallback});
>
>
> }
>
> Hope this helps.
> Christophe
>
>
>
> 2010/4/10 ChaosKnight
<rvanhoepen-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Hi, I am still very new to Ruby on Rails, but I''m busy with my
first
> RoR website, everything went well, until I realized that my images
> didn''t preload... On previous websites I used a simple JavaScript
> preloader that seemed to work very well:
>
>    function preloadImages() {
>      if (document.images) {
>        var imgFiles = preloadImages.arguments;
>        var preloadArray = new Array();
>        for (var i=0; i < imgFiles.length; i++) {
>          preloadArray[i] = new Image;
>          preloadArray[i].src = imgFiles[i];
>       }
>     }
>   }
>
> And I called it from within the HTML:
>
>    <body
onload="preloadImages(''/images/home.png'',....etc....)
>
> This is how I called the image rollover in the Rails code:
>    <%= link_to(image_tag(''home.png'',
:mouseover=>"home_ro.png"), ''/'',
> {:controller=>''home'',
:action=>''index''}) %>
>
> Can anyone please tell me why this didn''t work? I also heard that
> Prototype has it''s own built-in image preloader, is this true?
>
> Thanks I really appreciate your help!
>
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