Image preloading

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!

On Apr 10, 2010, at 9:32 AM, ChaosKnight wrote:

   for (var i=0; i < imgFiles.length; i++) {

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!

You’ll want to take the onload out of the body tag and do something like
this in your application.js file:

document.observe(“dom:loaded”, function()
{
preloadImages(’/images/home.png’,…etc…);
}
);

On Apr 11, 3:43 am, steve ross [email protected] wrote:

didn’t preload… On previous websites I used a simple JavaScript
}
Can anyone please tell me why this didn’t work? I also heard that
);
That still doesn’t work in Firefox… What about caching? Won’t that
help?
Or perhaps and alternative solution that works better?

On Apr 11, 2010, at 4:15 AM, ChaosKnight wrote:

 if (document.images) {

Or perhaps and alternative solution that works better?
You haven’t really said what “doesn’t work” means. What have you done to
determine that there is a problem?

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/http://www.google.com/url?sa=D&q=http://www.webreference.com/programming/javascript/gr/column3/&usg=AFQjCNFzGhxZUss7TN6EJ8L7Yt4qIqydAA

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 [email protected]

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

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