Preload images from an Ajax Call

Question

Could someone help me understand how to preload the images from random.php page so the first time it loads it fades in as it should. Currently its got an ugly bulk echo because they're not preloaded but once the page has been run once it fades on after the other perfectly...

//Loop through the images and print them to the page
for (var i=0; i < totalBoxes; i++){
  $.ajax({
    url: "random.php?no=",
    cache: false,
    success: function(html) {
      // following line I originally suggested, but let's make it better...
      //$('#bg').append(html).fadeIn('slow');
      // also note the fine difference between append and appendTo.
      var $d = $(html).hide().appendTo('#bg').fadeIn('slow');
      $('img', $d).hover(function() {
        var largePath = $(this).attr("rel");
        $(this).fadeOut("slow", function() {
          $(this).attr({ src: largePath }).fadeIn("slow");
        });
      });
    }
  });
}

Answer 1

Here's a trick that I like: on a page before random.php add at the bottom of the page an img tag which references the image you want to fade in on random.php. Add to the img tag a CSS class which is simply display: none. This will prime the user's browser cache with the image so that when they do go to random.php the image was already downloaded and your fade works as expected. Of course this only works if random.php isn't the site landing page. Another factor is how many images you're talking about and their size.

Answer 2

The other day I wrote up a quick plugin that will take an array of image URLs and preload them, while letting you specify what to do after each image is loaded and/or after all the images are finished loading.

jQuery.extend(jQuery, {
    imagesToLoad: 0,
    loadedImages: [],
    preloadImages: function(){
        var settings = {
            urls : [],
            begin : function(){},
            each : function(){},
            complete : function(){},
            scope: window
        };
        jQuery.extend(settings, arguments[0]);
        var images = [];

        jQuery.imagesToLoad = settings.urls.length;

        settings.begin.call(settings.scope, settings.urls);
        for(var i = 0; i < settings.urls.length; i++){
            images[i] = new Image();
            images[i].onload = function(){
                settings.each.call(settings.scope,this);
                jQuery.loadedImages.push(this);
                if(jQuery.loadedImages.length >= jQuery.imagesToLoad){
                    settings.complete.call(settings.scope,jQuery.loadedImages);
                }
            }
            images[i].src= settings.urls[i];
        }
    }
});

You can then use this in your code by doing something like:

$.preloadImages({
    urls: ['path/to/image/1.jpg', 'path/to/image/2.jpg'],
    begin: function(urls){
        console.log("loading images %o", urls);
    },
    each: function(image){
        console.log("finished loading %s", image.src);
    },
    complete: function(images){
        // do whatever after all the images are done loading
    }
});