用jCarousel预加载图像

Question

I have the following code which loads a JSON feed and creates all the HTML needed for the jCarousel to work. However, I'm not sure how to preload the images. Anyone have any idea's how to do this?

$(".banner ul").jcarousel({
    itemLoadCallback:loadTopBanner,
    auto: 6,
    wrap: 'circular',
    scroll: 1,
    animation:1000,
    itemFallbackDimension:10
});

function loadTopBanner(carousel, state){
    $.getJSON("get_top_banner.php", function(data){
        carousel.size( data.length );
            $.each(data, function(i){
                carousel.add(i, makeTag(this.imageURL, this.URL));
            });
        });
    }

function makeTag(img, url){
    return "<a href='" + url + "'><img src='" + img + "'></a>";
}

Answer 1

This should do the trick, however, it is untested, and can be further optimised:

function loadTopBanner(carousel, state) {
    $.getJSON("get_top_banner.php", function(data) {
        carousel.size(data.length);

        // make array of elements to which load events can be attached
        var imgs = [];

        $.each(data, function(i) {
            var img = $("<img/>").attr("src", this.imageURL);
            imgs.push(img);
        });

        // init a load counter
        var loadCounter = 0;

        $.each(imgs, function() {
            $(this).one("load", function() {
                loadCounter++

                // when all have loaded, add to carousel
                if (loadCounter == data.length) {
                    $.each(data, function(i) {
                        carousel.add(i, makeTag(this.imageURL, this.URL));
                    });
                }
            });
            if(this.complete) $(this).trigger("load");
        });
    });
}

Answer 2

May not be a good solution but you can try to load the images somewhere on the page in a hidden div so that they will get cached before you make a ajax call and use them in loadTopBanner method. This way the carousel will not show any delay in loading the images.

Answer 3

If you really want to preload images, you don't need to put them in a hidden div -- you can use the Image object:

var image = new Image();
image.src = "images/foo.jpg";