Preloading images using PHP and jQuery - Comma separated array?

Question

So I'm building a website that will use a lot of animated GIF's on mouse hover. There will be a lot of images, and I only want to load the images that will be used on any given page. It's using WordPress so I can use the conditional tags to only put out the URL's of the images I need on a certain page.

My problem was how to tell the browser/server which images needed to be preloaded. I decided to add a HTML5 data attribute to the containing element.

For example, I would have a DIV with this PHP in it:

<div id="home-container" class="preload" data-preload="<?php echo preloadImages(); ?>"></div>

Which would call this function in PHP:

function preloadImages() {
    global $post;
    $templateURL = get_template_directory_uri();
    $images = array();

    if( is_page('test') )
        $images[] = "'".$templateURL."/images/gifs-static/button-info-tab-close-off.gif'";
        $images[] = "'".$templateURL."/images/gifs-animated/button-info-tab-close.gif'";
    }

    return implode(", ", $images);
}

So the output would be this:

<div id="home-container" class="preload" data-preload="'http://example.com/images/gifs-static/button-info-tab-close-off.gif', 'http://example.com/images/gifs-animated/button-info-tab-close.gif'"></div>

And then I run this JavaScript:

jQuery('.preload').each(function(){

        var images = [
            // Comma separated list
            jQuery(this).data('preload')
        ];

        jQuery(images).each(function() {
            jQuery('<img />').attr('src', this).addClass('preloaded').appendTo('body').hide();
        });
});

The problem is that the JavaScript does not like the comma separated list. If I just write in the URLs, it works fine. So is there a better way to pass in those URLs? Or a better way in general?

Answer 1

You need to split the string of URLs by the deliminator commas into an array:

    var images = jQuery(this).data('preload').split(',');

jQuery(this).data('preload') is returning a string, and you can use .split() to split that string into an array: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String/Split

Also you can make your loop run a little faster by using $.each():

jQuery('.preload').each(function(){

        var images = [
            // Comma seperated list
            jQuery(this).data('preload')
        ];

        jQuery.each(images, function() {
            jQuery('<img />').attr('src', this).addClass('preloaded').appendTo('body').hide();
        });
});

Note that you shouldn't have anything other than the commas that separate the different urls in the data-attribute:

<div id="home-container" class="preload" data-preload="http://example.com/images/gifs-static/button-info-tab-close-off.gif,http://example.com/images/gifs-animated/button-info-tab-close.gif"></div>

Answer 2

Why don't you put the list directly in a Js array ?

Something like :

<script>
    var aPreloadImages = <?php echo preloadImages(); ?>;

    for (index in aPreloadImages) {
        jQuery('<img />').attr('src', aPreloadImages[index]).addClass('preloaded').appendTo('body').hide();
    }
</script>

And your PHP code :

function preloadImages() {
    global $post;
    $templateURL = get_template_directory_uri();
    $images = array();

    if( is_page('test') )
        $images[] = "'".$templateURL."/images/gifs-static/button-info-tab-close-off.gif'";
        $images[] = "'".$templateURL."/images/gifs-animated/button-info-tab-close.gif'";
    }

    return json_encode($images);
}

Answer 3

Make your php return the data to the element as an actual array, wrapped in []. Then, when you pull it from the data attribute using the .data() method, jquery will convert it into a real array.

div

<div class="preload" data-preload='["foo.jpg","bar.jpg","foobar.gif","barfoo.png"]'></div>

code

var images = jQuery(this).data('preload');