jQuery: create image markers with custom data attributes

If you use Facebook, Twitter or Flickr, you've probably noticed that a common feature provided by these web sites is the ability to add markers to images. These markers will be expanded to show a full description of an image's detail. Creating such markers is easy. A common problem, however, is related to the metadata storage for each marker. We can use jQuery and the new HTML5 data attributes to overcome this problem.

We'll use a well-formed JSON string which contains an array (called coords) which in turn contains several objects with the following three properties:

1. top – The vertical offset of each marker.
2. left – The horizontal offset of each marker.
3. text – The caption's text of each marker.

This string will be placed within a custom data attribute:

<div id="image-wrapper" data-captions='{"coords": [{"top":180,"left":180,"text":"iMac 1"},{"top":250,"left":300,"text":"iMac 2"}]}'>
    <img src="web-agency-company.jpg" alt=""/>
</div>​

Then we need to define our CSS styles:

#image-wrapper {
    width: 400px;
    height: 400px;
    position: relative;
    margin: 2em auto;
    background: #f6f6f6;
    border: 2px solid #ddd;
}

#image-wrapper img {
    display: block;
    margin: 25px auto;
}

span.marker {
    width: 20px;
    height: 20px;
    background: #f66;
    color: #fff;
    text-align: center;
    position: absolute;
    line-height: 20px;
    cursor: pointer;
}
span.marker:before {
    content: '+';
}

span.caption {
    width: 180px;
    background: #f66;
    color: #fff;
    padding: 4px;
    position: absolute;
    top: 20px;
    left: 0;
    display: none;
}
​

With jQuery we need to use the $.parseJSON() to turn the attribute's value into an actual JavaScript object. Then we can loop through the array of objects to extract the metadata we need:

var Markers = {
    fn: {
        addMarkers: function() {
            var target = $('#image-wrapper');
            var data = target.attr('data-captions');
            var captions = $.parseJSON(data);
            var coords = captions.coords;

            for (var i = 0; i < coords.length; i++) {
                var obj = coords[i];
                var top = obj.top;
                var left = obj.left;
                var text = obj.text;

                $('<span class="marker"/>').css({
                    top: top,
                    left: left
                }).html('<span class="caption">' + text + '</span>').
                appendTo(target);

            }
        },
        showCaptions: function() {
            $('span.marker').live('click', function() {
                var $marker = $(this),
                    $caption = $('span.caption', $marker);
                if ($caption.is(':hidden')) {
                    $caption.slideDown(300);

                } else {
                    $caption.slideUp(300);

                }

            });

        }
    },

    init: function() {
        this.fn.addMarkers();
        this.fn.showCaptions();

    }
};

$(function() {
    Markers.init();

});​

[view-example url="http://jsfiddle.net/gabrieleromanato/GhVkx/"]