Rewriting WordPress Gallery Shortcode with Bootstrap Carousel

In a recently personal theme project for publishing travel posts, I desired a slider-style image gallery rather than the standard column/list-style one WordPress uses out of the box.

I like the look-and-feel of the Bootstrap Carousel, so my challenge was this:

Is it possible to mash up the WordPress Gallery Shortcode with Bootstrap’s Carousel?

I’ve given my solution below, but would be interested to know what other people might suggest to improve the code and make it work better.

On a separate thread, I found out that it was indeed possible to customize the output of the Gallery Shortcode using some code in the functions.php.

The code below would be a fairly standard version of the slider without captions. You’d obviously need to make sure you include Jquery and Bootstrap files as you would normally do using Bootstrap.

// Remove built in shortcode
remove_shortcode('gallery', 'gallery_shortcode');

// Replace with custom shortcode
function shortcode_gallery($attr) {
    $post = get_post();

    static $instance = 0;
    $instance++;

    if (!empty($attr['ids'])) {
        if (empty($attr['orderby'])) {
            $attr['orderby'] = 'post__in';
        }
        $attr['include'] = $attr['ids'];
    }

    $output = apply_filters('post_gallery', '', $attr);

    if ($output != '') {
        return $output;
    }

    if (isset($attr['orderby'])) {
        $attr['orderby'] = sanitize_sql_orderby($attr['orderby']);
        if (!$attr['orderby']) {
            unset($attr['orderby']);
        }
    }

    extract(shortcode_atts(array(
        'order' => 'ASC',
        'orderby' => 'menu_order ID',
        'id' => $post->ID,
        'itemtag' => '',
        'icontag' => '',
        'captiontag' => '',
        'columns' => 3,
        'size' => 'thumbnail',
        'include' => '',
        'link' => '',
        'exclude' => ''
                    ), $attr));

    $id = intval($id);

    if ($order === 'RAND') {
        $orderby = 'none';
    }

    if (!empty($include)) {
        $_attachments = get_posts(array('include' => $include, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby));

        $attachments = array();
        foreach ($_attachments as $key => $val) {
            $attachments[$val->ID] = $_attachments[$key];
        }
    } elseif (!empty($exclude)) {
        $attachments = get_children(array('post_parent' => $id, 'exclude' => $exclude, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby));
    } else {
        $attachments = get_children(array('post_parent' => $id, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby));
    }

    if (empty($attachments)) {
        return '';
    }

    if (is_feed()) {
        $output = "\n";
        foreach ($attachments as $att_id => $attachment) {
            $output .= wp_get_attachment_link($att_id, $size, true) . "\n";
        }
        return $output;
    }

    //Bootstrap Output Begins Here
    //Bootstrap needs a unique carousel id to work properly. Because I'm only using one gallery per post and showing them on an archive page, this uses the $post->ID to allow for multiple galleries on the same page.

    $output .= '<div id="carousel-' . $post->ID . '" class="carousel slide" data-ride="carousel">'; 
    $output .= '<!-- Indicators -->';
    $output .= '<ol class="carousel-indicators">';

    //Automatically generate the correct number of slide indicators and set the first one to have be class="active".
    $indicatorcount = 0;
    foreach ($attachments as $id => $attachment) {
        if ($indicatorcount == 1) {
            $output .= '<li data-target="#carousel-' . $post->ID . '" data-slide-to="' . $indicatorcount . '" class="active"></li>';
        } else {
            $output .= '<li data-target="#carousel-' . $post->ID . '" data-slide-to="' . $indicatorcount . '"></li>';
        }
        $indicatorcount++;
    }

    $output .= '</ol>';
    $output .= '<!-- Wrapper for slides -->';
    $output .= '<div class="carousel-inner">';
    $i = 0;

    //Begin counting slides to set the first one as the active class
    $slidecount = 1;
    foreach ($attachments as $id => $attachment) {
        $link = isset($attr['link']) && 'file' == $attr['link'] ? wp_get_attachment_link($id, $size, false, false) : wp_get_attachment_link($id, $size, true, false);

        if ($slidecount == 1) {
            $output .= '<div class="item active">';
        } else {
            $output .= '<div class="item">';
        }

        $image_src_url = wp_get_attachment_image_src($id, $size);
        $output .= '<img src="' . $image_src_url[0] . '">';
        $output .= '    </div>';


        if (trim($attachment->post_excerpt)) {
            $output .= '<div class="caption hidden">' . wptexturize($attachment->post_excerpt) . '</div>';
        }

        $slidecount++;
    }

    $output .= '</div>';
    $output .= '<!-- Controls -->';
    $output .= '<a class="left carousel-control" href="#carousel-' . $post->ID . '" data-slide="prev">';
    $output .= '<span class="glyphicon glyphicon-chevron-left"></span>';
    $output .= '</a>';
    $output .= '<a class="right carousel-control" href="#carousel-' . $post->ID . '" data-slide="next">';
    $output .= '<span class="glyphicon glyphicon-chevron-right"></span>';
    $output .= '</a>';
    $output .= '</div>';
    $output .= '</dl>';
    $output .= '</div>';

    return $output;