You can piggyback off of the WP Gallery shortcode to render your slider. This example is adapted from the Owl Carousel Demo / Docs.
To use it, create a WP Gallery and add the attribute owl="true". Obviously there are better ways to enqueue scripts but this can guide you on gallery modifications.
NOTE: It’s best to put this shortcode in a plugin but it’s possible to add to your functions.php — at that point it will be theme specific. If you apply to a default theme and update then these changes will be overwritten. You have been warned.
For an easy location put it into wp-content/mu-plugins/gallery-owl-shortcode.php. It will always load and you don’t need to make a full plugin.
UPDATE:
I realized this code could repeat a lot of JS and CSS on a blog page if every post had gallery so I added a check to see if that portion has already been included. The downside is that all galleries have the same settings.
<?php
add_shortcode( 'gallery', 'modified_gallery_shortcode' );
function modified_gallery_shortcode($attr)
{
// Replace WP gallery with OWL Carousel using gallery shortcode -- just add `owl=true`
//
//
if( isset($attr['owl']))
{
$attr['itemtag']="span";
$output = gallery_shortcode($attr);
$output = strip_tags($output,'<a><img><span><figcaption>'); // remove extra tags, but keep these
$output = str_replace("span", "div", $output); // replace span for div -- removes gallery wrap
$output = str_replace('gallery-item', "item", $output); // remove class attribute
$output = "<div class=\"owl-demo\" >$output</div>"; // wrap in div
// begin styles and js
static $js_loaded; // only create once
if( empty ( $js_loaded )) {
ob_start();
?>
<style>
.owl-demo .item{
margin: 3px;
}
.owl-demo .item img{
display: block;
width: 100%;
height: auto;
}
</style>
<script defer src="http://owlgraphic.com/owlcarousel/owl-carousel/owl.carousel.js"></script>
<script>
jQuery('head').append('<link defer id="owl-carousel-css" rel="stylesheet" href="http://owlgraphic.com/owlcarousel/owl-carousel/owl.carousel.css" type="text/css" />');
jQuery('head').append('<link defer id="owl-theme-css" rel="stylesheet" href="http://owlgraphic.com/owlcarousel/owl-carousel/owl.theme.css" type="text/css" />');
jQuery(document).ready(function () {
// pulling example from -- including CSS
// http://owlgraphic.com/owlcarousel/demos/images.html
// notice I replaced #id with .class for when you want to have more than one on a page
jQuery(".owl-demo").owlCarousel({
autoPlay: 3000, //Set AutoPlay to 3 seconds
items: 1,
itemsDesktop: [1199, 1],
itemsDesktopSmall: [979, 1] // adaptive image count
});
});
</script>
<?php
$js_loaded = ob_get_clean(); // store in static var
// add the HTML output
$output .= $js_loaded;
}
}
else
{
// default gallery
$output = gallery_shortcode($attr);
}
return $output; // final html
ALTERNATE
So the above code works but it might be nice to create a completely new interface rather than hacking at the gallery that WP provides. Since the IDs are passed in the attributes you can build your own solution from scratch — including a custom link — just make sure to register the link field.
if( isset($attr['owl']))
{
$image_ids = explode(',', $attr['ids']);
$output="";
$size = isset($attr['size']) ? $attr['size'] : 'medium';
foreach($image_ids as $attachment_id) {
// http://wordpress.stackexchange.com/questions/1051/how-to-retrieve-an-image-attachments-alt-text
$img_src="";//esc_url( wp_get_attachment_image_url( $attachment_id, $size ) );
$img_srcset = esc_attr( wp_get_attachment_image_srcset( $attachment_id, $size ) );
$alt = esc_attr ( get_post_meta($attachment_id, '_wp_attachment_image_alt', true) );
$link = esc_attr ( get_post_meta($attachment_id, '_gallery_link_url', true) );
$the_attachment = get_post($attachment_id); // post
$caption = esc_attr ( $the_attachment->post_excerpt );
$img = "<img src=\"{$img_src}\" srcset=\"{$img_srcset}\"
sizes=\"auto\" alt=\"{$alt}\">";
$link = empty($link) ? $img : "<a href=\"{$link}\" >{$img}</a>";
$caption = "<figcaption>{$caption}</figcaption>";
$item = "<div class=\"item\">{$link}{$caption}</div>";
$output .= $item;
}
$output = "<div class=\"owl-demo\" >$output</div>"; // wrap in div
//... include JS & CSS stuff
}
Register Custom Link field on gallery attachment
add_filter( 'attachment_fields_to_edit', 'apply_filter_attachment_fields_to_edit', null, 2 );
add_filter( 'attachment_fields_to_save', 'apply_filter_attachment_fields_to_save', null , 2 );
function apply_filter_attachment_fields_to_edit( $form_fields, $post )
{
// Gallery Link URL field
$form_fields['gallery_link_url'] = array(
'label' => 'Gallery Link URL',
'input' => 'text',
'value' => get_post_meta( $post->ID, '_gallery_link_url', true ),
);
return $form_fields;
}
function apply_filter_attachment_fields_to_save( $post, $attachment ) {
// Save our custom meta fields
if( isset( $attachment['gallery_link_url'] ) ) {
update_post_meta( $post['ID'], '_gallery_link_url', $attachment['gallery_link_url'] );
}
return $post;
} // End function apply_filter_attachment_fields_to_save()