Add attribute to caption shortcode from custom attachment field

Question

Here’s a solution that leverages the shortcode_atts_{shortcode} filter, where caption is the shortcode in this case.

This works by pulling the attachment id from $out['id'] then getting the value of the class via get_post_meta( $attachment_id, 'img_class_field', true ).

With this approach, it is not necessary to modify the output of the caption shortcode in the editor. The adding of the custom class is handled behind the scenes. It also still works if a user has manually added a class attribute to the caption shortcode.

/**
 * Adds class to caption shortcode output via img_class_field attachment meta.
 *
 * @param array  $out       The output array of shortcode attributes.
 * @param array  $pairs     The supported attributes and their defaults.
 * @param array  $atts      The user defined shortcode attributes.
 * @param string $shortcode The shortcode name.
 */
add_filter( 'shortcode_atts_caption', 'wpse_shortcode_atts_caption', 10, 4 );
function wpse_shortcode_atts_caption( $out, $pairs, $atts, $shortcode ) {
    // Get the attachment id. It should be available via $out['id'], but
    // it will be in the format 'attachment_xxxx' where xxxx is the id.
    // We'll try to get the id portion and we'll bail if this doesn't work.
    $attachment_id = isset( $out['id'] ) && ( $out['id'] ) ? $out['id'] : false;
    $attachment_id = (int) preg_replace( '/^attachment_/', '', $attachment_id );
    if ( ! $attachment_id ) {
        return $out;
    }

    // Get the custom image class and add it to the existing classes
    $extra_image_class = get_post_meta( $attachment_id, 'img_class_field', true );
    if ( $extra_image_class ) {
        $spacer = isset( $out['class'] ) && ( $out['class'] ) ? ' ' : '';
        $out['class'] .= esc_attr( $spacer . $extra_image_class );
    }

    return $out;
}

Usage

Custom classes added to attachment meta data (I used 2 for good measure):

some-class another-class

Example of unmodified caption shortcode added via media modal:

<a href="http://example.com/wp-content/uploads/2017/05/image.jpg"><img src="http://example.com/wp-content/uploads/2017/05/image.jpg" alt="" width="480" height="360" class="size-full wp-image-2397" /></a> This is the caption!

Example output 1: (theme supports HTML5 for captions):

<figure id="attachment_2397" style="width: 480px" class="wp-caption alignnone some-class another-class">
    <a href="http://example.com/">
        <img src="http://example.com/wp-content/uploads/2017/05/image.jpg" alt="" width="480" height="360" class="size-full wp-image-2397" srcset="http://example.com/wp-content/uploads/2017/05/image.jpg 480w, http://example.com/wp-content/uploads/2017/05/image-300x225.jpg 300w" sizes="(max-width: 480px) 100vw, 480px">
    </a>
    <figcaption class="wp-caption-text">This is the caption!</figcaption>
</figure>

Example output 2: (theme does not support HTML5 for captions):

<div id="attachment_2397" style="width: 490px" class="wp-caption alignnone some-class another-class">
    <a href="http://example.com/wp-content/uploads/2017/05/image.jpg">
        <img src="http://example.com/wp-content/uploads/2017/05/image.jpg" alt="" width="480" height="360" class="size-full wp-image-2397" srcset="http://example.com/wp-content/uploads/2017/05/image.jpg 480w, http://example.com/wp-content/uploads/2017/05/image-300x225.jpg 300w" sizes="(max-width: 480px) 100vw, 480px">
    </a>
    <p class="wp-caption-text">This is the caption!</p>
</div>

Usage

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