Adds a new field in the Novelist plugin for Cover Artist.

novelist-cover-artist.php

<?php
/*
 * Plugin Name: Novelist Cover Artist
 * Plugin URI: https://novelistplugin.com
 * Description: Adds a new field in the Novelist plugin for Cover Artist.
 * Version: 1.0
 * Author: Nose Graze
 * Author URI: https://www.nosegraze.com
 * License: GPL2
 * 
 * @package novelist-cover-artist
 * @copyright Copyright (c) 2016, Nose Graze Ltd.
 * @license GPL2+
*/

/**
 * Add the "Cover Artist" field to the available book layout options.
 *
 * @param array $fields
 *
 * @return array
 */
function novelist_cover_artist_field( $fields ) {
	$fields['cover_artist'] = array(
		'name'        => __( 'Cover Artist', 'novelist-cover-artist' ),
		'placeholder' => '[cover_artist]',
		'label'       => sprintf( __( '<strong>Cover Artist:</strong> %s', 'novelist-cover-artist' ), '[cover_artist]' ),
		'linebreak'   => 'on'
	);

	return $fields;
}

add_filter( 'novelist/book/available-fields', 'novelist_cover_artist_field' );

/**
 * Render the admin area text box
 *
 * @param Novelist_Book $book     Book object.
 * @param WP_Post       $post     Current post object.
 * @param array         $settings Array of settings for this field. Includes the 'label' and 'linebreak'.
 *
 * @return void
 */
function novelist_cover_artist_text_box( $book, $post, $settings ) {
	?>
	<div class="novelist-box-row">
		<label for="novelist_cover_artist"><?php _e( 'Cover Artist', 'novelist' ); ?></label>
		<div class="novelist-input-wrapper">
			<input type="text" id="novelist_cover_artist" name="novelist_cover_artist" value="<?php echo esc_attr( $book->get_value( 'novelist_cover_artist' ) ); ?>">
		</div>
	</div>
	<?php
}

add_action( 'novelist/meta-box/display-field-cover_artist', 'novelist_cover_artist_text_box', 10, 3 );

/**
 * Add 'novelist_cover_artist' as a field to be saved.
 *
 * @param array $fields Array of name attributes to be located in $_POST and saved.
 *
 * @return array
 */
function novelist_add_cover_image_to_save( $fields ) {
	$fields[] = 'novelist_cover_artist';

	return $fields;
}

add_filter( 'novelist/book/meta-box/saved-fields', 'novelist_add_cover_image_to_save' );

/**
 * Sanitize our field with 'sanitize_text_field'.
 */
add_filter( 'novelist/book/meta-box/sanitize/novelist_cover_artist', 'sanitize_text_field' );

/**
 * Render Cover Artist
 *
 * @param string        $value          Current value for this field
 * @param string        $key            The key that is being filtered
 * @param array         $all_fields     All available book fields
 * @param array         $enabled_fields Array of the enabled book fields
 * @param Novelist_Book $book           Object for the current book
 *
 * return string
 */
function novelist_render_cover_artist( $value, $key, $all_fields, $enabled_fields, $book ) {

	$value = $book->get_value( 'novelist_cover_artist' );

	return $value;

}

add_filter( 'novelist/book/pre-render/cover_artist', 'novelist_render_cover_artist', 10, 5 );