WP Meta Box

/**
 * Extend RW_Meta_Box class
 * Add field types: 'taxonomy', 'post_type'
 */
class RW_Meta_Box_Extend extends RW_Meta_Box {

    function add_missed_values() {
        parent::add_missed_values();

        // add 'multiple' option to taxonomy field with checkbox_list type
        foreach ($this->_meta_box['fields'] as $key => $field) {
            if ('taxonomy' == $field['type'] && 'checkbox_list' == $field['options']['type'] || 'custom_post' == $field['type'] && 'checkbox_list' == $field['options']['type']) {
                $this->_meta_box['fields'][$key]['multiple'] = true;
            }
        }
    }

    // show taxonomy list
    function show_field_taxonomy($field, $meta) {
        global $post;

        if (!is_array($meta)) $meta = (array) $meta;

        $this->show_field_begin($field, $meta);

        $options = $field['options'];
        $terms = get_terms($options['taxonomy'], $options['args']);

        // checkbox_list
        if ('checkbox_list' == $options['type']) {
            foreach ($terms as $term) {
                echo "<input type='checkbox' name='{$field['id']}[]' value='$term->slug'" . checked(in_array($term->slug, $meta), true, false) . " /> $term->name<br/>";
            }
        }
        // select
        else {
            echo "<select name='{$field['id']}" . ($field['multiple'] ? "[]' multiple='multiple' style='height:auto'" : "'") . ">";

            foreach ($terms as $term) {
                echo "<option value='$term->slug'" . selected(in_array($term->slug, $meta), true, false) . ">$term->name</option>";
            }
            echo "</select>";
        }

        $this->show_field_end($field, $meta);
    }

    // show custom post list
    function show_field_custom_post($field, $meta) {
        global $post;

        if (!is_array($meta)) $meta = (array) $meta;

        $this->show_field_begin($field, $meta);



        $options = $field['options'];

        $customs = get_posts($options['args']);

        // checkbox_list
        if ('checkbox_list' == $options['type']) {
            foreach ($customs as $custom) {
                $galleryurl = wp_get_attachment_image_src( get_post_thumbnail_id( $custom->ID ), 'full');
                echo "<input type='checkbox' name='{$field['id']}[]' value='$custom->post_title'" . checked(in_array($custom->post_title, $meta), true, false) . " />$custom->post_title<br/>";
            }
        }
        // select
        else {
            echo "<select name='{$field['id']}" . ($field['multiple'] ? "[]' multiple='multiple' style='height:auto'" : "'") . ">";

            foreach ($customs as $custom) {
                echo "<option value='$custom->post_title'" . selected(in_array($custom->post_title, $meta), true, false) . ">$custom->post_title</option>";
            }
            echo "</select>";
        }

        $this->show_field_end($field, $meta);
    }
}


// Show post_type meta box
array(
    'id' => $prefix . 'custom',
    'type' => 'custom_post',
    'options' => array(
        'type' => 'checkbox_list',
        'args' => array(          // arguments for get posts http://goo.gl/OVtT
            'post_type' => 'artist',
            'posts_per_page' => 3
                )
            ),
            'multiple' => true
        )



// Renamed the class here with the new one

foreach ($meta_boxes as $meta_box) {
    $my_box = new RW_Meta_Box_Extend($meta_box);
}