title: Adding Meta Boxes to Custom Post Type not working
body: <p>I've tried many things to fix this, I'm fairly new to coding a plugin but I can't seem to get this to work.</p>
  
  <p>I have created a custom post type ("Book") perfectly fine. Now I'm trying to add meta-boxes into it. I can't seem to get the coding to connect to each other properly. </p>
  
  <p>I've tried adding the slugs of each meta box to the "support", I've tried various instances of function and add_meta_box as well as putting different things under  "register_meta_box_cb" and nothing I'm doing seems to be working. </p>
  
  <p>Any help is appreciated.</p>
  
  <pre><code>// Register Custom Post Type
  function post_type_book() {
  
  
      $args = array(
          'label'                 =&gt; __( 'book', 'author_station' ),
          'description'           =&gt; __( 'Custom Book Entry', 'author_station' ),
          'supports'              =&gt; array('title'),
          'taxonomies'            =&gt; array( 'genres', 'series', 'tags' ),
           'register_meta_box_cb' =&gt; ('as_add_book' ),
          'labels'                =&gt; $labels,
          'hierarchical'          =&gt; false,
          'public'                =&gt; true,
          'show_ui'               =&gt; true,
          'show_in_menu'          =&gt; false,
          'menu_position'         =&gt; 20,
          'show_in_admin_bar'     =&gt; true,
          'show_in_nav_menus'     =&gt; true,
          'can_export'            =&gt; true,
          'has_archive'           =&gt; true,
          'exclude_from_search'   =&gt; false,
          'publicly_queryable'    =&gt; true,
           'rewrite' =&gt; array('slug' =&gt; 'Book'),
          'capability_type'       =&gt; 'page',
  
      );
      register_post_type( 'as_book', $args );
  
  }
  
  function as_add_book( $meta_boxes ) {
      add_meta_box(
          'add_meta_boxes',
          array( $this, 'as_add_book_boxes' ));
  
       $types = array('post', 'page', 'book');
  
     if (in_array($types)) {
        add_meta_box(
          'as_add_book',
          'add_meta_boxes',
          'Book',
          'as_add_book_callback',
          $types,
          'normal',
          'high'
        );
  
     }
      $prefix = 'as_';
  
  
      $meta_boxes[] = array(
          'id' =&gt; 'as_add_book',
          'title' =&gt; esc_html__( 'Book', 'author_station_book' ),
          'pages'=&gt; array('as_book'),
          'context' =&gt; 'advanced',
          'priority' =&gt; 'high',
          'autosave' =&gt; 'false',
          'fields' =&gt; array(
              array(
                  'id' =&gt; $prefix . 'book_cover',
                  'type' =&gt; 'image',
                  'name' =&gt; esc_html__( 'Book Cover', 'author_station_book' ),
              ),
              array(
                  'id' =&gt; $prefix . 'book_title',
                  'type' =&gt; 'text',
                  'name' =&gt; esc_html__( 'Title', 'author_station_book' ),
              ),
  
      );
  
      return $meta_boxes;
  }
  add_filter( 'rwmb_meta_boxes', 'as_add_book' );
  </code></pre>