Restrict category drop down to a specific post type

   <?php
    $exclusive_post_type = 'book';
    $exclude_cats        = array();
    $nonemptycats        = get_terms( 'category' );
    foreach( $nonemptycats as $term ){
      $term_id                 = $term->term_id;
      $term_objects            = get_objects_in_term( $term_id, 'category' );
      $exclude_term            = true;
      foreach( $term_objects as $post_id ){
        if ( $exclusive_post_type === get_post_type( $post_id ) ){
          $exclude_term = false;
          break;
        }
      }
      if ( $exclude_term ) $exclude_cats[] = $term_id;
    }

    $args = array(
      'orderby'         => 'name',
      'show_count'      => false,
      'hide_if_empty'   => true,
      'show_option_all' => 'All categories',
      'depth'           => 1,
      'exclude'         => $exclude_cats,
    );
  ?>
  Categories: <?php wp_dropdown_categories( $args ); ?>