public function add_columns( $columns ) {

		foreach ( $this->meta_columns as $meta_field ) {
			if ( 'checkbox' === $meta_field->type && isset( $meta_field->extra['options'] ) ) {
				foreach ( $meta_field->extra['options'] as $option ) {
					$key = $meta_field->slug . '_' . sanitize_title( $option );

					$columns[ $key ] = "{$meta_field->label}: {$option}";
				}
				continue;
			}

			$columns[ $meta_field->slug ] = $meta_field->label;
		}

		return $columns;
	}