data.
	 */
	public function get_item_schema() {
		if ( $this->schema ) {
			return $this->add_additional_fields_schema( $this->schema );
		}

		$options = $this->get_registered_options();

		$schema = array(
			'$schema'    => 'http://json-schema.org/draft-04/schema#',
			'title'      => 'settings',
			'type'       => 'object',
			'properties' => array(),
		);

		foreach ( $options as $option_name => $option ) {
			$schema['properties'][ $option_name ]                = $option['schema'];
			$schema['properties'][ $option_name ]['arg_options'] = array(
				'sanitize_callback' => array( $this, 'sanitize_callback' ),
			);
		}

		$this->schema = $schema;

		return $this->add_additional_fields_schema( $this->schema );
	}

	/**
	 * Custom sanitize callback used for all options to allow the use of 'null'.
	 *
	 * By default, the schema of settings will throw an error if a value is set to
	 * `null` as it's not a valid value for something like "type => string". We
	 * provide a wrapper sanitizer to allow the use of `null`.
	 *
	 * @since 4.7.0
	 *
	 * @param mixed           $value   The value for the setting.
	 * @param WP_REST_Request $request The request object.
	 * @param string          $param   The parameter name.
	 * @return mixed|WP_Error
	 */
	public function sanitize_callback( $value, $request, $param ) {
		if ( is_null( $value ) ) {
			return $value;
		}

		return rest_parse_request_arg( $value, $request, $param );
	}

	/**
	 * Recursively add additionalProperties = false to all objects in a schema
	 * if no additionalProperties setting is specified.
	 *
	 * This is needed to restrict properties of objects in settings values to only
	 * registered items, as the REST API will allow additional properties by
	 * default.
	 *
	 * @since 4.9.0
	 * @deprecated 6.1.0 Use {@see rest_default_additional_properties_to_false()} instead.
	 *
	 * @param array $schema The schema array.
	 * @return array
	 */
	protected function set_additional_properties_to_false( $schema ) {
		_deprecated_function( __METHOD__, '6.1.0', 'rest_default_additional_properties_to_false()' );

		return rest_default_additional_properties_to_false( $schema );
	}
}