Sample class that extends WP rest controller

rest-controller-extend.php

<?php
/**
 * Add rest api endpoint for category listing
 */

/**
 * Class Category_List_Rest
 */
class Category_List_Rest extends WP_REST_Controller {
	/**
	 * The namespace.
	 *
	 * @var string
	 */
	protected $namespace;
	/**
	 * Rest base for the current object.
	 *
	 * @var string
	 */
	protected $rest_base;

	/**
	 * Category_List_Rest constructor.
	 */
	public function __construct() {

		$this->namespace = 'category-list/v1';
		$this->rest_base = 'order';
	}
	/**
	 * Register the routes for the objects of the controller.
	 */
	public function register_routes() {

		register_rest_route( $this->namespace, '/' . $this->rest_base, array(

			array(
				'methods'             => WP_REST_Server::READABLE,
				'callback'            => array( $this, 'get_items' ),
				 'permission_callback' => array( $this, 'get_items_permissions_check' ),
			),
			array(
				'methods'         => WP_REST_Server::EDITABLE,
				'callback'        => array( $this, 'update_item' ),
				'permission_callback' => array( $this, 'update_item_permissions_check' ),
				'args'            => $this->get_endpoint_args_for_item_schema( false ),
			),
			'schema' => null,

		) );

	}
	/**
	 * Check permissions for the read.
	 *
	 * @param WP_REST_Request $request get data from request.
	 *
	 * @return bool|WP_Error
	 */
	public function get_items_permissions_check( $request ) {
		if ( ! current_user_can( 'read' ) ) {
			return new WP_Error( 'rest_forbidden', esc_html__( 'You cannot view the category resource.' ), array( 'status' => $this->authorization_status_code() ) );
		}
		return true;
	}
	/**
	 * Check permissions for the update
	 *
	 * @param WP_REST_Request $request get data from request.
	 *
	 * @return bool|WP_Error
	 */
	public function update_item_permissions_check( $request ) {
		if ( ! current_user_can( 'manage_options' ) ) {
			return new WP_Error( 'rest_forbidden', esc_html__( 'You cannot update the category resource.' ), array( 'status' => $this->authorization_status_code() ) );
		}
		return true;
	}
	/**
	 * Grabs all the category list.
	 *
	 * @param WP_REST_Request $request get data from request.
	 *
	 * @return mixed|WP_REST_Response
	 */
	public function get_items( $request ) {

		$cat_order = get_option( 'category_order' );
		$data = get_categories();
		$res = [];
		if ( $cat_order ) {
			$index = 0;
			foreach ( $cat_order as $cat ) {

				$temp = [];
				$temp['name'] = get_cat_name( $cat );
				$temp['id']   = $cat;
				$temp['order'] = $index;
				$res[] = $temp;
				$index++;

			}
		} elseif ( ! empty( $data ) ) {

			$index = 0;
			foreach ( $data as $list ) {

				if ( 'category' === $list->taxonomy ) {

					$temp = [];
					$temp['name'] = $list->name;
					$temp['id']   = $list->term_id;
					$temp['order'] = $index;
					$res[] = $temp;
					$index++;
				}
			}
		}

		// Return all of our comment response data.
		return rest_ensure_response( $res );
	}

	/**
	 * Update category order
	 *
	 * @param WP_REST_Request $request get data from request.
	 *
	 * @return mixed|WP_Error|WP_REST_Response
	 */
	public function update_item( $request ) {

		$data = [];
		if ( ! isset( $request['order'] ) ) {
			return new WP_Error( 'invalid_data', __( 'Cannot update category order.' ), array( 'status' => 400 ) );
		}
		$res = update_option( 'category_order', $request['order'] );

		if ( $res ) {
			$data['msg'] = __( 'Category order updated', '' );
		} else {
			return new WP_Error( 'cant update', __( 'Please provide proper data' ), array( 'status' => 400 ) );
		}

		return rest_ensure_response( $data );
	}
	/**
	 * Sets up the proper HTTP status code for authorization.
	 *
	 * @return int
	 */
	public function authorization_status_code() {

		$status = 401;

		if ( is_user_logged_in() ) {
			$status = 403;
		}

		return $status;
	}
}
/**
 * Function to register our new routes from the controller.
 */
function register_cat_list_controller() {
	$controller = new Category_List_Rest();
	$controller->register_routes();
}

add_action( 'rest_api_init', 'register_cat_list_controller' );