unction get_permission_callback( $request ) {
		$is_multi = (bool) $request->get_param( 'is_multi' );

		$result = false;

		// The function is public since endpoint need to access it.
		// Utilize 'WP_REST_Controller' get_permission_check methods.
		switch ( $request->get_method() ) {
			case 'GET':
				$result = $is_multi ? $this->get_items_permissions_check( $request ) : $this->get_item_permissions_check( $request );
				break;
			case 'POST':
				$result = $is_multi ? $this->create_items_permissions_check( $request ) : $this->create_item_permissions_check( $request );
				break;
			case 'UPDATE':
			case 'PUT':
			case 'PATCH':
				$result = $is_multi ? $this->update_items_permissions_check( $request ) : $this->update_item_permissions_check( $request );
				break;

			case 'DELETE':
				$result = $is_multi ? $this->delete_items_permissions_check( $request ) : $this->delete_item_permissions_check( $request );
				break;
		}

		if ( $result instanceof \WP_Error ) {
			throw new WP_Error_Exception( $result );
		}

		return $result;
	}

	/**
	 * Checks if a given request has access to create items.
	 **
	 *
	 * @param \WP_REST_Request $request Full details about the request.
	 *
	 * @return true|\WP_Error True if the request has access to create items, WP_Error object otherwise.
	 */
	public function create_items_permissions_check( $request ) {
		return new \WP_Error( 'invalid-method', sprintf( "Method '%s' not implemented. Must be overridden in subclass.", __METHOD__ ), [ 'status' => 405 ] );
	}

	/**
	 * Checks if a given request has access to update items.
	 *
	 * @param \WP_REST_Request $request Full details about the request.
	 *
	 * @return true|\WP_Error True if the request has access to update the item, WP_Error object otherwise.
	 */
	public function update_items_permissions_check( $request ) {
		return new \WP_Error( 'invalid-method', sprintf( "Method '%s' not implemented. Must be overridden in subclass.", __METHOD__ ), [ 'status' => 405 ] );
	}

	/**
	 * Checks if a given request has access to delete items.
	 *
	 * @param \WP_REST_Request $request Full details about the request.
	 *
	 * @return true|\WP_Error True if the request has access to delete the item, WP_Error object otherwise.
	 */
	public function delete_items_permissions_check( $request ) {
		return new \WP_Error( 'invalid-method', sprintf( "Method '%s' not implemented. Must be overridden in subclass.", __METHOD__ ), [ 'status' => 405 ] );
	}

	public function get_items( $request ) {
		return $this->get_controller_index();
	}

	/**
	 * Creates multiple items.
	 *
	 * @param \WP_REST_Request $request Full data about the request.
	 *
	 * @return \WP_Error|\WP_REST_Response Response object on success, or WP_Error object on failure.
	 */
	public function create_items( $request ) {
		return new \WP_Error( 'invalid-method', sprintf( "Method '%s' not implemented. Must be overridden in subclass.", __METHOD__ ), [ 'status' => 405 ] );
	}

	/**
	 * Updates multiple items.
	 *
	 * @param \WP_REST_Request $request Full data about the request.
	 *
	 * @return \WP_Error|\WP_REST_Response Response object on success, or WP_Error object on failure.
	 */
	public function update_items( $request ) {
		return new \WP_Error( 'invalid-method', sprintf( "Method '%s' not implemented. Must be overridden in subclass.", __METHOD__ ), [ 'status' => 405 ] );
	}

	/**
	 * Delete multiple items.
	 *
	 * @param \WP_REST_Request $request Full data about the request.
	 *
	 * @return \WP_Error|\WP_REST_Response Response object on success, or WP_Error object on failure.
	 */
	public function delete_items( $request ) {
		return new \WP_Error( 'invalid-method', sprintf( "Method '%s' not implemented. Must be overridden in subclass.", __METHOD__ ), [ 'status' => 405 ] );
	}

	/**
	 * Get the parent controller.
	 *
	 * @return \Elementor\Data\V2\Base\Controller|null
	 */
	public function get_parent() {
		return $this->parent;
	}

	/**
	 * Get sub controller(s).
	 *
	 * @return \Elementor\Data\V2\Base\Controller[]
	 */
	public function get_sub_controllers() {
		return $this->sub_controllers;
	}

	/**
	 * Get processors.
	 *
	 * @param string $command
	 *
	 * @return \Elementor\Data\V2\Base\Processor[]
	 */
	public function get_processors( $command ) {
		$result = [];

		if ( isset( $this->processors[ $command ] ) ) {
			$result = $this->processors[ $command ];
		}

		return $result;
	}

	/**
	 * Register processors.
	 */
	public function register_processors() {
	}

	/**
	 * Register index endpoint.
	 */
	protected function register_index_endpoint() {
		if ( ! $this->parent ) {
			$this->register_endpoint( new Endpoint\Index( $this ) );

			return;
		}

		$this->register_endpoint( new Endpoint\Index\Sub_Index_Endpoint( $this ) );
	}

	/**
	 * Register endpoint.
	 *
	 * @param \Elementor\Data\V2\Base\Endpoint $endpoint
	 *
	 * @return \Elementor\Data\V2\Base\Endpoint
	 */
	protected function register_endpoint( Endpoint $endpoint ) {
		$command = $endpoint->get_full_command();

		if ( $endpoint instanceof Endpoint\Index ) {
			$this->index_endpoint = $endpoint;
		} else {
			$this->endpoints[ $command ] = $endpoint;
		}

		$format = $endpoint->get_format();

		// `$e.data.registerFormat()`.
		Manager::instance()->register_endpoint_format( $command, $format );

		return $endpoint;
	}

	/**
	 * Register a processor.
	 *
	 * That will be later attached to the endpoint class.
	 *
	 * @param Processor $processor
	 *
	 * @return \Elementor\Data\V2\Base\Processor $processor_instance
	 */
	protected function register_processor( Processor $processor ) {
		$command = $processor->get_command();

		if ( ! isset( $this->processors[ $command ] ) ) {
			$this->processors[ $command ] = [];
		}

		$this->processors[ $command ] [] = $processor;

		return $processor;
	}

	/**
	 * Register.
	 *
	 * Endpoints & processors.
	 */
	protected function register() {
		$this->register_index_endpoint();
		$this->register_endpoints();

		// Aka hooks.
		$this->register_processors();
	}

	/**
	 * Get collection params by 'additionalProperties' context.
	 *
	 * @param string $context
	 *
	 * @return array
	 */
	protected function get_collection_params_by_additional_props_context( $context ) {
		$result = [];

		$collection_params = $this->get_collection_params();

		foreach ( $collection_params as $collection_param_key => $collection_param ) {
			if ( isset( $collection_param['additionalProperties']['context'] ) && $context === $collection_param['additionalProperties']['context'] ) {
				$result[ $collection_param_key ] = $collection_param;
			}
		}

		return $result;
	}

	/**
	 * When `$this->get_parent_name` is extended, the controller will have a parent, and will know to behave like a sub-controller.
	 *
	 * @param string $parent_name
	 */
	private function act_as_sub_controller( $parent_name ) {
		$this->parent = Manager::instance()->get_controller( $parent_name );

		if ( ! $this->parent ) {
			trigger_error( "Cannot find parent controller: '$parent_name'", E_USER_ERROR ); // phpcs:ignore
		}

		$this->parent->sub_controllers [] = $this;
	}

	/**
	 * Controller constructor.
	 *
	 * Register endpoints on 'rest_api_init'.
	 */
	public function __construct() {
		$this->namespace = static::get_default_namespace() . '/v' . static::get_default_version();
		$this->rest_base = $this->get_name();

		add_action( 'rest_api_init', function () {
			$this->register(); // Because 'register' is protected.
		} );

		/**
		 * Since all actions were removed for custom internal REST server.
		 * Re-add the actions.
		 */
		add_action( 'elementor_rest_api_before_init', function () {
			add_action( 'rest_api_init', function () {
				$this->register();
			} );
		} );

		$parent_name = $this->get_parent_name();
		if ( $parent_name ) {
			$this->act_as_sub_controller( $parent_name );
		}
	}
}