tatic function ( $classes, $form_data ) use ( $id, $attr ) {

			if ( (int) $form_data['id'] !== $id ) {
				return $classes;
			}

			$cls = [];

			// Add custom class to form container.
			if ( ! empty( $attr['className'] ) ) {
				$cls = array_map( 'esc_attr', explode( ' ', $attr['className'] ) );
			}

			// Add classes to identify that the form displays inside the block.
			$cls[] = 'wpforms-block';

			if ( ! empty( $attr['clientId'] ) ) {
				$cls[] = 'wpforms-block-' . $attr['clientId'];
			}

			return array_unique( array_merge( $classes, $cls ) );
		};

		if ( empty( $this->callbacks[ $id ] ) ) {
			add_filter( 'wpforms_frontend_container_class', $class_callback, 10, 2 );
		}

		$this->callbacks[ $id ][] = $class_callback;
	}

	/**
	 * Get content.
	 *
	 * @since 1.8.1
	 *
	 * @param int   $id    Form id.
	 * @param bool  $title Form title is not empty.
	 * @param bool  $desc  Form desc is not empty.
	 * @param array $attr  Form attributes.
	 *
	 * @return string
	 */
	private function get_content( $id, $title, $desc, $attr ): string {

		/**
		 * Filter allow render block content flag.
		 *
		 * @since 1.8.8
		 *
		 * @param bool $allow_render Allow render flag. Defaults to `true`.
		 */
		$allow_render = (bool) apply_filters( 'wpforms_integrations_gutenberg_form_selector_allow_render', true );

		if ( ! $allow_render ) {
			return '';
		}

		ob_start();

		// phpcs:disable WPForms.PHP.ValidateHooks.InvalidHookName

		/**
		 * Fires before Gutenberg block output.
		 *
		 * @since 1.5.8.2
		 */
		do_action( 'wpforms_gutenberg_block_before' );

		/**
		 * Filter block title display flag.
		 *
		 * @since 1.5.8.2
		 *
		 * @param bool $title Title display flag.
		 * @param int  $id    Form id.
		 */
		$title = (bool) apply_filters( 'wpforms_gutenberg_block_form_title', $title, $id );

		/**
		 * Filter block description display flag.
		 *
		 * @since 1.5.8.2
		 *
		 * @param bool $desc Description display flag.
		 * @param int  $id   Form id.
		 */
		$desc = (bool) apply_filters( 'wpforms_gutenberg_block_form_desc', $desc, $id );

		$this->output_css_vars( $attr );
		$this->output_custom_css( $attr );

		wpforms_display( $id, $title, $desc );

		/**
		 * Fires after Gutenberg block output.
		 *
		 * @since 1.5.8.2
		 */
		do_action( 'wpforms_gutenberg_block_after' );

		// phpcs:enable WPForms.PHP.ValidateHooks.InvalidHookName

		$content = (string) ob_get_clean();

		if ( ! $this->is_gb_editor() ) {
			return $content;
		}

		if ( empty( $content ) ) {
			return '<div class="components-placeholder"><div class="components-placeholder__label"></div>' .
						'<div class="components-placeholder__fieldset">' .
						esc_html__( 'The form cannot be displayed.', 'wpforms-lite' ) .
						'</div></div>';
		}

		/**
		 * Unfortunately, the inline 'script' tag cannot be executed in the GB editor.
		 * This is the hacky way to trigger custom event on form loaded in the Block Editor / GB / FSE.
		 */

		// phpcs:disable WordPress.PHP.DevelopmentFunctions.error_log_var_export
		$content .= sprintf(
			'<img src="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" onLoad="
				window.top.dispatchEvent(
					new CustomEvent(
						\'wpformsFormSelectorFormLoaded\',
						{
							detail: {
								formId: %1$s,
								title: %2$s,
								desc: %3$s,
								block: this.closest( \'.wp-block\' )
							}
						}
					)
				);
			" class="wpforms-pix-trigger" alt="">',
			absint( $id ),
			var_export( (bool) $title, true ),
			var_export( (bool) $desc, true )
		);

		// phpcs:enable WordPress.PHP.DevelopmentFunctions.error_log_var_export

		return $content;
	}

	/**
	 * Checking if is Gutenberg REST API call.
	 *
	 * @since 1.5.7
	 *
	 * @return bool True if is Gutenberg REST API call.
	 */
	public function is_gb_editor(): bool {

		// TODO: Find a better way to check if is GB editor API call.
		// phpcs:ignore WordPress.Security.NonceVerification.Recommended
		return defined( 'REST_REQUEST' ) && REST_REQUEST && ! empty( $_REQUEST['context'] ) && $_REQUEST['context'] === 'edit';
	}

	/**
	 * Disable form fields if called from the Gutenberg editor.
	 *
	 * @since 1.7.5
	 *
	 * @return void
	 */
	private function disable_fields_in_gb_editor() { // phpcs:ignore WPForms.PHP.HooksMethod.InvalidPlaceForAddingHooks

		add_filter(
			'wpforms_frontend_container_class',
			static function ( $classes ) {

				$classes[] = 'wpforms-gutenberg-form-selector';

				return $classes;
			}
		);
		add_action(
			'wpforms_frontend_output',
			static function () {

				echo '<fieldset disabled>';
			},
			3
		);
		add_action(
			'wpforms_frontend_output',
			static function () {

				echo '</fieldset>';
			},
			30
		);
	}

	/**
	 * Output CSS variables for the particular form.
	 *
	 * @since 1.8.1
	 *
	 * @param array $attr Attributes passed by WPForms Gutenberg block.
	 */
	private function output_css_vars( $attr ) {

		if ( empty( $this->css_vars_obj ) || ! method_exists( $this->css_vars_obj, 'get_vars' ) ) {
			return;
		}

		$this->css_vars_obj->output_root();

		if ( $this->render_engine === 'classic' || $this->disable_css_setting !== 1 ) {
			return;
		}

		$css_vars = $this->css_vars_obj->get_customized_css_vars( $attr );

		if ( empty( $css_vars ) ) {
			return;
		}

		$style_id = "#wpforms-css-vars-{$attr['formId']}-block-{$attr['clientId']}";

		/**
		 * Filter the CSS selector for output CSS variables for styling the GB block form.
		 *
		 * @since 1.8.1
		 *
		 * @param string $selector The CSS selector for output CSS variables for styling the GB block form.
		 * @param array  $attr     Attributes passed by WPForms Gutenberg block.
		 * @param array  $css_vars CSS variables data.
		 */
		$vars_selector = apply_filters(
			'wpforms_integrations_gutenberg_form_selector_output_css_vars_selector',
			"#wpforms-{$attr['formId']}.wpforms-block-{$attr['clientId']}",
			$attr,
			$css_vars
		);

		$this->css_vars_obj->output_selector_vars( $vars_selector, $css_vars, $style_id, $this->current_form_id );
	}

	/**
	 * Output custom CSS styles.
	 *
	 * @since 1.8.8
	 *
	 * @param array $attr Attributes passed by WPForms Gutenberg block.
	 */
	private function output_custom_css( $attr ) {

		if ( wpforms_get_render_engine() === 'classic' ) {
			return;
		}

		$custom_css = trim( $attr['customCss'] ?? '' );

		if ( empty( $custom_css ) ) {
			return;
		}

		$style_id = "#wpforms-custom-css-{$attr['formId']}-block-{$attr['clientId']}";

		printf(
			'<style id="%1$s">
				%2$s
			</style>',
			sanitize_key( $style_id ),
 			esc_html( $custom_css )
		);
	}
}