Example.php

<?php

namespace JPry;

use JPry\Exception\InvalidField;

class Example {

    /**
	 * Generate the sub-field objects for this field.
	 *
	 * @since %VERSION%
	 *
	 * @throws InvalidField When an invalid field class is provided through the filter.
	 */
	protected function generate_sub_fields() {
		$classes        = array_merge( $this->classes, $this->get_classes() );
		$default_fields = $this->get_default_fields();
		foreach ( $default_fields as $field => $settings ) {
			$settings = wp_parse_args( $settings, [
				'label'    => ucwords( str_replace( [ '_', '-' ], ' ', $field ) ),
				'class'    => Types::TEXT,
				'required' => true,
			] );

			// Set up the field ID, depending on whether the field is repeatable.
			$id = $this->id . ( $this->repeatable ? '[]' : '' ) . "[{$field}]";

			// Instantiate the sub field.
			$this->sub_fields[] = new $settings['class'](
				$id,
				$settings['label'],
				$classes,
				(bool) $settings['required']
			);

			// Ensure the class extends the Field interface.
			if ( ! ( end( $this->sub_fields ) instanceof Field ) ) {
				throw InvalidField::from_field( $settings['class'] );
			}
		}
	}
}

Exception.php

<?php

namespace JPry\Exception;

interface Exception {}

InvalidField.php

<?php

namespace JPry\Exception;

class InvalidField extends \InvalidArgumentException implements Exception {

	/**
	 * Create a new instance of the exception for a field class name that is not recognized.
	 *
	 * @param string $field Class name of the service that was not recognized.
	 *
	 * @return static
	 */
	public static function from_field( $field ) {
		$message = sprintf(
			'The field class "%s" is not recognized and cannot be used.',
			is_object( $field )
				? get_class( $field )
				: (string) $field
		);

		return new static( $message );
	}

	/**
	 * Create a new instance of the exception for a field ID that is invalid.
	 *
	 * @param string $id The invalid field ID.
	 *
	 * @return static
	 */
	public static function invalid_id( $id ) {
		$message = sprintf(
			'The ID "%s" is invalid. The ID must be a simple string, or a single depth array',
			$id
		);

		return new static( $message );
	}
}