class EnvironmentSupportValidator

final class EnvironmentSupportValidator implements EventSubscriberInterface {
    use BaseRequirementValidatorTrait {
        getSubscribedEvents as private getSubscribedEventsFromTrait;
    }
    use StringTranslationTrait;
    
    /**
     * The name of the environment variable to check.
     *
     * This environment variable, if defined, should be parseable by
     * \Drupal\Core\Url::fromUri() and link to an explanation of why Package
     * Manager is not supported in the current environment.
     *
     * @var string
     */
    public const VARIABLE_NAME = 'DRUPAL_PACKAGE_MANAGER_NOT_SUPPORTED_HELP_URL';
    
    /**
     * Checks that this environment supports Package Manager.
     */
    public function validate(SandboxValidationEvent $event) : void {
        $message = $this->t('Package Manager is not supported by your environment.');
        $help_url = getenv(static::VARIABLE_NAME);
        if (empty($help_url)) {
            return;
        }
        // If the URL is not parseable, catch the exception that Url::fromUri()
        // would generate.
        try {
            $message = $this->t('<a href=":url">@message</a>', [
                ':url' => Url::fromUri($help_url)->toString(),
                '@message' => $message,
            ]);
        } catch (\InvalidArgumentException) {
            // No need to do anything here. The message just won't be a link.
        }
        $event->addError([
            $message,
        ]);
        // If Package Manager is unsupported, there's no point in doing any more
        // validation.
        $event->stopPropagation();
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        // Set priority to run before BaseRequirementsFulfilledValidator, and even
        // before other base requirement validators.
        // @see \Drupal\package_manager\Validator\BaseRequirementsFulfilledValidator
        return array_map(fn() => [
            'validate',
            BaseRequirementsFulfilledValidator::PRIORITY + 1000,
        ], static::getSubscribedEventsFromTrait());
    }

}