groupField.php

<?php

class SingleGroupQuery extends AbstractField
{
    use ServiceLocatorAwareTrait;

    /**
     * {@inheritdoc}
     */
    public function getType()
    {
        return new GroupType();
    }

    /**
     * {@inheritdoc}
     */
    public function getName()
    {
        return 'group';
    }

    public function build(FieldConfig $config)
    {
        parent::build($config);

        $this->addArgument('id', [
            'type' => new NonNullType(new IdType())
        ]);
    }

    /**
     * {@inheritdoc}
     */
    public function resolve($value, array $args, ResolveInfo $info)
    {
        if (!array_key_exists('id', $args) || !is_string($args['id'])) {
            throw new InvalidArgumentException('Argument ID must be a a string');
        }

        /** @var GroupServiceInterface $groupService */
        $groupService = $this->getServiceLocator($info)->get(GroupServiceInterface::class);

        return $groupService->find($args['id']);
    }
}