src/Type/Definition/UnionType.php
public function resolveType($objectValue, $context, ResolveInfo $info)
    {
        if (isset($this->config['resolveType'])) {
            $fn = $this->config['resolveType'];

            return $fn($objectValue, $context, $info);
        }

        return null;
    }
public function resolveType($objectValue, $context, ResolveInfo $info)
    {
        if (isset($this->config['resolveType'])) {
            $fn = $this->config['resolveType'];

            return $fn($objectValue, $context, $info);
        }

        return null;
    }
Resolves concrete ObjectType for given object value

@param object $objectValue
@param mixed  $context

@return callable|null