Here is the base resolver class in graphql in magento 2.

<?php

namespace Magesan\Extension\Model\Resolver;

use Magento\Framework\GraphQl\Config\Element\Field;
use Magento\Framework\GraphQl\Exception\GraphQlInputException;
use Magento\Framework\GraphQl\Query\ResolverInterface;
use Magento\Framework\GraphQl\Schema\Type\ResolveInfo;

class BaseResolver implements ResolverInterface
{
    /**
     * @inheritdoc
     */
    public function resolve(
        Field $field,
        $context,
        ResolveInfo $info,
        array $value = null,
        array $args = null
    ) {
        if (!isset($args['post_id']) || empty($args['post_id'])) {
            throw new GraphQlInputException(__('Invalid Data.'));
        }

         return $this;
    }
}