Minimal GraphQL example for Flow for interface types

GraphQlMiddleware.php

<?php
declare(strict_types=1);
namespace Some\App;

use GraphQL\Error\DebugFlag;
use GraphQL\GraphQL;
use GraphQL\Utils\BuildSchema;
use GuzzleHttp\Psr7\Response;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;

final class GraphQlMiddleware implements MiddlewareInterface
{

    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        // TODO handle exceptions
        $input = json_decode(file_get_contents('php://input'), true, 512, JSON_THROW_ON_ERROR);

        $typeConfigDecorator = static function(array $typeConfig, $typeDefinitionNode) {

            $typeConfig['resolveType'] = static fn($a) => 'CustomerAccount';
            return $typeConfig;
        };
        // TODO cache this (see https://webonyx.github.io/graphql-php/type-system/type-language/#performance-considerations)
        $schema = BuildSchema::build(file_get_contents('resource://Some.App/Private/schema.graphql'), $typeConfigDecorator);
        $rootValue = [
            'node' => fn($rootValue, array $args, $context) => [
                'id' => $args['id'],
                'customerName' => 'John Doe',

            ],
        ];
        // TODO handle exceptions
        $result = GraphQL::executeQuery($schema, $input['query'], $rootValue, null, $input['variables'] ?? null)
            // TODO remove debug flag
            ->toArray(DebugFlag::INCLUDE_DEBUG_MESSAGE);

        // TODO handle exceptions
        $responseBody = \json_encode($result, JSON_THROW_ON_ERROR);

        return new Response(200, ['Content-Type' => 'application/json'], $responseBody);
    }
}

schema.graphql

interface Node {
    id: ID!
}

type CustomerAccount implements Node {
    id: ID!
    customerName: String!
}

type Query {
    node(
        id: ID!
    ): Node
}

Settings.yaml

Neos:
  Flow:
    http:
      middlewares:
        'graphql':
          position: 'start'
          middleware: 'Some\App\GraphQlMiddleware'

The corresponding query could look like this:

{
  node(id: "foo") {
    id
    ... on CustomerAccount {
      customerName
    }
  }
}