要自定义API Platform的GraphQL输出，你可以使用Symfony的事件系统。下面是一个示例解决方法：

首先，创建一个事件监听器类：

// src/EventListener/CustomGraphQLOutputListener.php

namespace App\EventListener;

use ApiPlatform\Core\EventListener\EventPriorities;
use ApiPlatform\Core\GraphQl\Executor\Promise\PromiseAdapterInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\ViewEvent;
use Symfony\Component\HttpKernel\KernelEvents;

class CustomGraphQLOutputListener implements EventSubscriberInterface
{
    private $promiseAdapter;

    public function __construct(PromiseAdapterInterface $promiseAdapter)
    {
        $this->promiseAdapter = $promiseAdapter;
    }

    public function onKernelView(ViewEvent $event): void
    {
        $result = $event->getControllerResult();
        $request = $event->getRequest();

        // 检查请求是否为GraphQL请求
        if (!$request->attributes->get('_graphql')) {
            return;
        }

        // 自定义GraphQL输出
        $result['data']['customField'] = 'Custom value';

        // 转换为Promise对象
        $event->setControllerResult($this->promiseAdapter->createFulfilled($result));
    }

    public static function getSubscribedEvents(): array
    {
        return [
            KernelEvents::VIEW => ['onKernelView', EventPriorities::POST_SERIALIZE],
        ];
    }
}

然后，注册事件监听器：

# config/services.yaml

services:
    App\EventListener\CustomGraphQLOutputListener:
        tags:
            - { name: kernel.event_subscriber }

最后，通过在GraphQL查询中添加customField字段来测试自定义输出：

query {
  books {
    edges {
      node {
        id
        title
        customField
      }
    }
  }
}

这样，GraphQL查询的输出中将包含customField字段，并且值为Custom value。

希望以上解决方法对你有帮助！