Question

在CustomDecorator中，如何访问Nest.js中定义的服务实例？

export const CustomDecorator = (): MethodDecorator => {
  return (
    target: Object,
    propertyKey: string | symbol,
    descriptor: PropertyDescriptor
    ) => {

    // Here, is possibile to access a Nest.js service (i.e. TestService) instance?

    return descriptor;
  }
};

Answer 1

该晚了，但是由于我遇到了类似的问题（Use global nest module in decorator），因此偶然发现了这个问题。

import { Inject } from '@nestjs/common';
export function yourDecorator() {
  const injectYourService = Inject(YourServiceClass);

  return (target: any, propertyKey: string, propertyDescriptor: PropertyDescriptor) => {
    // this is equivalent to have a constructor like constructor(yourservice: YourServiceClass)
    // note that this will injected to the instance, while your decorator runs for the class constructor
    injectYourService(target, 'yourservice');

    // do something in you decorator

    // we use a ref here so we can type it
    const yourservice: YourServiceClass = this.yourservice;
    yourservice.someMethod(someParam);
  };
}

Answer 2

我们有几点：

将创建在decorated instance之前执行的属性修饰符。
装饰者要使用some instance的注入器解析的decorated instance。

一种简单的方法-使用some instance注入的decorated instance。

@Injectable()
export class CatsService {
  constructor(public myService: MyService){}

  @CustomDecorator()
  foo(){}
}

export const CustomDecorator = (): MethodDecorator => {
  return (
    target: Object,
    propertyKey: string | symbol,
    descriptor: PropertyDescriptor
  ) => {

    const originalMethod = descriptor.value;

    descriptor.value = function () {
      const serviceInstance = this;
      console.log(serviceInstance.myService);

    }

    return descriptor;
  }
};

PS我认为可以somehow使用Injector实例来获取任何所需的实例（例如angular does）。

Answer 3

问这个问题，花了整整一天的时间来找出一个好的答案。这可能并不适合每个用例，但是我能够复制Nest核心软件包中的通用模式来满足我的需求。

我想创建自己的装饰器，以注释处理事件的控制器方法（例如@Subscribe('some.topic.key') async handler() { ... })）。

要实现此目的，我的装饰器使用了SetMetadata中的@nestjs/common来注册一些我需要的元数据（正在应用的方法名称，所属的类，对该方法的引用）。

export const Subscribe = (topic: string) => {
  return (target: any, propertyKey: string, descriptor: PropertyDescriptor) => {
    SetMetadata<string, RabbitSubscriberMetadataConfiguration>(
      RABBITMQ_SUBSCRIBER,
      {
        topic,
        target: target.constructor.name,
        methodName: propertyKey,
        callback: descriptor.value,
      },
    )(target, propertyKey, descriptor);
  };
};

从那里，我能够创建自己的模块，该模块连接到Nest的生命周期挂钩中，以查找用装饰器装饰过的所有方法，并对其应用一些逻辑，例如：

@Module({
  imports: [RabbitmqChannelProvider],
  providers: [RabbitmqService, MetadataScanner, RabbitmqSubscriberExplorer],
  exports: [RabbitmqService],
})
export class RabbitmqModule implements OnModuleInit {
  constructor(
    private readonly explorer: RabbitmqSubscriberExplorer,
    private readonly rabbitmqService: RabbitmqService,
  ) {}

  async onModuleInit() {
    // find everything marked with @Subscribe
    const subscribers = this.explorer.explore();
    // set up subscriptions
    for (const subscriber of subscribers) {
      await this.rabbitmqService.subscribe(
        subscriber.topic,
        subscriber.callback,
      );
    }
  }
}

Explorer服务使用@nestjs/core中的一些实用工具对容器进行内部检查，并处理使用其元数据查找所有修饰的函数的情况。

@Injectable()
export class RabbitmqSubscriberExplorer {
  constructor(
    private readonly modulesContainer: ModulesContainer,
    private readonly metadataScanner: MetadataScanner,
  ) {}

  public explore(): RabbitSubscriberMetadataConfiguration[] {
    // find all the controllers
    const modules = [...this.modulesContainer.values()];
    const controllersMap = modules
      .filter(({ controllers }) => controllers.size > 0)
      .map(({ controllers }) => controllers);

    // munge the instance wrappers into a nice format
    const instanceWrappers: InstanceWrapper<Controller>[] = [];
    controllersMap.forEach(map => {
      const mapKeys = [...map.keys()];
      instanceWrappers.push(
        ...mapKeys.map(key => {
          return map.get(key);
        }),
      );
    });

    // find the handlers marked with @Subscribe
    return instanceWrappers
      .map(({ instance }) => {
        const instancePrototype = Object.getPrototypeOf(instance);
        return this.metadataScanner.scanFromPrototype(
          instance,
          instancePrototype,
          method =>
            this.exploreMethodMetadata(instance, instancePrototype, method),
        );
      })
      .reduce((prev, curr) => {
        return prev.concat(curr);
      });
  }

  public exploreMethodMetadata(
    instance: object,
    instancePrototype: Controller,
    methodKey: string,
  ): RabbitSubscriberMetadataConfiguration | null {
    const targetCallback = instancePrototype[methodKey];
    const handler = Reflect.getMetadata(RABBITMQ_SUBSCRIBER, targetCallback);
    if (handler == null) {
      return null;
    }
    return handler;
  }
}

我不认为这是处理此问题的最佳方法，但对我来说效果很好。使用此代码需要您自担风险，它应该可以帮助您开始:-)。我修改了可从此处获得的代码：https://github.com/nestjs/nest/blob/5.1.0-stable/packages/microservices/listener-metadata-explorer.ts

Answer 4

我试图在ParamDecorator中使用我的配置服务，所以我通过创建服务的新实例来访问我的服务：

export const MyParamDecorator = createParamDecorator((data, req) => {

  // ...
  const configService = new ConfigService(`${process.env.NODE_ENV || 'default'}.env`);
  const myConfigValue = configService.getMyValue();
  // ...
});

在Nest.js中，如何在装饰器中获取服务实例？

4 个答案: