在Nest.js中,如何在装饰器中获取服务实例？

Question

在CustomDecorator,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

聚会迟到了，但由于我遇到了类似的问题（在装饰器中使用全局嵌套模块）并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

遇到这个问题并花了一天时间试图找出一个好的答案。这可能不适合所有用例，但我能够复制 Nest 核心包中的通用模式以满足我的需求。

我想创建我自己的装饰器来注释控制器方法来处理事件（例如，@Subscribe('some.topic.key') async handler() { ... })）。

为了实现这一点，我的装饰器使用SetMetadatafrom@nestjs/common来注册一些我需要的元数据（它被应用到的方法名称、它所属的类、对方法的引用）。

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 的生命周期钩子，以查找我用装饰器装饰的所有方法，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,
      );
    }
  }
}

资源管理器服务使用一些实用程序@nestjs/core来内省容器并处理查找所有带有元数据的修饰函数。

@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;
  }
}

我并不认为这是处理这个问题的最佳方式，但它对我来说效果很好。使用此代码风险自负，它应该可以帮助您入门:-)。我修改了此处提供的代码：https : //github.com/nestjs/nest/blob/5.1.0-stable/packages/microservices/listener-metadata-explorer.ts

Answer 3

我们有几点:

• 之前执行的属性装饰器decorated instance将被创建.
• 装饰者希望使用some instance由Injector解决的decorated instance.

作为一种直截了当的方式 - 使用some instance注入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我认为在某种程度上可以使用Injector实例来获取任何所需的实例(如angular do).