event-subscribers.loader.ts

import {
  Injectable,
  Logger,
  OnApplicationBootstrap,
  OnApplicationShutdown,
} from '@nestjs/common';
import {
  ContextIdFactory,
  DiscoveryService,
  MetadataScanner,
  ModuleRef,
} from '@nestjs/core';
import { Injector } from '@nestjs/core/injector/injector';
import {
  ContextId,
  InstanceWrapper,
} from '@nestjs/core/injector/instance-wrapper';
import { Module } from '@nestjs/core/injector/module';
import { EventEmitter2 } from 'eventemitter2';
import { EventsMetadataAccessor } from './events-metadata.accessor';
import { OnEventOptions } from './interfaces';

@Injectable()
export class EventSubscribersLoader
  implements OnApplicationBootstrap, OnApplicationShutdown
{
  private readonly injector = new Injector();
  private readonly logger = new Logger('Event');

  constructor(
    private readonly discoveryService: DiscoveryService,
    private readonly eventEmitter: EventEmitter2,
    private readonly metadataAccessor: EventsMetadataAccessor,
    private readonly metadataScanner: MetadataScanner,
    private readonly moduleRef: ModuleRef,
  ) {}

  onApplicationBootstrap() {
    this.loadEventListeners();
  }

  onApplicationShutdown() {
    this.eventEmitter.removeAllListeners();
  }

  loadEventListeners() {
    const providers = this.discoveryService.getProviders();
    const controllers = this.discoveryService.getControllers();
    [...providers, ...controllers]
      .filter(wrapper => wrapper.instance && !wrapper.isAlias)
      .forEach((wrapper: InstanceWrapper) => {
        const { instance } = wrapper;
        const prototype = Object.getPrototypeOf(instance) || {};
        const isRequestScoped = !wrapper.isDependencyTreeStatic();
        this.metadataScanner.scanFromPrototype(
          instance,
          prototype,
          (methodKey: string) =>
            this.subscribeToEventIfListener(
              instance,
              methodKey,
              isRequestScoped,
              wrapper.host as Module,
            ),
        );
      });
  }

  private subscribeToEventIfListener(
    instance: Record<string, any>,
    methodKey: string,
    isRequestScoped: boolean,
    moduleRef: Module,
  ) {
    const eventListenerMetadatas =
      this.metadataAccessor.getEventHandlerMetadata(instance[methodKey]);
    if (!eventListenerMetadatas) {
      return;
    }

    for (const eventListenerMetadata of eventListenerMetadatas) {
      const { event, options } = eventListenerMetadata;
      const listenerMethod = this.getRegisterListenerMethodBasedOn(options);

      if (isRequestScoped) {
        this.registerRequestScopedListener({
          event,
          eventListenerInstance: instance,
          listenerMethod,
          listenerMethodKey: methodKey,
          moduleRef,
          options,
        });
      } else {
        listenerMethod(
          event,
          (...args: unknown[]) =>
            this.wrapFunctionInTryCatchBlocks(instance, methodKey, args),
          options,
        );
      }
    }
  }

  private getRegisterListenerMethodBasedOn(options?: OnEventOptions) {
    return Boolean(options?.prependListener)
      ? this.eventEmitter.prependListener.bind(this.eventEmitter)
      : this.eventEmitter.on.bind(this.eventEmitter);
  }

  private registerRequestScopedListener(eventListenerContext: {
    listenerMethod: EventEmitter2['on'];
    event: string | symbol | (string | symbol)[];
    eventListenerInstance: Record<string, any>;
    moduleRef: Module;
    listenerMethodKey: string;
    options?: OnEventOptions;
  }) {
    const {
      listenerMethod,
      event,
      eventListenerInstance,
      moduleRef,
      listenerMethodKey,
      options,
    } = eventListenerContext;

    listenerMethod(
      event,
      async (...args: unknown[]) => {
        const contextId = ContextIdFactory.create();

        this.registerEventPayloadByContextId(args, contextId);

        const contextInstance = await this.injector.loadPerContext(
          eventListenerInstance,
          moduleRef,
          moduleRef.providers,
          contextId,
        );
        return this.wrapFunctionInTryCatchBlocks(
          contextInstance,
          listenerMethodKey,
          args,
        );
      },
      options,
    );
  }

  private registerEventPayloadByContextId(
    eventPayload: unknown[],
    contextId: ContextId,
  ) {
    /*
      **Required explanation for the ternary below**

      We need the conditional below because an event can be emitted with a variable amount of arguments.
      For instance, we can do `this.eventEmitter.emit('event', 'payload1', 'payload2', ..., 'payloadN');`

      All payload arguments are internally stored as an array. So, imagine we emitted an event as follows:

      `this.eventEmitter.emit('event', 'payload');

      if we registered the original `eventPayload`, when we try to inject it in a listener, it'll be retrieved as [`payload`].
      However, whoever is using this library would certainly expect the event payload to be a single string 'payload', not an array,
      since this is what we emitted above.
    */

    const payloadObjectOrArray =
      eventPayload.length > 1 ? eventPayload : eventPayload[0];

    this.moduleRef.registerRequestByContextId(payloadObjectOrArray, contextId);
  }

  private async wrapFunctionInTryCatchBlocks(
    instance: Record<string, any>,
    methodKey: string,
    args: unknown[],
  ) {
    try {
      return await instance[methodKey].call(instance, ...args);
    } catch (e) {
      this.logger.error(e);
    }
  }
}