NestJS Metadata Discovery Module

Description

A NestJS Module that simplifies the creation of services that can discover metadata from methods to be consumed by decorated handlers from other classes.

Installation

pnpm nestjs-metadata-discovery@latest

Usage

1. Create a service that implements registerHandler. Here we have an example of a class that extends EventEmitter that listens for events and calls the handler that was registered in the Service1 class.

   @Injectable()
   export class Service1 extends EventEmitter {
     @Inject('OPTIONS')
     private options: any;
   
     // Map of handlers to the tuple of Context and Decorated Method
     private map_of_handlers = new Map<string, [any, FunctionController]>();
   
     constructor() {
       super();
       // Listen for Some Events
       this.on('SOME_EVENT', this.handleSomeEvent.bind(this));
     }
   
     // Catch all Handler
     async handleSomeEvent(...args) {
       // Get handler from map of registered handlers
       const handler = this.map_of_handlers.get(metadata_value);
       if (!handler) {
         throw new Error('Handler not found');
       }
       const [context, method] = handler;
       // Call handler in the context of the class where it was defined.
       return method.apply(context, args);
     }
   
     registerHander(
       metadata_key: string,
       metadata_value: string,
       handler: FunctionConstructor,
       instance: any
     ) {
       // Register handler to map of registered handlers
   
       this.map_of_handlers.set(metadata_value, [instance, handler]);
     }
   }

2. Create a decorator that will be used to decorate the method that will be called when the event is emitted.

   export function Handle(some_key: string) {
     return applyDecorators(SetMetadata('EVENT_HANDLER', some_key));
   }

3. Decorate your Service Methods with your decorator.

   @Injectable()
   export class Service2 {
     @Handle('SOME_EVENT')
     async handleSomeEvent(...args) {
       // Do something
     }
   }

4. Use the MetadataDiscoveryModule to register your handlers.

   import { MetadataDiscoveryModule } from '@app/metadata-discovery';
   
   @Module({
     imports: [
       MetadataDiscoveryModule.forRoot(
         {
           metadata_key: 'EVENT_HANDLER', // Metadata Key to discover
           discover_controllers: true, // Discover Controllers
           discover_services: true, // Discover Services
         },
         Service1 // Service that implements `registerHandler`
       ),
       // Import other modules
     ],
     providers: [
       // Import other providers,
       Service2,
     ],
   })
   export class AppModule {}

5. When the app runs and SOME_EVENTS events are emitted, the Service1 will call the methods decorated in Service2 .