travetto: Dependency Injection

Dependency injection is provided as a best practice, and as a framework primitive. Dependency injection can be achieved in multiple ways to support different software patterns. Dependency injection only supports classes as the class is the lookup key.

General Usage

Dependency injection revolves around the following paradigms:

• @Injectable decorator denotes that a class can be injected.

  @Injectable()
  class CustomService {
    async coolOperation() {
      ... work ...
    }
  }

• @InjectableFactory decorator denotes a static class method that produces an @Injectable

  class Config {
    @InjectableFactory()
    static initService(): CoolService {
      return new CoolService();
    }
  }

• @Inject decorator denotes a desire to inject a value directly

  @Injectable()
  class CustomService {
    @Inject()
    private dependentService: DependentService;
    
    async coolOperation() {
      await this.dependentService.doWork();
    }
  }

• @Injectable constructor params

  @Injectable()
  class CustomService {
    constructor (private dependentService: DependentService) {}
    
    async coolOperation() {
      await this.dependentService.doWork();
    }
  }

• @InjectableFactory params

  class Config {
    @InjectableFactory()
    static initService(dependentService: DependentService): CustomService {
      return new CustomService(dependentService);
    }
  }

Manual Invocation

Some times you will need to lookup a dependency dynamically, or you want to control the injection process at a more granular level. To achieve that you will need to directly access the DependencyRegistry.

The registry allows for requesting a dependency by class reference.

@Injectable()
class Complex {}

class ManualLookup {
  async invoke() {
    const complex = await DependencyRegistry.getInstance(Complex);
  }
}

Advanced Usage

In addition to the standard operations, you are able to create multiple versions of the same dependency by providing a qualifier at registration time and at injection time. This allows you to have multiple database connections, or multiple email services.

const USER_DB = Symbol('USER_DB');
const ASSET_DB = Symbol('ASSET_DB');

class DBConfig {
  host: string;
}

class Factory {
  @InjectableFactory(USER_DB)
  getUserConfig(): DBConfig {
    const conf = new DBConfig();
    conf.host = 'user';
    return conf;
  }

  @InjectableFactory(USER_DB)
  getAssetConfig(): DBConfig {
    const conf = new DBConfig();
    conf.host = 'asset';
    return conf;
  }
  
}

class UserService {
  constructor(@Inject(USER_DB) config:DBConfig) {}
}