When to use Angular 2 factory functions?

Question

I can't imagine a situation where I need to use a factory provider.

According to the offical docs https://angular.io/docs/ts/latest/guide/dependency-injection.html the situation is that one may not be able to access a service (service-b) from within another service (service-a), but, the factory function does (have access to service-b). So, when would something like this really happen?

Answer 1

You can register for a provider by just passing the class

providers: [MyService]

This only works if Angulars DI can instantiate MyService.

If you have for example

@Injectable()
class MyService {
  constructor(private http: Http, private String configVal) {}
}

then DI is not able to create an instance because String is not a valid key for a provider (primitive types don't work as provider key.

If you need this you can use a factory function like

providers: [
    {
      provide: MyService, 
      useFactory: (http) => {
        return new MyService(http, 'http://mydbserver.com:12345');
      },
      deps: [Http]
    }
]

This way you fully control how a new instance is created and Angulars DI only needs to know that it needs to call the factory function with an instance of Http.