Why use `deps` property in DI

Question

Here is the code snippet from angular.io:

{ provide: RUNNERS_UP,    useFactory:  runnersUpFactory(2), deps: [Hero, HeroService] }

...

export function runnersUpFactory(take: number) {
  return (winner: Hero, heroService: HeroService): string => {
    /* ... */
  };
};

My question is why deps property is used here? What are the general cases for using deps?

Billi

See the manual angular.io/docs/ts/latest/api/core/index/… , it is quite exhaustive on this. – Faso

@estus, thanks, I've seen the manual. I usually ask a question to get a broader picture, while the manual just says that it's used with factory providers. But is it limited to it? Are there any other use cases? etc. These are the questions I have in my head – Billi

Yes, it is limited to factories. The benefit of TS is that properties don't come out of nowhere, they are defined with interfaces (FactoryProvider here). – Faso

@estus, got itl, thanks – Billi

Answer 1

This is a way to tell Angular dependency injections what dependencies it needs to inject to the factory function returned by runnersUpFactory.

For services there is the @Injectable() class to tell DI that it needs to analyze the constructor parameter of this class (same for @Component(), @Directive(), and @Pipe()), but this seems not to work for functions. Therefore they introduced the deps parameter.

DI will look up a provider using the key Hero and another one using HeroService and then will pass them as parameters to the factory function in the same order.

https://angular.io/docs/ts/latest/api/core/index/FactoryProvider-interface.html

deps : any[] A list of tokens which need to be resolved by the injector. The list of values is than used as arguments to the useFactory function.

Dismay

thanks, so it's basically only relevant when useFactory is used, correct? when the other providers are used, like useClass, useValue and useExisting, it has no purpose? – Billi

Exactly - only for useFactory – Alvord

Great, thanks. Just a quick follow-up, is there any other provider besides the ones I mentioned in my previous comment? – Billi

That's the kind of inconsistency I don't like on Angular 2 – Goodhen

github.com/angular/angular/blob/… – Alvord

@Goodhen A function cannot be annotated for DI with decorators, that's TS limitation. If you're after consistency, you can always annotate it like factoryFn.parameters = [[new Inject(dep)]] instead of deps: [dep], that's common syntax for all DI. – Faso

@Estus, Gunther! Thanks to both for very useful insights ;-) – Goodhen

@GünterZöchbauer Are you sure they're only for useFactory ? I've asksed it yesterday and was told - no. – Kajdan

@RoyiNamir seems reasonable. Might be handy in rare (I guess) situations. Haven't seen it before. – Alvord

Its not documented well, but deps can be used with useClass: #48595444 – Chilli

@pixelbits thanks for the info. Royi linked to your answer already. It makes sense, it just didn't occur to me it could be useful, but I can imagine scenarios. – Alvord