Angular 2 @Injectable() - how it works

Question

I'm trying to understand the necessity of adding the @Injectable() decoration in services in angular 2.

From the documentation: https://angular.io/docs/ts/latest/guide/dependency-injection.html

Why don't we add @Injectable() to the HeroesComponent? We can add it if we really want to. It isn't necessary because the HeroesComponent is already decorated with @Component. TypeScript generates metadata for any class with a decorator and any decorator will do.

So basically you only need to add @Injectable() if no other decoration is available, because if there is a decoration of any type available, the typescript compiler will automatically generate dependency information based on the variables that you passed in constructor eg.: constructor(private logger: Logger)

Is this right? Thanks

Answer 1

I think the name is a bit wierd and i really don't get why they tell you to add it for best practice. If adding it to every class is best practice, i don't get why it's needed at all. It would be easier to have the framework check every class if the constructor needs stuff injected or not. This could have been solved with a commandline property on typescript to always generate (mock) metadata.

The advice to always add it, i guess is similar to have the compiler always add metadata (which is not possible as far as i know). You could also (with vigilance) only (and always) add it to the classes that needs stuff injected - even if they have another decorator. That would probably be the most explicit solution - but hey - being sloppy can be both a bad thing and a good thing. You'll be the judge in this case.

TL;DR If you have no other decorators AND you if your constructor needs services/etc injected, then you need it.