Angular 6 now has injectable providers which is the new recommended way of injecting services, and it works really well except I'm having a problem when using a service which extends another service. So as an example, suppose I have
@Injectable({
providedIn: 'root'
})
export class ParentAppService { ... }
@Injectable({
providedIn: 'root'
})
export class ChildAppService extends ParentAppService { ... }
The problem is that no matter what I ask for in a component, the parent class is always injected.
So if you ask for
constructor(private childAppService: ChildAppService) { ... }
you will still be provided an instance of ParentAppService, which isn't expected.
A very simple workaround would be to just register the providers in your module the old fashioned way, and this works:
@NgModule({
providers: [AppService, ChildAppService]
})
But that's basically the old way of doing things, and doesn't have the benefits of better tree-shaking and cleaner testing like the new providedIn registration process.
So my question is, what's the right way to do this? Is there a better way to register a provider so I can get the desired behavior (maybe specifying a provider token somehow?).
I've setup a super simple stackblitz example to show what's going on.
https://stackblitz.com/edit/angular-lacyab?file=src%2Fapp%2Fapp.component.ts
You'll notice it says "Hello I AM APP SERVICE!" even though the component asked to be provided the child service. If in the app module we register the providers the old way (see the commented out code), all of a sudden the proper provider gets injected.
Thanks for your help!
Update:
There is already a pull request for that https://github.com/angular/angular/pull/25033
Original version
The problem: seems new angular treeshakable services don't respect inheritance:
First angular defines(1) ngInjectableDef
property on AppService
function. Then you inherit ChilAppService
from AppService
so that child class contains all properties from parent class. And finally when angular tries to define(2) ngInjectableDef
property on ChildAppService
it can't because it already exists(3) thanks to javascript prototypical inheritance.
In order to fix it you can either
1) workaround it through defining undefined ngInjectableDef
property on child service so that it won't read already filled from parent class property and angular will be able to define ngInjectableDef
on child class:
@Injectable({
providedIn: 'root'
})
export class ChildAppService extends AppService {
static ngInjectableDef = undefined;
constructor() {
super();
this.name = 'CHILD SERVICE';
}
}
Forked stackblitz
2) or report issue in github
3) or use composition instead of inheritance as was suggested in comments.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With