Angular: lookup all providers implementing a specific interface

Question

In my Angular (4+) application, I want to create a basic plugin mechanism:

• The extension point is defined as an interface
• Extensions implement that interface
• To find all extensions, I need to discover the implementations of that interface at runtime.

Example: interface SearchStrategy with implementations PersonSearchStrategy and DocumentSearchStrategy (both services, registered as providers).

Is there a way to dynamically get instances of these services by querying for implementations of the SearchStrategy interface? How?

(might be some Injector related functionality?)

Answer 1

It's kinda possible, provided if you register the services with InjectionToken and use provide multi.

You can create an injection token with interface.

export const SearchStrategyToken = new InjectionToken<SearchStrategy[]>('SearchStrategy');

In your module register:

providers: [
  {provide: SearchStrategyToken, useClass: PersonSearchStrategy, multi: true}, 
  {provide: SearchStrategyToken, useClass: DocumentSearchStrategy, multi: true},
]

In your component or service:

constructor(private injector: Injector) {

  const services = this.injector
  .get(SearchStrategyToken); // return 2 items [ PersonSearchStrategy, DocumentSearchStrategy ]

  const personSearch = services.find(x => x.constructor.name === 'PersonSearchStrategy');

  const docSearch = services.find(x => x.constructor.name === 'DocumentSearchStrategy');

}

Code example provided here: https://stackblitz.com/edit/angular-clxr6k.

However, it would be good if you provide more details on your use case. Probably there are better solution than going for the above route.