我可以为类的方法定义通用的打字稿接口吗？

Question

I'm cleaning up the types of my Angular 4 Application and I would like to define an interface to the Database classes in charge of the API calls.

Those classes will have methods receiving Criteria objects, and returning Observables of ResultItems[] that are widely different. So I've tried to define this Interface:

import { Observable } from 'rxjs/Observable';

export interface Database<C> {
  [methodName: string]: (args: C) => Observable<any>;
}

to cover the methods receiving a Criteria and returning any thing.

Then I try to have this kind of implementation:

@Injectable()
export class ItemsDatabase<ItemsCriteria> implements Database<ItemsCriteria> {
  constructor(private api: ApiService) {}

  fetch(args: ItemsCriteria): Observable<APIResponse[]> {
    return this.api.fetchItems(args);
  }

  summary(args: ItemsCriteria): Observable<SummaryResponse[]> {
    return this.api.itemsSummary(args);
  }
}

but Typescript denies it throwing:

Class 'ItemsDatabase<ItemsCriteria>' incorrectly implements interface 'Database<ItemsCriteria>'.
  Index signature is missing in type 'ItemsDatabase<ItemsCriteria>'

Any advice? or this is just not possible? Thanks in advance!

Answer 1

You have to add the index signature to the class

@Injectable()
export class ItemsDatabase<ItemsCriteria> implements Database<ItemsCriteria> {
  constructor(private api: ApiService) {}
  [methodName: string]: (args: ItemsCriteria) => Observable<any>;
  fetch(args: ItemsCriteria): Observable<APIResponse[]> {
    return this.api.fetchItems(args);
  }

  summary(args: ItemsCriteria): Observable<SummaryResponse[]> {
    return this.api.itemsSummary(args);
  }
}