Today I encountered the following problem while trying to implement an abstract http service. This service should be base to extend for all other http services.
The implementation is following so far, skipping other methods for ilustration:
@Injectable()
export abstract class HttpWrapper<T> {
private options: RequestOptions;
constructor(private http: Http, private endpointUrl: string) {
let headers = new Headers();
headers.append('Content-Type', 'application/json');
this.options = new RequestOptions({ headers: headers });
}
public getAll(): Observable<T[]>{
return this.http.get(this.endpointUrl, this.options)
.map(this.extractAll)
.catch(this.handleError);
}
abstract handleError(error: any):Observable<Response>;
abstract extractOne(res: Response):T;
abstract extractAll(res: Response):T[];
}
Now if i want to use the abstract HttpWrapper i do the following:
@Injectable()
export class BlastReportService extends HttpWrapper<Item> {
constructor(http: Http) {
super(http,'/api/items');
}
handleError(error: any):Observable<Response>{
//Handling error
return Observable.throw(error);
}
extractAll(res: Response):Item[]{
let body = res.json();
let formatedBody = body.map((item: Item) => {
item = new Item(item);
return blastReport;
});
return formatedBody || [{}];
}
}
But doing this I get following compilation error:
Type 'Observable<Response>' is not assignable to type 'Observable<T[]>'.
Type 'Response' is not assignable to type 'T[]'.
Property 'find' is missing in type 'Response'.
I can't wrap my head around this because the method extractAll clearly returns Item[] and is used while mapping the results returned from the server.
I decided to implement this abstract HttpWrapper "to stay DRY". I'm not sure if it's the best way to do so.
