Support Observable return types for Nestia SDK

[sourav-bhar]

Feature Request

When using the Nestia Client SDK, it can properly infer the controller method return value types when returning synchronously or returning a Promise. However, if we try to return an Observable, then the SDK is not able to generate types properly.

Since NestJS has first-class support for Observables, many times we work with Observables exclusively and would just like to return the Observable from the controller method instead of converting it to a Promise and then returning the Promise. However, returning an Observable type breaks Nestia's ability to properly type the response type.

Workaround's we need to use today:

1. Use async / await and only work with Promises
2. Use the firstValueFrom rxjs operator to convert Observable to Promise before returning the response from the controller.

This works fine:

   @TypedRoute.Get('user/:id')
   async getUser(@Param('id') id: string): Promise<User> {
     return firstValueFrom(this.userService.findById(id));
   }

This does not:

   @TypedRoute.Get('user/:id')
   getUser(@Param('id') id: string): Observable<User> {
     return this.userService.findById(id);
   }

[samchon]

Can you give me an example repo?

I can't image the Observable type being used in the SDK, because in my memory, it does not have any property mebers and there're only member methods in the type.

[sourav-bhar]

Ok, I will try to provide a sample repo tomorrow. Basically, when Nestia sees the Observable<ReturnType> being returned, it should essentially consider it the same as Promise<ReturnType> since it is irrelevant if Promise or Observable is being returned by the controller method. The fetch functions used by the Nestia SDK will always return a Promise as expected. However, all information about ReturnType is lost when returning Observable<ReturnType> from the controller method, but it works fine when returning Promise<ReturnType>.