如何将多个 Observable 的发射值合并为一个 Observable 发射？

Question

I have a function that receives a DTO object and transforms it into an observable that emits the single transformed Entity. It has the following header:

dtoTransformer(dto: DTO): Observable<Entity>

I also have a function that returns an Observable emitting a list of DTOs:

getDTOs(): Observable<DTO[]>

Using these two I need to write a function that returns an observable of the Entity list, transformed from the DTOs received:

getEntyties():Observable<Entity[]>

My problem is applying the dtoTransformer to the values of the DTO list inside the observable. Is there an operator that can help me? I don't know how to approach this.

Answer 1

You can use a combination of forkJoin and concatMap . This is under the assumption that these are simple httpClient (completing) calls:

getEntities(): Observable<Entity[]> {
  return this.getDtos().pipe(
    concatMap((dtos) => forkJoin(
      dtos.map((dto) => this.dtoTransformer(dto))
    )
  )
}

What happens here:

1. Call http to get the DTO[]
2. Map this response to an array of http calls which by using map
3. Pass this array of observables to forkJoin which will do parallel calls and returns when all are completed (and not errored)

Obviously there are things that you need to take into account, when one http call errors, it doesn't return anything, so you need to add some fallback. If the DTO[] array is very big, you can hit a request limit, so perhaps some throttling can be put in place, or chunking by making a fixed amount in parallel and when those are done continue with the next batch