如何在rxjava中等待改造响应

Question

I use retrofit2 with rxjava extension.

I have a list of REST API urls and want to do this:

• for each
  • check whether a corresponding file locally exists
  • if yes: call the API and store the response or the HTTP error
  • if not: store a customized error
• return the list of those results

My problem is: apply returns (with an empty RequestResult ) before the server response is received. I think, I understand why, but I don't know how to fix it, because I need to return a RequestResult and not the Retrofit observable.

How can this be solved?

Here is my code:

@GET
Observable<Response<ResponseBody>> enroll(@Url String url);


class RequestResult {
    CustomException error;
    Response<ResponseBody> response;
}

Observable<ClassOfListItem> observable = Observable.fromIterable(listOfItems);

observable
    .flatMap(new Function<ClassOfListItem, ObservableSource<RequestResult>>() {

        @Override
        public ObservableSource<RequestResult> apply(ClassOfListItem listItem) throws Exception {

            RequestResult requestResult = new RequestResult();
            if (fileExists(listItem.url))   {
                Observable<Response<ResponseBody>> callObservable = restAPI.enroll(listItem.url)
                    .subscribeOn(Schedulers.io());

                callObservable
                    .subscribe(new DisposableObserver<Response<ResponseBody>>() {
                        @Override
                        public void onNext(Response<ResponseBody> responseBodyResponse) {
                            onPremiseEnrollmentResult.response = responseBodyResponse;
                        }
                        @Override
                        public void onError(Throwable e) {
                            onPremiseEnrollmentResult.error = new CustomException(e);
                        }
                        @Override
                        public void onComplete() {
                        }
                    });
            }
            else {
                requestResult.error = new CustomException("file not found");
            }
            return Observable.just(requestResult);
        }
    }
    .toList()
    .observerOn(AndroidScheduler.mainThread())
    .subscribe(new DisposableSingleObserver<List<RequestResult>>() {
        @Override
        public void onError(Throwable e) {
            Log.d("onError", e.getMessage());
        }
        @Override
        public void onSuccess(List<RequestResult> requestResults) {
            // parse results
        }
    }
 )

Answer 1

The flatMap() operator allows you to turn one observable into a different observable. You have a nested observer chain inside your apply() which is not part of the observer chain, so it will be empty because it has not completed yet.

To fix this, when the file exists, return the observable.

observable
  .flatMap(new Function<ClassOfListItem, ObservableSource<RequestResult>>() {
    @Override
    public ObservableSource<RequestResult> apply(ClassOfListItem listItem) throws Exception {
        RequestResult requestResult = new RequestResult();
        if (fileExists(listItem.url))   {
            return restAPI.enroll(listItem.url)
                .subscribeOn(Schedulers.io());
        }
        return Observable.error( new CustomException("file not found") );
    }
}
.toList()
.observerOn(AndroidScheduler.mainThread())
.subscribe(new DisposableSingleObserver<List<RequestResult>>() {
    @Override
    public void onError(Throwable e) {

        Log.d("onError", e.getMessage());
    }

    @Override
    public void onSuccess(List<RequestResult> requestResults) {
        // parse results
    }
}

If you need to capture both errors and successes into the list, then you can add map() operator to wrap RequestResult around the response and onErrorResumeNext() to wrap RequestResult around the error before the toList() operator.

Answer 2

If you are making api call on background thread then what you can do is invoke it synchronously....in your case your retrofit api method would change to following

Call<Response<ResponseBody>> enroll(@Url String url);

and you'd invoke by calling restAPI.enroll(listItem.url).execute()