RxJava

    @Override public Observable<IWorkoutDetailsDataModel> getWorkoutDetails(int workoutId) {
        return userApi.getWorkoutCategories(token)
                .flatMap(response -> {
                    if (!response.isSuccessful() || response.body() == null) {
                        return Observable.just(categories);
                    }

                    categories = response.body();
                    return Observable.just(categories);
                })
                .flatMapIterable(categories -> categories)
                .flatMap(category ->  userApi.getWorkouts(token, category.getId())
                        .flatMap(response -> {
                            if (!response.isSuccessful() || response.body() == null) {
                                WorkoutDetailsDataModel dataModel = WorkoutDetailsDataModel.newInstance(null);
                                return Observable.just(dataModel);
                            }

                            List<Workout> workouts = response.body();
                            cache.setWorkouts(workouts);
                            if(cache.getWorkout(workoutId) != null) {
                                return Observable.just(WorkoutDetailsDataModel.newInstance(cache.getWorkout(workoutId)));
                            }

                            return Observable.fromIterable(cache.getWorkouts())
                                    .filter(item -> item.getId() == workoutId)
                                    .flatMap(result -> {
                                        DownloadWorkoutService.DownloadState workoutState = DownloadWorkoutService.isWorkoutDownloaded(result);

                                        result.setDownloading(workoutState == DownloadWorkoutService.DownloadState
                                                .DOWNLOADING);
                                        result.setDownloaded(workoutState == DownloadWorkoutService.DownloadState
                                                .DOWNLOADED);

                                        return Observable.just(WorkoutDetailsDataModel.newInstance(result));
                                    });
                        })
                        .onErrorResumeNext(throwable -> {
                            WorkoutDetailsDataModel dataModel = WorkoutDetailsDataModel.newInstance(null);
                            return Observable.just(dataModel);
                        }));
    }