code: public <T> void adapt(@NonNull final Observable<T> subscriber, @Nullable final Callback<T> callback) {
          subscriber.subscribeOn(Schedulers.io())
                  .observeOn(AndroidSchedulers.mainThread())
                  .subscribe(new Subscriber<T>() {
  
                      @Override
                      public void onCompleted() {
  
                      }
  
                      @Override
                      public void onError(Throwable e) {
                          if (callback != null) {
                              callback.error(e);
                          }
                      }
  
                      @Override
                      public void onNext(T result) {
                          if (callback != null) {
                              callback.success(result);
                          }
                      }
                  });
      }
docs: Changes observables into callbacks.
  
  @param subscriber Observable to subscribe to.
  @param callback Callback where onError and onNext from Observable will be delivered
  @param <T> Class of the result.