How do I delay RxJava2 subscription?

Question

I have the following code, which returns a lot of data in the response, so much that I get a NullPointerException when it's being loaded in the android Activity when I scroll down too fast (since not all the data has been initialized yet), no problems if I wait a second and then scroll.

I want a way to delay the subscribe part, so that the Response<GetFeedTopicsResponseBody> is entirely populated with data (none is not initialized) when I call setAdapter . I tried checking response.isSuccessful but that does not work because no problem with the response itself, just the data takes time to deserialize into Java objects from JSON. I also tried onComplete in subscribe but that does not work either.

So I want either a way in RxJava2 to have a boolean value switch to notify the following subscription once it is complete, it will subscribe.

        mGetFeedTopicsDisposable = ApiClient.getInstance()
            .doGetFeedTopicsQuery(request)
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread());
            .subscribe((Response<GetFeedTopicsResponseBody> response) -> {
                if (response.body() != null) {
                    List<Topic> topics = response.body().getTopics();
                    if (topics != null) {
                        mBinding.fragmentTopicListRecyclerTopics.setAdapter(TopicListAdapter.getInstance(topics));
                        if (response.body().isPaginated()) {
                            mRequestBuilder.setCursor(response.body().getCursor());
                        }
                    }
                }
            }, (Throwable ex) -> {
                Log.e(TAG, ex.getMessage());
            });

The error message I specifically got was:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.models.User.getThumbnailImageUrl()' on a null object reference

where this User object is set as a field of the Topic object which is added into the list of topics retrieved with getTopics() . If I don't scroll, I don't get this NullPointerException and the thumbnail urls for the Users are loaded properly.

Answer 1

Question : How do I delay RxJava2 Subscription?

Example :

I have added repeat(...) for better understanding.

io.reactivex.Observable
                    .just(new Object())
                    .subscribeOn(Schedulers.io())
                    .observeOn(AndroidSchedulers.mainThread())
                    .delay(10, TimeUnit.SECONDS)
                    .repeat(2)
                    .doOnSubscribe(disposable -> Log.d("Delay Example ","Observer subscribed at : "+  DateFormat.getDateTimeInstance().format(new Date()) + " and execute after 10 seconds"))
                    .subscribe(new DefaultObserver<Object>() {
                        @Override
                        public void onNext(Object o) {
                            Log.d("Delay Example ","on Next : "+  DateFormat.getDateTimeInstance().format(new Date()));
                        }
                        @Override
                        public void onError(Throwable e) {}
                        @Override
                        public void onComplete() {
                            Log.d("Delay Example ","on Complete : "+ DateFormat.getDateTimeInstance().format(new Date()));
                        }
                    });

Output:

You see, on Next is called twice with 10 second delay.

Here, you can do adapter related operations in onComplete . :)

Hope this answers the question that you've asked.