By Tomek Polański —

Reactive Frustrations

7 minutes read

Most of us, who use Rx in daily projects, really enjoy it. That’s why we and so many others talk and write about Rx. Most material out there about Rx is aimed towards new adopters, to encourage them to give it a try. This kind of material, introduces Rx in a really positive light, but like with everything, Rx is not unicorns and rainbows. It’s not a silver bullet, you will encounter things that will annoy you.

In this post, I would like to share some of our frustrations that we’ve encountered while using RxJava and show that despite that, it is an awesome tool, but there will be some bumps along the way.


When you are new to Rx, the first thing that you will do is to read the documentation. In RxJava you will encounter this kind of documentation:

Converts an Observable that emits Observables into an Observable that emits the items emitted by the most recently emitted of those Observables.

- switchOnNext

It is completely accurate. For an experienced Rx developer, it is helpful. Unfortunately, a new adopter might be really confused.

Learning Rx is hard. I would encourage you to find an experienced developer that could explain things to you. RxMarbles is also a useful tool to visualize the operator behaviors.

Anonymous classes

This is not so much related to Rx, but rather about how we are used to developing for Android. In the past we were fine with using anonymous inner classes like this:

button.setOnClickListener(new View.OnClickListener() {
public void onClick(final View v) {
Log.d("", "Hi");

Before Rx, we didn’t need so many of them, but now almost every operator in RxJava requires a function.

Do yourself a favour and use either Retrolambda, Kotlin or Jack (when it’s ready):

button.setOnClickListener(v -> Log.d("", "Hi"));

Forgetting to subscribe

Time and time again, you write your reactive code and after you’re done, you will run your application. Then the Observable won’t trigger. It won’t emit any event and you will wonder why.

The first thing you need to check is if you have subscribed to the Observable.

Observables are lazy, they will not emit events unless you subscribe. Don’t feel too bad; forgetting to subscribe is a mistake made by both junior and senior Rx developers.

Reasoning about the code

The biggest challenge when working with Rx is to understand the code, to know how it will behave at runtime.

How does the following code behave?

(f, s) -> f + s)
.subscribe(result -> Log.d("", result));

Here we combine the last emissions of first and second Observables and combine them. Just by looking at this code, we do not know if the Log.d call will be ever executed. combineLatest requires both Observables to emit at least one item. Here we would need to go up the stream for both source Observables to know if they would ever emit any event.

Here’s another example:

Observable.concat(first, second)
.subscribe(result -> Log.d("", result));

concat subscribes to first and emits its values. If first completes, then it would subscribe to second and do the same thing. The question that we could ask here is: will elements from second be ever emitted?

This question is valid as we do not know if first will ever complete. Again we would need to check upstream to see how first is created to have the answer.

There are two approaches to tackle this problem: with types or with proper naming. Here I will suggest our naming conventions.

In our current project, we use three main naming conventions:

  • ...Once - when there is only one value emitted and after that, the complete event will come (similar to Single)

  • ...Stream - the stream might emit values or might never emit any value, but we are sure that it never completes

  • ...OnceAndStream - the stream will emit value as fast as it can after subscription. After that, it might emit values, but it will never complete.

With this approach it’s way easier to reason about the code:

(f, s) -> f + s)
.subscribe(result -> Log.d("", result));

Here we can see that the combineLatest will for sure emit at least one value. Other example:

Observable.concat(firstOnce, secondStream)
.subscribe(result -> Log.d("", result));

And here we see that the secondStream will be subscribed to as firstOnce completes.

On the other hand, if we have code like this:

Observable.concat(firstStream, secondStream)
.subscribe(result -> Log.d("", result));

We know that something is not right as the firstStream will never complete and there is something wrong with this code.

Using these naming conventions with Observables and Flowables improve code clarity, but they should not be used with other Rx types like Single, Completable, and Maybe. operators

RxJava contains a massive amount of methods/operators. Among those operators, there are some that at the first glance look and behave the same way, but in reality, they should be used in different cases and could be easily misused.

flatMap, switchMap, and concatMap

If you do not know the difference, let me draw an analogy it:

At a developer conference, there are multiple speakers and one announcer. The role of the announcer is to introduce speakers and to invite them on the stage. After speakers are introduced, they come on the stage and give their speech.


In the case of flatMap, whenever the announcer decides, she invites a speaker on the stage. That means that she can allow multiple speakers on the stage, at the same time, resulting in a really noisy conference.


The announcer could be a bit impatient. Whenever she chooses to invite a speaker, she will interrupt the current speaker on stage and ask them to stop. Next, she will invite a new speaker to take the floor. In this case, only one talk at a time is possible.


With concatMap, the announcer is fair - she might want to invite a new speaker while the previous talk hasn’t finished, but she won’t. She will wait for the current one to finish, even if that speaker takes too long, and then will invite the next one. Again, only one speech at a time.

That’s it for now. Hope you’ve learnt something new and it will prevent you from repeating our mistakes. Stay tuned for more in the next post.