RxJava Pro-Tip: How flatMap and switchMap handle terminate events
In this article, we will discuss the confusing question of, “How does a flatMap or switchMap handle a terminate event such as Observable.empty() or Completable.complete().” We will also discuss how we can handle null state inside of a RxJava chain.
Before RxJava and Kotlin became standard practice, developers would use null to represent NOTHING. This “nothing” state has been used as a stop or return early trigger. What about with RxJava? How do we achieve an equivalent null or NOTHING state in the modern immutable, reactive world? Luckily, the combination of the Optional class and a reactive “empty” state solve this problem.
When dealing with Observable.empty(), Completable.complete(), and similar RxJava terminate events, there is usually some misunderstanding around how they work inside of flatMap and switchMap.
When I return an Observable.empty() in the flatMap or switchMap, do I terminate the entire stream?
The answer is NO, it won’t. Let me tell you why:
Analyzing the result in the chain
Let’s run a small unit-test to see the result.
In the snippet, we send out an Observable.empty(). This function calls the observer’s onComplete immediately. Typically, this disposes the chain, thus never emitting future values. However, in this test, we see that the second value is indeed emitted. The observer still gets the value! The stream is still alive. How is this possible?
Diving into flatMap
When you call flatMap, you are required to create a new Observable that is used as the new ObservableSource. This source is used to create an ObservableFlatMap which internally creates another Observer that is subscribed to the upstream. This Observer acts as a man-in-the-middle by observing the upstream and redirect the result to the downstream. What we really care about is the code inside the subscribeInner function as below:
Both Observable.just() and Observable.empty() produce a Callable Observable. We can see that this source is handled by the first if-statement, which does not propagate the complete. What about the else-statement? If the returned Observable is custom, it will subscribe to the returned Observable with an inner observer, also ignoring the complete.
Insights
As can be seen, we can safely use Observable.empty() to indicate a NOTHING state in flatMap or switchMap.
Note:
For those who prefer to use RxRelay to avoid any accidental stream termination, this safeguard doesn’t necessarily require the RxRelay.
Lastly, the flatMap and switchMap are similar, so I won’t show the switchMap here.
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