RXKotlin Lambda Understanding

Question

This never completes:

Completable.complete()
        .andThen{ Completable.complete() }
        .test()
        .assertComplete()

This does complete:

Completable.complete()
        .andThen(Completable.complete())
        .test()
        .assertComplete()

According to Jake Wharton:

"You want andThen(Completable.complete()). Note the use of parenthesis and not curly braces. The latter creates a lambda that doesn't call its emitter."

Reference: https://github.com/ReactiveX/RxJava/issues/5551

Can anyone explain this in more detail? I thought I understood lambda's but this has really thrown me.

score 3 · Accepted Answer · answered Jan 30 '18 at 18:48

With the curly braces, you're using SAM conversion and defining an anonymous CompletableSource instance by describing its single method inside the lambda.

This is the same as doing the following:

Completable.complete()
        .andThen(object: CompletableSource {
            override fun subscribe(cs: CompletableObserver) {
                Completable.complete()
            }
        })
        .test()
        .assertComplete()

As you can see, inside this CompletableSource you're doing nothing but creating a Completable instance and throwing away its result. For the chain to continue, you'd have to call methods on the CompleteableObserver that you've received as a parameter (for example, you could call onNext() and onComplete() on it).

Although it's almost the same syntax, here's the Java version, in case it helps you understand things:

Completable.complete()
        .andThen(new CompletableSource() {
            @Override
            public void subscribe(CompletableObserver cs) {
                Completable.complete();
            }
        })
        .test()
        .assertComplete();

Great great explanation, I must have been very tired, this makes complete sense. — Thomas Cook, Jan 30 '18 at 19:02
It's one of those gotchas that everyone using Kotlin and Rx are bound to spend a couple hours debugging, unfortunately. It happens with other Rx classes and operators too. — zsmb13, Jan 30 '18 at 19:04
Brain melt :D it's the same thing that's hurting my brain trying to learn Haskell — Thomas Cook, Jan 30 '18 at 19:05

score 1 · Answer 2 · answered Jan 30 '18 at 18:49

The valid example in parentheses works as follows:

Completable.complete() is executed and then passed as an argument to andThen.
The invalid one does simply pass a lambda in curly braces as an argument to andThen, which becomes an implementation (thanks to SAM Conversion) of CompletableSource. It never gets executed. The following shows what you do:
```
val compl = CompletableSource { Completable.complete() }
Completable.complete()
    .andThen (compl)
    .test()
    .assertComplete()
    .assertComplete()
```

RXKotlin Lambda Understanding

2 Answers2