Can I create a Kotlin extension method for adding an rxJava Subscription to a CompositeSubscription?

Question

I've been playing with Kotlin/RxJava and tried to create an extension method for adding a Subscription to a CompositeSubscription, that would work like:

search.subscribe {
       //do stuff
}.addToComposite(compositeSubscription)

This is my attempt so far:

fun Subscription.addToComposite(composite: CompositeSubscription) = { composite.add(this) }

It compiles and runs without errors but doesn't seem to actually add the Subscription to the CompositeSubscription. Am I doing something wrong?

Answer 1

Your function is defined incorrectly. Compare:

fun f() { println("hello") }

This function executes the single statement println("hello") and is a simple Unit-returning function (same to void in Java).

fun g() = { println("hello") }

This function is a single-expression function, it just returns value of expression
{ println("hello") }, that is a lambda expression. g's return type is () -> Unit, and it doesn't execute the lambda body!

This is also explained here.

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To fix your code, you can just remove = from your function declaration:

fun Subscription.addToComposite(composite: CompositeSubscription) { composite.add(this) }