How to improve kotlin lambda syntax?

Question

I'm trying to use kotlin M12 in android project and during the work I got this piece of code subscribe({onSuccess(it)}, {onFailure(it)})

AppObservable.bindActivity(this, api.get(id)).subscribe({onSuccess(it)}, {onFailure(it)})

fun onSuccess(str: String) {}

fun onFailure(tr: Throwable) {}

This is not so bad, but I think it would/should be better. How can I improve it?

Answer 1

First, create a helper extension method like this:

fun<T, R> Observable<T>.subscribe(
        receiver: R,
        onSuccess: R.(T) -> Unit, 
        onFailure: R.(Throwable) -> Unit) {
    subscribe({ receiver.onSuccess(it) }, { receiver.onFailure(it) })
}

Now you can use your new method like this:

AppObservable.bindActivity(this, api.get(id)).subscribe(this, ::onSuccess, ::onFailure)

The :: operator creates a method reference. Once KT-6947 gets resolved, we can omit the helper method and write subscribe(this::onSuccess, this::onFailure) directly. This is already possible in Java 8.

To work around this issue we can alternatively implement the following helper method that binds a receiver to an extension method:

fun <T, R, E> T.bind(reference: T.(R) -> E): (R) -> E = { this.reference(it) }

and use it like this:

AppObservable.bindActivity(this, api.get(id)).subscribe(bind(Foo::onSuccess), bind(Foo::onFailure))

but this is hardly more elegant than your initial solution.

EDIT1:

Thy syntax ::onSuccess is forbidden for now, so you have to use 'Foo::onSuccess' where Foo is your class name.

EDIT2:

You can have a little fun with operator overloading by declaring the same method as

operator fun <T, R, E> T.plus(reference: T.(R) -> E): (R) -> E = { this.reference(it) }

Then you can use it like so:

AppObservable.bindActivity(this, api.get(id)).subscribe(this + Foo::onSuccess, this + Foo::onFailure)