3.x: Proposal: ObservableConverter interface and friends

[ZacSweers]

For the compose() operators, ObservableTransformer was introduced to allow for classes to implement multiple transformer interfaces for composite implementations. I'd like to propose doing the same for the to() operator by introducing ObservableConverter (and corresponding ones for others). This would allow for composite converters as well.

If you're open to this, I can contribute a PR!

[akarnokd]

That would break binary compatibility and otherwise to(Function<? super Observable<T>, R>) isn't that much of a type mess as lift or compose is.

[ZacSweers]

This would be an alternative to, not a replacement for. I'm not sure what type mess means, the larger issue is that you can't have something implement Function multiple times with different generics

[akarnokd]

A to overload will lead to type ambiguity when one writes to(f -> somefunc(f)).

[ZacSweers]

ah, good point. Would you be against a different name? Like convertTo? Or would this be more of an RxJava 3.0 discussion?

[akarnokd]

Yes, 3.x API cleanup can include this type of change. I otherwise always have separate converters for the base reactive classes.

/cc @JakeWharton, @vanniktech, @artem-zinnatullin

[vanniktech]

I remember we had the discussion about to() already in 2.0.0-RC days. The closed I could find is - #4672 - if I recall correctly it was decided to not change the to operator.

I don't have a strong opinion about changing it in 3.x though.

[artem-zinnatullin]

In general, I'd like RxJava to have as less interfaces as possible, especially if generic ones like Function can be used instead.

But Java limitations kinda force us to have them here. So I'm ok if 3.x will have dedicated *Converter interfaces.

[akarnokd]

Retagged as 3.x.

[ZacSweers]

I'd like to lean on this for a 2.x consideration again, seeing as 3.x discussion seems to be settling on "not necessary anytime soon".

Proposal, to protect against ambiguity, would be to add a new convert or convertTo method, which retains the semantics while introducing the new interfaces to allow for composition.

The old to() methods could optionally be deprecated too to avoid confusion

[akarnokd]

Project reactor chose the as() naming. It's short, which I like, and could be added without deprecating to.

public <R> R as(ObservableConverter<T, R> converter);
public <R> R as(FlowableConverter<T, R> converter);
public <R> R as(SingleConverter<T, R> converter);
public <R> R as(MaybeConverter<T, R> converter);
public <R> R as(CompletableConverter<R> converter);

[ZacSweers]

I like it! Should I send a PR?

[akarnokd]

Sure.

[ZacSweers]

Done in #5729

[akarnokd]

This has to be still resolved in 3.x where I suggest having only to() with the custom interfaces just introduced.

[ZacSweers]

Sounds good

[phatnhse]

Hi guys, I have a concern which related to this ticket. Here is my code:

   inline fun <reified T> withSchedulers(subscribeOn: Scheduler, observeOn: Scheduler): T {
       when (T::class) {
           FlowableTransformer::class -> return FlowableTransformer<Any, Any> {
               it.subscribeOn(subscribeOn).observeOn(observeOn)
           } as T
           ObservableTransformer::class -> return ObservableTransformer<Any, Any> {
               it.subscribeOn(subscribeOn).observeOn(observeOn)
           } as T
           SingleTransformer::class -> return SingleTransformer<Any, Any> {
               it.subscribeOn(subscribeOn).observeOn(observeOn)
           } as T
           CompletableTransformer::class -> return CompletableTransformer {
               it.subscribeOn(subscribeOn).observeOn(observeOn)
           } as T
           MaybeTransformer::class -> return MaybeTransformer<Any, Any> {
               it.subscribeOn(subscribeOn).observeOn(observeOn)
           } as T
       }
       throw IllegalArgumentException("not a valid Transformer type")
   }

Are there any ways to reduce the boilerplate code? or any better approaches to achieve this? Or just write 5 functions for 5 types, something like to operator above?

Thanks.

[akarnokd]

Closing via #6514.