Dumb quick question: can we implement an extension...

# multiplatform

[Sylvain Patenaude]

Dumb quick question: can we implement an extension method with the expect / actual pattern?

EDIT: Looks like it's doable, but the method I want to split is not an extension method after all. So my question is rather:
Can we have only some methods of a class that are prefixed with expect or does it have to be the full class at that point?
For instance, I would have a class where pretty much all methods would be 'common' except for a few that would need to be implemented natively.

[russhwolf]

An expect class needs to have only expect declarations. Follow/star https://youtrack.jetbrains.com/issue/KT-20427 to change that. There’s a couple strategies you can use as workarounds such as having your expect class extend another class/interface that includes your default/shared implementations, or putting your common implementations in extension functions.

[Dmitri Sh]

exactly my question from last night. came across the same problem: ith an expected class in shared, not possible to have any method implementation? All methods must be actualized in platform specific actuals? I have a class (decorator) that can have some of its methods actually implemented in shared - but am forced to duplicate the code in actuals. Is this (no pun intended) "expected"? Seems like a waste.

[ankushg]

@Dmitri Sh Would declaring extension functions on your expected class in your common code help reduce duplication?

Edit: looks like Russell beat me to it 😊

[russhwolf]

It’s the current state of things but it won’t be that way forever. This sort of rough edge is what comes with being an early adopter to a new technology like this.

[Sylvain Patenaude]

Good point @russhwolf. I will use extension functions as needed. I can't extend another class since I'm using `object`s. My methods are kind of static, more like helper functions.

[russhwolf]

Objects can extend interfaces if that helps, but extension functions work just as well if you don’t need to be able to override.

[Sylvain Patenaude]

I know that objects can extend interfaces, but that would mean that my interface would have some default implementation in this case? While feasible, I think the code will be clearer with extension functions. Interfaces with only default code (without real abstract methods) would look weird to me.

[Sylvain Patenaude]

Can we have static methods (or companion methods in Kotlin terms) in an interface? Basically my pattern would look like this:

expect object Helper : HelperCommon { // HelperCommon would either be a class or interface with static methods only
    expect fun function1() // specialized natively
    expect fun function2() // specialized natively
    ...
}

Now from another class, I would like to be able to call the functions in HelperCommon through Helper like this:

val someValue = Helper.functionCommon1(...) // functionCommon1 would be static/companion in HelperCommon

[Sylvain Patenaude]

At first I wanted to use extension functions, but they behave like member functions while my functions are meant to be static.

[russhwolf]

You can’t have an expect companion object in a non-expect class/interface. But you could have a non-expect object Helper and have its methods delegate to internal expect functions which then have separate implementations per-platform.

[Sylvain Patenaude]

But the internal expect functions would be at module level instead of object level, right? So I wouldn't be able to call Helper.functionThatIsNativelySpecialized().

[russhwolf]

Well, doesn’t have to be internal, I just figured you might want to limit scope

[russhwolf]

internal would mean consumers can only interact with it through Helper

[Sylvain Patenaude]

I may be stubborn, but I'm looking for a workaround for the current Kotlin limitation that we mention above and offer a unified interface to calling clients like this:
val someVal1 = Helper.commonFunction()
val someVal2 = Helper.specializedFunction() //expect/actual

[russhwolf]

Right I might be losing track of your actual use-case

[Sylvain Patenaude]

So you're saying internal would do this? I thought it was just a visibility modifier for the callers in same module.

[russhwolf]

No not quite saying that

[Sylvain Patenaude]

My other option is to have 2 separate objects, like HelperNative and HelperCommon for instance, but if I can avoid it that would be better.

[russhwolf]

object Helper {
    fun commonFunction() {
        // .. common impl
    }
    fun specializedFunction() = specializedFunctionInternal()
}

expect fun specializedFunctionInternal() // Will have separate impl

[russhwolf]

you can do something like that where you delegate so the caller only needs to talk to one object

[russhwolf]

You might make specializedFunctionInternal have internal visibility so people don’t call it directly, but you don’t have to

[russhwolf]

and you could wrap specializedFunctionInternal() in a HelperNative object if you want but it’s not necessary

[Sylvain Patenaude]

That would work. The only minor caveat is that clients from the same module/package could still call it directly if they wanted, but it's no big deal.

[russhwolf]

Yeah that’s where you could make it internal and have Helper live in its own module

[Sylvain Patenaude]

Unless I put the specializedFunctionInternal() function private? Can you have a private expect fun function()?

[russhwolf]

No I think it needs to be public or internal

[Sylvain Patenaude]

Ah ok, I tried. 😉 I miss the good old days when I was using private virtual functions in C++. Thank you very much for your help @russhwolf.

[Dmitri Sh]

@ankushg not sure how extensions would help me avoid code duplications here, can you clarify? Btw -I was searching the other day for a way to add Parcelable support to a common data class, and came (I think, based on the name) your article on that - and was able to do exactly that, so thanks for that!

[Dmitri Sh]

@russhwolf sure, I understand I am an early adopter and this sort of thing comes with the price - I was not so much complaining as asking if I misunderstood something

[ankushg]

@Dmitri Sh In some simple cases, you can take an expect class SpecialClass and define an extension fun SpecialClass.doSomething() in common code.

This way, if your implementation of doSomething is identical across platforms, you can still define it once in Common code, even though you aren't technically allowed to have it inside your expect class declaration.

Any yup that was me 😊 Glad it was helpful!

[Dmitri Sh]

interesting, I didn't realize I could do that in common code - previously I defined extensions for working with dates in android and iOS code, maybe I can move that to common. And yes, that article was very helpful!