Hi. What will be the proper way to create an extension function for an Enum? I saw this question, bu...

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[Vitali Plagov]

Hi.
What will be the proper way to create an extension function for an Enum?
I saw this question, but I’m not sure if it’s a correct approach to define a extension for an Enum.
https://stackoverflow.com/q/35966447

[Vitali Plagov]

My use case is to extract this function I have in one Enum class into an extension function:

companion object {
   fun methodByValue(value: String) = values().first { it.value == methodValue }
}

[marstran]

You could do something like this. It requires that the enum has a companion object that implements the EnumCompanion interface though.

interface EnumCompanion<T : Enum<T>>

enum class Test {
    A, B, C;
    
    companion object : EnumCompanion<Test>
}

fun EnumCompanion<Test>.print() = print(this)

fun main() {
   Test.print()
}

[marstran]

I actually wrote this answer on SO some time ago that kind of answers your question: https://stackoverflow.com/a/55019478/4137489

[Vitali Plagov]

Not sure I understand this approach either 😀

[marstran]

The problem is, you cannot create a function directly on the enum like that without it having a companion object.

[marstran]

And to be able to create a generic extension function on enum companion objects, you have to make the companion objects implement an interface.

[Vitali Plagov]

I don’t understand this function - fun EnumCompanion<Test>.print() = print(this). Where is it located?

[marstran]

Just on the top level. It just prints the companion object. In your case, you could implement this function:

fun <T> EnumCompanion<T>.methodByValue(value: String) = TODO()

[Vitali Plagov]

Is it a correct way to define an extension function?

fun <T> EnumCompanion<T>.methodByValue(value: String) = this.values().first { it.value == methodValue }

It doesn’t compile

[marstran]

Right. You need to use enumValues<T>() , and that requires T to be reified. Look that the definition of getByKey in my Stack Overflow answer.

[marstran]

Also, it.value will not compile, because the generic T doesn't have value defined.

[Vitali Plagov]

No other ways to check it’s value?

[marstran]

Look at my Stack Overflow answer.. That's why I define val T.key: K inside the EnumWithKey interface.

[Vitali Plagov]

Ok, thanks.
I think I will postpone defining the extension fun for an Enum. It’s too complicated for me now 🙂

[David Eriksson]

@marstran out of curiosity (not at the right computer to try it myself now) would it be possible make an extension function fun KClass<MyEnum>.MyExtensionFunction() so that it could be called as MyEnum::class.MyExtensionFunction()?

[marstran]

@David Eriksson Tested it. That works too 🙂 The drawback is using reflections for it though.

[David Eriksson]

::class still counts as reflection, right 😞

[David Eriksson]

I'm no stranger to reflection on the JVM though :)

[marstran]

Yes. You also have to do an import kotlin.reflect.KClass 😉

[David Eriksson]

Yeah the EnumCompanion<> way seem superior