how i can initialized var in interface?

# android

[morozov]

how i can initialized var in interface?

[dknapp]

https://kotlinlang.org/docs/reference/interfaces.html#properties-in-interfaces

[pablisco]

Or you can do an extension property like:

val MyInterface.prop: Int by lazy { 123 }

[yoavst]

That's actually not possible ☝️

[pablisco]

or

val MyInterface.prop: Int 
    get() = 123

But it’ll be always fetched but you can do it with an object.

[pablisco]

@yoavst Is it not?

[yoavst]

Where will the delegate object be stored?

[pablisco]

It creates the value every time, ideally you want to have an object that provides it

[pablisco]

If you want to leave it localised you can do this too:

interface MyInterface {
    object Prop {
        operator fun getValue(thisRef: Any, property: KProperty<*>): Int = 123
    }
}

val MyInterface.prop: Int by MyInterface.Prop

[pablisco]

Also worth to say that it’s a global. If you want to do it for each subclass it’s a bit more complicated (especially if you want a var instead of a val)
You may have to start using a companion cache and such, potentially giving you leaks (unless you use WeakReferences)

[yoavst]

Your first version with the lazy delegation, does it compile?

[pablisco]

yeah, beacuse lazy is a property delegate so it has the getValue operator

[villela]

in many libs we see this default value for fields issue solved by using annotations

[villela]

without knowing the problem is hard to give a good answer but the closest I see to get what you want is this:

[villela]

interface MyInterface {
var _prop: Int?
var prop: Int
get() = if (_prop!=null) _prop!! else 1
set(value) {
_prop = value
}
}

[yoavst]

wow @pablisco , I didn't know that you could use a delegate on extension properties (of course it would have been global, but still). Good to know that for DSL stuff!

[pablisco]

Yeah! Delegation is equal part amazing and dangerous 😁