Interface with generic type of the class implementing the interface?

Question

Having a class that implement a interface with generic type of "This" class

Is there a way to do this without a cast?

Simple code:

interface Triggerable<This: Triggerable<This>> {
    var trigger: (This) -> Unit
    fun triggerNow() = trigger(this as This)
}
class Test : Triggerable<Test>{
    override var trigger: (Test) -> Unit = { /*...*/ }
}

The same a little more complex:

interface TriggerInterface<T> {
    val trigger: (T) -> Unit
    fun triggerNow()
}
interface Triggerable<T: Triggerable<T>>: TriggerInterface<T> {
    override fun triggerNow() = trigger(this as T)
}
interface Signalable<T>: TriggerInterface<T> {
    var value: T
    override fun triggerNow() = trigger(value)
}
class Test : Triggerable<Test>{
    override val trigger: (Test) -> Unit = { /*...*/ }
}

EpicPandaForce · Accepted Answer · 2020-02-26 12:16:14Z

3

Should be possible like this

 interface TriggerInterface<T: TriggerInterface<T>> {
    val trigger: (T) -> Unit
    fun triggerNow()

    fun getThis(): T
 }

 interface Triggerable<T: TriggerInterface<T>>: TriggerInterface<T> {
     override fun triggerNow() = trigger(getThis())
 }

 class Test : Triggerable<Test>{
     override fun getThis(): Test = this

     override val trigger: (Test) -> Unit = { /*...*/ }
 }

Check http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#FAQ206

I would also personally recommend to reconsider if you really need both TriggerInterface AND Triggerable (where one inherits from the other).

edited Feb 26, 2020 at 12:16

answered Feb 26, 2020 at 8:14

EpicPandaForce

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2 Comments

Robot Over a year ago

It's needed because there is another interface which trigger another value

Robot Over a year ago

I accepted your answer because it's the best solution but not what I'm looking for. I ended using cast, because if the generic type argument "This" is well defined there will be no problem

Alex Krafts · Accepted Answer · 2020-02-26 07:59:41Z

0

Calling override fun triggerNow() = trigger(this as T) you are trying to cast Triggerable<T> to T thats why compiler warns you about unchecked cast

having

    val trigger: (TriggerInterface<T>) -> Unit

will allow you to call trigger with no casting

    override fun triggerNow() = trigger(this)

to actually execute

override val trigger: (Test) -> Unit = { /*...*/ }

you need an instance of Test to pass inside of trigger which is not declared anywhere in your code yet

answered Feb 26, 2020 at 7:59

Alex Krafts

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