is it possible to do something like this in kotlin ```interface ClientEvents { ready: () => voi...

# getting-started

[Yusuf.I]

is it possible to do something like this in kotlin

interface ClientEvents {
  ready: () => void;
}

export declare interface Client {

  on<Event extends keyof ClientEvents>(
    event: Event,
    listener: ClientEvents[Event],
  ): this;
  off<Event extends keyof ClientEvents>(
    event: Event,
    listener: ClientEvents[Event]
  ): this;
  emit<Event extends keyof ClientEvents>(
    event: Event,
    ...args: Parameters<ClientEvents[Event]>
  ): boolean;
}

[Yusuf.I]

This code is from type script

[Yusuf.I]

https://github.com/stuyy/Katana/blob/master/src/client/Client.ts#L15

[August Lilleaas]

I don’t think so, there’s no structural typing in Kotlin afaik

[Yusuf.I]

would there be a way to do anything similar

[Yusuf.I]

like

client.on<ReadyEvent> { c -> {
}

[Yusuf.I]

@August Lilleaas

[August Lilleaas]

not sure exactly what you’re asking, but if I understand you correctly, then yes, it’s possible to specify the types for lambda arguments 🙂

[Yusuf.I]

so i am trying to create an event listener system where once i want to use client.on to recieve the event

[August Lilleaas]

you could create an inline extension function with a reified type for the event. That way, you can use the type of the event to run the code you want

[August Lilleaas]

so, instead of onFooEvent, onBarEvent, etc, you can use a generic type and call on<FooEvent>, on<BarEvent>, etc

[Yusuf.I]

ah i see

[Yusuf.I]

thank you for the help

[August Lilleaas]

(it doesn’t have to be an extension function, by the way)

[Yusuf.I]

can you give me a ruff example please

[August Lilleaas]

what are you trying to do, and what errors are you getting?

[August Lilleaas]

ah, I see the problem now. The type of the event needs to define the type of the lambda

[Yusuf.I]

yeah

[Yusuf.I]

inline fun <reified T : Event> EvenTester.on(
    crossinline consumer: suspend EventListener.(T) -> Unit
) {

}

[Yusuf.I]

does this look right?

[August Lilleaas]

you can use typealiases for the events themselves. After all, the only thing the type needs to indicate, is the type of the lambda handlers

[Yusuf.I]

how do typealiases work

[August Lilleaas]

actually, that won’t work, as typealiases are subject to type erasure even for reified types

[Yusuf.I]

oh ok

[August Lilleaas]

I’m not an expert, but it seems to me that the only way to do this in Kotlin is to have a function for each event type

[Yusuf.I]

ok

[Yusuf.I]

will try that out and let you know if it works