An API question - guard based on event parameter

[sorokod]

I have a MyDataEvent that is a DataEvent<Int>. A transition should occur only if data is non negative (for example).

It looks like a guard thing but I don't see a way of accessing the event data from a guard.

transition<MyDataEvent> {
    guard = { event.data >= 0 } // ????
    targetState = SomeState
}

Am I missing something? Is there a better approach?

[nsk90]

Hello! Check out this sample https://github.com/nsk90/kstatemachine/blob/master/samples/src/commonMain/kotlin/ru/nsk/samples/TypesafeTransitionSample.kt

It looks very close to your code, but if you use DataState/DataEvent api, I assume that you should use dataTransition instead of simple transition function.

[sorokod]

Ok, interestingly in this case the targetState must be a DataState. Something like:

dataTransition<MyDataEvent, Int>  {
    guard = { event.data >=0 }
    targetState = MyDataState
}

There may be use cases where this is not necessary.

[nsk90]

Yes, DataEvent is designed to be used in a couple with DataState. That is the way how I can guarantee at compile time that state receives expected data. If you don't need compile time guarantee or this API does not completely fit your use case, you can subclass simple Event, not DataEvent.

like this:

data class MyEvent(val data: Int) : Event

This block should work in this case guard = { event.data >= 0 }

[sorokod]

Nice, that is what I was looking for.

[nsk90]

.