I was reading someone's code for some thing and fo...

# announcements

[otakusenpai]

I was reading someone's code for some thing and found this strange class definition

class Usage(from: List<UsageArgument> = listOf(), val runsAt: List<RunAt> = listOf()) : List<UsageArgument> by from {
    override fun toString(): String {
        val result = joinToString(" ") { it.format() }

        return if (result.isBlank()) "none" else result
    }
}

can anyone explain this to me?
especially the parameters of the class def and the ending "by from". what does it mean?
the use case is like a list or something i suppose?

[Chills]

strange?

[Foso]

"by" is a keyword for Delegation https://kotlinlang.org/docs/reference/delegation.html

[Chills]

delegation is a hard topic

[Chills]

to wrap your head around it , look at delegation pattern before hand.

[Ray Eldath]

off-topic: i remembered KotlinConf 2018 Closing Panel someone pointed delegation as their most hateful feature. and in ktconf 2019 Andrej says delegation is what he want to redesign most, IIRC.