Hello guys, I don't know how to use the Koltin delegation, can anyone help me understand it and what...

# android

[vimlesh yadav]

Hello guys, I don't know how to use the Koltin delegation, can anyone help me understand it and what are the use cases for it?

[CLOVIS]

https://kotlinlang.org/docs/delegation.html and https://kotlinlang.org/docs/delegated-properties.html

[CLOVIS]

If you have specific questions, ask in #getting-started

[vimlesh yadav]

Ok Thanks

[John Matthews]

This explanation for me helped better. I just wish they would tell me why use this over Inheritance≥. I always struggle with the why part in Kotlin. It seems they like to invent things but the why and when I would use this is a mystery over proven patterns.

[CLOVIS]

@John Matthews let's imagine you have :

interface Game {
    val name: String

    /** `true` if [move] is legal. */
    fun canPlay(move: Move): Boolean

    fun play(move: Move)

    // some other methods…
}

class Game1 : Game {
    override val name get() = "Game 1"
    …
}

class Game2 : Game {
    override val name get() = "Game 2"
    …
}

What if you wanted to create variants of games where you can only play 3 moves? With regular inheritance you'd have to create a new class for each Game subclass! With composition (implemented using delegation):

// Takes a Game implementation as parameter, and adds the maximum move count rule to it
class LimitedGame(private val game: Game, private val maxCount: Int) : Game by game {
    private var currentCount = 0

    override val canPlay(move: Move): Boolean {
        if (currentCount >= maxCount)
            return false
        currentCount++
        return game.canPlay(move)
    }

    // no need to implement the other methods, they are all delegated to the original game instance
}

[CLOVIS]

over proven patterns

Composition is a proven pattern. For example, it's item 16 in Effective Java, which doesn't even have composition at the language feature. It's also mentioned in the original Design Patterns book from 1994

[CLOVIS]

(btw I recommend reading that item of Effective Java, it gives other examples of when to use it and describes in more details why it's useful)

[CLOVIS]

See also: https://en.wikipedia.org/wiki/Composition_over_inheritance

[John Matthews]

I forgot about the link I read. But thanks this makes it better to understand the when and why. https://en.wikipedia.org/wiki/Delegation_pattern