Lightweight events in Kotlin

In one of my applications I needed some minimalistic event system and all I can found was require creating a ton of classes or use a lot of annotations, or was magick-scala-like with overloaded -=
, +=
and ()
just because author can do it.

So I decided to write something very simple, with interface like:

ClickEvent on {
    println("CLICK: ${it.x} ${it.y}")
}

ClickEvent(10, 20).emit()

And it was implemented easily:

open class Event<T> {
    var handlers = listOf<(T) -> Unit>()

    infix fun on(handler: (T) -> Unit) {
        handlers += handler
    }

    fun emit(event: T) {
        for (subscriber in handlers) {
            subscriber(event)
        }
    }
}

data class ClickEvent(val x: Int, val y: Int) {
    companion object : Event<ClickEvent>()

    fun emit() = Companion.emit(this)
}

ClickEvent on {
    println("CLICK: ${it.x} ${it.y}")
}

ClickEvent(10, 20).emit()

>>> CLICK: 10 20

You can notice a bit of boilerplate, main reason to have it is because in Kotlin we don’t have static methods, we can’t use inheritance with data classes and also we can’t use companion object of interface (or even parent class) in class that implements it. So in each event we have to duplicate some redundant code:

data class MyEvent(val data: String) {
    companion object : Event<MyEvent>()

    fun emit() = Companion.emit(this)
}

Back to positive side, let’s implement a few other events and try it in action. Start from event without arguments, it can’t be a data class:

class ConnectionLostEvent {
    companion object : Event<ConnectionLostEvent>()

    fun emit() = Companion.emit(this)
}

ConnectionLostEvent on {
    println("Oh, snap!")
}

ConnectionLostEvent().emit()

>>> Oh, snap!

What if we want events hierarchy, we can use sealed class for event type and few classes for events. They also can’t be data classes:

sealed class MouseEvent {
    companion object : Event<MouseEvent>()

    class Click(val x: Int, val y: Int) : MouseEvent() {
        fun emit() = Companion.emit(this)
    }

    class Move(val fromX: Int, val fromY: Int, val toX: Int, val toY: Int) : MouseEvent() {
        fun emit() = Companion.emit(this)
    }
}

MouseEvent on {
    when (it) {
        is MouseEvent.Click -> println("Click x:${it.x} y:${it.y}")
        is MouseEvent.Move -> println(
                "Move from ${it.fromX}:${it.fromY} to ${it.toX}:${it.toY}")
    }
}

MouseEvent.Click(50, 40).emit()
MouseEvent.Move(0, 0, 9, 12).emit()

>>> Click x:50 y:40
>>> Move from 0:0 to 9:12

As you can see it isn’t that nice that it can be implemented with Scala or Clojure. Kotlin isn’t rich language, but it’s simple and works pretty well on Android.

Also you notice that code highlighting in the article looks wrong, it’s because rouge, highlighting engine used by github pages, doesn’t support Kotlin right now
, and I just used one for Scala.

nvbn blog稿源:nvbn blog (源链) | 关于 | 阅读提示

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