[Kotlin/Native] CallbackUtils.kt

CallbackUtils.kt

import kotlinx.cinterop.ObjCAction
import kotlinx.cinterop.StableRef
import platform.AppKit.NSControl
import platform.Foundation.NSSelectorFromString
import platform.darwin.NSObject
import platform.objc.OBJC_ASSOCIATION_RETAIN
import platform.objc.objc_setAssociatedObject

/**
 * Specify a [callback][onAction] for an [NSControl] action, instead of setting
 * [NSControl.target] and [NSControl.action] with a delegate.
 */
fun NSControl.action(
    onAction: () -> Unit
) =
    ActionCallback(onAction).bindTo(this)

private class ActionCallback(
    private val onAction: () -> Unit
) : NSObject() {

    @ObjCAction
    private fun invokeAction() = onAction()

    fun bindTo(view: NSControl) {
        view.target = this
        view.action = NSSelectorFromString(::invokeAction.name)

        /** attach to the view lifecycle, since [NSControl.target] is a weak ref! */
        objc_setAssociatedObject(
            `object` = view,
            key = associatedObjectKey,
            value = this@ActionCallback,
            policy = OBJC_ASSOCIATION_RETAIN
        )
    }

    private companion object {
        private val associatedObjectKey = StableRef.create(Any()).asCPointer()
    }
}