Android LifecycleProperty delegate

LifecycleProperty.kt

/**
 * A property that automatically clears its reference according to the Android lifecycle.
 *
 * When the value for this property is set, it will be scheduled to be cleared in the opposing
 * lifecycle event. For example, a property whose value is set during `onCreate()` will be cleared
 * during `onDestroy()`.
 */
class LifecycleProperty<T> : LifecycleObserver {
    companion object {
        private val opposingEvents = mapOf(
                Lifecycle.State.INITIALIZED to Lifecycle.Event.ON_DESTROY,
                Lifecycle.State.CREATED to Lifecycle.Event.ON_DESTROY,
                Lifecycle.State.STARTED to Lifecycle.Event.ON_STOP,
                Lifecycle.State.RESUMED to Lifecycle.Event.ON_PAUSE
        )
    }

    private var value: T? = null
    private var destroyEvent: Lifecycle.Event? = null

    operator fun getValue(thisRef: LifecycleOwner, property: KProperty<*>): T
        = value ?: throw LifecyclePropertyDestroyedException()

    operator fun setValue(thisRef: LifecycleOwner, property: KProperty<*>, value: T) {
        this.value = value
        destroyEvent = opposingEvents[thisRef.lifecycle.currentState]
                ?: throw IllegalStateException("Value set outside of the Android lifecycle")
        thisRef.lifecycle.addObserver(this)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
    fun onEvent(owner: LifecycleOwner, event: Lifecycle.Event) {
        if (event == destroyEvent) {
            value = null
        }
    }

    class LifecyclePropertyDestroyedException : Exception("Property accessed outside of its valid lifecycle")
}

/**
 * Perform an action and ignore any exceptions raised because a property is
 * being accessed after its terminal lifecycle event.
 *
 * @param block the action to perform
 */
inline fun <T> T.lifecycleCheck(block: T.() -> Unit): Unit {
    try {
        block()
    } catch (e: LifecycleProperty.LifecyclePropertyDestroyedException) {}
}