Kotlin Lifecycle-aware fields

LifecycleDelegate.kt

import android.arch.lifecycle.*
import android.support.v4.app.ActivityCompat
import android.support.v4.app.Fragment
import android.support.v7.app.AppCompatActivity
import kotlin.reflect.KProperty

fun <T> Fragment.selfClearing(valInitializer: (() -> T)? = null) = LifecycleDelegate(this.lifecycle, valInitializer)

fun <T> AppCompatActivity.selfClearing(valInitializer: (() -> T)? = null) = LifecycleDelegate(this.lifecycle, valInitializer)

class LifecycleDelegate<T>(
        lifecycle: Lifecycle,
        private val initializer: (() -> T)? = null
): LifecycleObserver {

    private var value: Any = EMPTY
    private var initializerInvoked = false
    private var withinLifecycle = false
    private object EMPTY

    init {
        lifecycle.addObserver(this)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun onResume() {
        withinLifecycle = true
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun onPause() {
        withinLifecycle = false
    }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T? {
        if (!withinLifecycle) return null
            
        if (value == EMPTY) {
            if (!initializerInvoked && initializer != null) {
                value = initializer.invoke()!!
                initializerInvoked = true

                @Suppress("UNCHECKED_CAST")
                return value as T
            }
        }

        @Suppress("UNCHECKED_CAST")
        return value as T
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T?) {
        if (withinLifecycle && value != null) {
            this.value = value
        }
    }
}

TestActivity.kt

class TestActivity: AppCompatActivity() {
    var testString: String? by selfClearing()
    
    override fun onCreate() {
        super.onCreate()
        testString = "Test"
    }
}