Kotlin: lateinit to val, or, alternatively, a var that can set once

In this solution you implement a custom delegate and it becomes a separate property on your class. The delegate has a var inside, but the controlObj property has the guarantees you want.

class X {
    private val initOnce = InitOnce<View>()
    private val controlObj: View by initOnce

    fun readWithoutInit() {
        println(controlObj)
    }

    fun readWithInit() {
        initOnce.initWith(createView())
        println(controlObj)
    }

    fun doubleInit() {
        initOnce.initWith(createView())
        initOnce.initWith(createView())
        println(controlObj)
    }
}

fun createView(): View = TODO()

class InitOnce<T : Any> {

    private var value: T? = null

    fun initWith(value: T) {
        if (this.value != null) {
            throw IllegalStateException("Already initialized")
        }
        this.value = value
    }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T =
            value ?: throw IllegalStateException("Not initialized")
}

BTW if you need thread safety, the solution is just slightly different:

class InitOnceThreadSafe<T : Any> {

    private val viewRef = AtomicReference<T>()

    fun initWith(value: T) {
        if (!viewRef.compareAndSet(null, value)) {
            throw IllegalStateException("Already initialized")
        }
    }

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T =
            viewRef.get() ?: throw IllegalStateException("Not initialized")
}

You can implement own delegate like this:

class InitOnceProperty<T> : ReadWriteProperty<Any, T> {

    private object EMPTY

    private var value: Any? = EMPTY

    override fun getValue(thisRef: Any, property: KProperty<*>): T {
        if (value == EMPTY) {
            throw IllegalStateException("Value isn't initialized")
        } else {
            return value as T
        }
    }

    override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
        if (this.value != EMPTY) {
            throw IllegalStateException("Value is initialized")
        }
        this.value = value
    }
}

After that you can use it as following:

inline fun <reified T> initOnce(): ReadWriteProperty<Any, T> = InitOnceProperty()

class Test {

     var property: String by initOnce()

     fun readValueFailure() {
         val data = property //Value isn't initialized, exception is thrown
     }

     fun writeValueTwice() {
         property = "Test1" 
         property = "Test2" //Exception is thrown, value already initalized
     }

     fun readWriteCorrect() {
         property = "Test" 
         val data1 = property
         val data2 = property //Exception isn't thrown, everything is correct
     }

}

In case when you try to access value before it is initialized you will get exception as well as when you try to reassign new value.