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.