How to create a nullable delegate property?

6

With provideDelegate feature and unchecked cast I was able to make it as a universal solution:

import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty

class Person {
  var name: String by DaoProperty()
  var address: String? by DaoProperty()
  var age: Int by DaoProperty()
  var height: Int? by DaoProperty()
}

class DaoProperty<T> {
  operator fun provideDelegate(thisRef: Any, property: KProperty<*>): ReadWriteProperty<Any, T> {
    val result: Any = if (property.returnType.isMarkedNullable) NullableDaoProperty<T>() else NotNullDaoProperty<T>()
    @Suppress("UNCHECKED_CAST")
    return result as ReadWriteProperty<Any, T>
  }
}

private class NotNullDaoProperty<T> : ReadWriteProperty<Any, T> {
  private var value: T? = null

  override fun getValue(thisRef: Any, property: KProperty<*>): T {
    if (value == null) throw UninitializedPropertyAccessException(property.name)
    return value!!
  }

  override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
    this.value = value
  }
}

private class NullableDaoProperty<T> : ReadWriteProperty<Any, T?> {
  private var value: T? = null
  private var initialized = false

  override fun getValue(thisRef: Any, property: KProperty<*>): T? {
    if (!initialized) throw UninitializedPropertyAccessException(property.name)
    return value
  }

  override fun setValue(thisRef: Any, property: KProperty<*>, value: T?) {
    this.value = value
    this.initialized = true
  }
}