```interface Foo<T> { fun getValue(): T ...

# getting-started

[Mark]

interface Foo<T> {
    fun getValue(): T
    fun removeValue()
    fun setValue(value: T) // is there a way to only allow non-null values?
}

interface NonNullFoo<T: Any>: Foo<T>
interface NullableFoo<T: Any>: Foo<T?>

[Sam]

Definitely-non-nullable types! K

fun setValue(value: T & Any)

[Mark]

That’s really good to know thanks, but how can I then define a delegated property?

val <T> Foo<T>.valueProperty: ReadWriteProperty<Any, T>
    get() = object : ReadWriteProperty<Any, T> {
        override fun getValue(thisRef: Any, property: KProperty<*>): T {
            return getValue()
        }

        override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
            setValue(value) // compile error: require T & Any
        }
    }

[Mark]

Oh it looks like you can just do a runtime check for value == null

val <T> Foo<T>.valueProperty: ReadWriteProperty<Any, T>
    get() = object : ReadWriteProperty<Any, T> {
        override fun getValue(thisRef: Any, property: KProperty<*>): T {
            return getValue()
        }

        override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
            if (value == null) {
                removeValue()
            } else {
                setValue(value)
            }
        }
    }