Solution: Mutable lazy delegate
fun <T> mutableLazy(
initializer: () -> T
): ReadWriteProperty<Any?, T> = MutableLazy(initializer)
private class MutableLazy<T>(
private var initializer: (() -> Any?)? = null,
): ReadWriteProperty<Any?, T> {
private var value: Any? = null
override fun getValue(
thisRef: Any?,
property: KProperty<*>
): T {
if (initializer != null) {
value = initializer?.invoke()
initializer = null
}
return value as T
}
override fun setValue(
thisRef: Any?,
property: KProperty<*>,
value: T
) {
this.value = value
this.initializer = null
}
}
This solution lacks thread safety. The simplest way to make it thread safe is to use a
synchronizedblock. A more efficient solution is to use theAtomicReferenceandcompareAndSetfunctions.
Example solution in playground
import org.junit.Assert
import org.junit.Test
import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty
import kotlin.system.measureTimeMillis
fun <T> mutableLazy(
initializer: () -> T
): ReadWriteProperty<Any?, T> = MutableLazy(initializer)
private class MutableLazy<T>(
private var initializer: (() -> Any?)? = null,
): ReadWriteProperty<Any?, T> {
private var value: Any? = null
override fun getValue(
thisRef: Any?,
property: KProperty<*>
): T {
if (initializer != null) {
value = initializer?.invoke()
initializer = null
}
return value as T
}
override fun setValue(
thisRef: Any?,
property: KProperty<*>,
value: T
) {
this.value = value
this.initializer = null
}
}
class MutableLazyTest {
@Test
fun `Do not initialize if initialized`() {
val time = measureTimeMillis {
var game: Game? by mutableLazy { readGameFromSave() }
game = Game()
print(game)
}
assert(time in 0..100)
}
@Test
fun `Initializes if not initialized`() {
val time = measureTimeMillis {
val game: Game? by mutableLazy { readGameFromSave() }
print(game)
}
assert(time in 450..550)
}
@Test
fun `Do not initialize again if already initialized`() {
val time = measureTimeMillis {
val game: Game? by mutableLazy { readGameFromSave() }
print(game)
print(game)
print(game)
}
assert(time in 450..550)
}
@Test
fun `MutableLazy should accept nullable values`() {
val lazy by mutableLazy<String?> { null }
Assert.assertNull(lazy)
var lazy2 by mutableLazy<String?> { "A" }
lazy2 = null
Assert.assertNull(lazy2)
}
private class Game()
private fun readGameFromSave(): Game? {
Thread.sleep(500)
return Game()
}
}
