Memoized based on Spek implementation

memoized.kt

import kotlin.properties.ReadOnlyProperty
import kotlin.reflect.KProperty

interface Memoized {
  fun <T> memoized(factory: () -> T): Memoizable<T>
}

class DefaultMemoized : Memoized {
  override fun <T> memoized(factory: () -> T): Memoizable<T> {
    return MemoizableAdapter(factory)
  }
}

interface Memoizable<out T> : ReadOnlyProperty<Any?, T> {
  operator fun invoke(): T
}

class MemoizableAdapter<T>(val factory: () -> T) : Memoizable<T> {

  var cached: T? = null

  override fun getValue(thisRef: Any?, property: KProperty<*>): T = invoke()
  override fun invoke(): T {
    if (cached == null) {
      cached = factory()
    }
    return cached!!
  }
}