Lifecycle-aware Lazy

LifecycleAwareLazy.kt

fun <T> LifecycleOwner.lifecycleAwareLazy(initializer: () -> T): Lazy<T> = LifecycleAwareLazy(this, initializer)

private object UninitializedValue

class LifecycleAwareLazy<out T>(
  private val owner: LifecycleOwner,
  initializer: () -> T
) : Lazy<T>, Serializable, LifecycleObserver {

  private var initializer: (() -> T)? = initializer

  private var _value: Any? = UninitializedValue

  @Suppress("UNCHECKED_CAST")
  override val value: T
    @MainThread
    get() {
      if (_value === UninitializedValue) {
        _value = initializer!!()
        attachToLifecycle()
      }

      return _value as T
    }

  @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
  fun resetValue() {
    _value = UninitializedValue
    detachFromLifecycle()
  }

  private fun attachToLifecycle() {
    if (getLifecycleOwner().lifecycle.currentState == Lifecycle.State.DESTROYED) {
      throw IllegalStateException("Initialization failed because lifecycle has been destroyed!")
    }
    getLifecycleOwner().lifecycle.addObserver(this)
  }

  private fun detachFromLifecycle() {
    getLifecycleOwner().lifecycle.removeObserver(this)
  }

  private fun getLifecycleOwner() = when (owner) {
    is Fragment -> owner.viewLifecycleOwner
    else -> owner
  }

  override fun isInitialized(): Boolean = _value !== UninitializedValue

  override fun toString(): String = if (isInitialized()) value.toString() else "Lazy value not initialized yet."
}