// Allow to set null value to LiveData
val liveData = MutableLiveData<Boolean>()
liveData.value = null

// LiveDataKtx doesn't allow to null value
val liveData = MutableLiveDataKtx<Boolean>()
liveData.value = null // doesn't allow

// Set nullable in LiveDataKtx
val liveData = MutableLiveDataKtx<Boolean?>()
liveData.value = null

val liveData = LiveData<Boolean>()
val liveDataKtx = liveData.toKtx()
val nullableLiveDataKtx = liveData.toNullableKtx()
val anotherNullableLiveDataKtx = LiveDataKtx<Boolean?>()

val mutableLiveData = MutableLiveData<Boolean>()
val mutableLiveDataKtx = mutableLiveData.toKtx()
val nullableMutableLiveDataKtx = mutableLiveData.toNullableKtx()
val anotherNullableMutableLiveDataKtx = MutableLiveDataKtx<Boolean?>()

val mediatorLiveData = MediatorLiveData<Boolean>()
val mediatorLiveDataKtx = mediatorLiveData.toKtx()
val nullableMediatorLiveDataKtx = mediatorLiveData.toNullableKtx()
val anotherNullableMediatorLiveDataKtx = MediatorLiveDataKtx<Boolean?>()

val liveData = MutableLiveDataKtx<Boolean>()
liveData
  .filter { it == false }
  .map { true }
  .observe(lifecycleOwner, Observer { result ->
    // result is non-null and always true
  })

val liveData = PublishLiveDataKtx<Int>()
val actual = mutableListOf<Int>()
val observer = Observer<Int> { actual.add(it) }
liveData.value = -1

liveData.observeForever(observer)
liveData.value = 0
liveData.removeObserver(observer)

assertEquals(listOf(0), actual)
actual.clear()

liveData.value = 1
liveData.value = 2
liveData.observeForever(observer)
liveData.value = 3

assertEquals(listOf(3), actual)

val liveDataKtx = MutableLiveDataKtx<Boolean>()

// Throw NPE if the value hasn't been set yet as it is defined as nonNull <Boolean>
liveDataKtx.value

// This works fine as the value has been set
liveDataKtx.value = true
liveDataKtx.value

// safeValue always returns nullable value and does not throw NPE
liveDataKtx.safeValue

// observe doesn't throw NPE even when the value hasn't been set
liveDataKtx.observe(...)

/**
 * filter
 */
private class FilterOperator<T>(val predicate: (T) -> Boolean) : Operator<T, T> {

    override fun run(output: MediatorLiveDataKtx<T>, value: T) {
        if (predicate.invoke(value)) {
            output.value = value
        }
    }
}

fun <T> LiveDataKtx<T>.filter(predicate: (T) -> Boolean): LiveDataKtx<T> =
    Extension.create(this, FilterOperator(predicate))

fun <T> MutableLiveDataKtx<T>.filter(predicate: (T) -> Boolean): MutableLiveDataKtx<T> =
    Extension.create(this, FilterOperator(predicate))

fun <T> MediatorLiveDataKtx<T>.filter(predicate: (T) -> Boolean): MediatorLiveDataKtx<T> =
    Extension.create(this, FilterOperator(predicate))