Why is the getValue of LiveData<Long> Nullable? - kotlin

Question

The Code A is based https://github.com/android/architecture-components-samples/blob/master/LiveDataSample/app/src/main/java/com/android/example/livedatabuilder/LiveDataViewModel.kt
I modified some code.
I think that emit(timeStampToTime(currentTime.value)) will be correct, but the system report error, so I have to use emit(timeStampToTime(currentTime.value?:0))
1: I'm very strange that why val currentTime isn't Nullable and currentTime.value is Nullable .
2: Why does the original code use liveData { emit(timeStampToTime(it)) } instead of liveData { emit(timeStampToTime(it?:0)) }
Code A
class LiveDataViewModel(
private val dataSource: DataSource
) : ViewModel() {
val currentTime = dataSource.getCurrentTime()
val currentTimeTransformed: LiveData<String> = liveData {
//emit(timeStampToTime(currentTime.value)) // It will cause error
emit(timeStampToTime(currentTime.value?:0))
}
//Original Code
// Coroutines inside a transformation
// val currentTimeTransformed = currentTime.switchMap {
// timeStampToTime is a suspend function so we need to call it from a coroutine.
// liveData { emit(timeStampToTime(it)) }
// }
private suspend fun timeStampToTime(timestamp: Long): String {
delay(500) // Simulate long operation
val date = Date(timestamp)
return date.toString()
}
}
class DefaultDataSource(private val ioDispatcher: CoroutineDispatcher) : DataSource {
override fun getCurrentTime(): LiveData<Long> =
liveData {
while (true) {
emit(System.currentTimeMillis())
delay(1000)
}
}
...
}

Answer 1

It's nullable because there is a constructor
LiveData()
Creates a LiveData with no value assigned to it.
and calling e.g.
val data: LiveData<Long> = LiveData()
data.value
will return null.
Or to consider your code:
liveData {
while (true) {
emit(System.currentTimeMillis())
delay(1000)
}
}
You emit a value immediately, but if you change the order
liveData {
while (true) {
delay(1000)
emit(System.currentTimeMillis())
}
}
then there is no value emitted during the first second and calling currentTime.value will return null.
(Note: is it intentional that you create a new LiveData on every currentTime access?)