为什么将 MutableLiveData 转换为 LiveData 时会出现“无用转换”？

Question

I have an array of feedback channels because (outside of question scope) in my ViewModel .

Now, I don't want to expose my MutableLiveData to outside my Viewmodel .

So, i make a private list of LiveData objects, but compiler complains of "Useless Cast"

private val _feedbackChannels = Array(10) { MutableLiveData<FeedbackEvent>() }
val feedbackChannels
    get() = _feedbackChannels.map{
        @Suppress("USELESS_CAST") // it is not useless as it no longer exposes the mutableLiveData
        it as LiveData<*>
    }

Why do I get USELESS_CAST warning?

Answer 1

Compiler doesn't realize you're doing it only to force implication of property type.

Just specify type explicitly and you'll be able to drop the cast entirely. You won't even have to use map, a simple toList() will do:

private val _feedbackChannels = Array(10) { MutableLiveData<FeedbackEvent>() }
val feedbackChannels : List<LiveData<FeedbackEvent>>
    get() = _feedbackChannels.toList()

Answer 2

Clearly the compiler doesn't understand the point of the cast. In order to do this in a more explicit way and remove the costly map function, you can just upcast it like this:

private val _feedbackChannels = Array(10) { MutableLiveData<FeedbackEvent>() }
val feedbackChannels: Array<out LiveData<FeedbackEvent>>
    get() = _feedbackChannels

Edit

If you wanted to expose a List specifically (avoid exposing a mutable array) then you should probably just create one in the first place:

private val _feedbackChannels = List(10) { MutableLiveData<FeedbackEvent>() }
val feedbackChannels: List<out LiveData<FeedbackEvent>>
    get() = _feedbackChannels