How to create a non-nullable LiveData that can save state

Question

When we have a liveData as below, we cannot _liveData.value++ , as the value is nullable.

class MainViewModel(savedStateHandle: SavedStateHandle): ViewModel() {
    private val _liveData: MutableLiveData<Int> =
        savedStateHandle.getLiveData("SomeKey", 0)

    val liveData: MutableLiveData<Int> = _liveData

    fun triggerLiveData() {
        _liveData.value++
    }
}

The article https://proandroiddev.com/improving-livedata-nullability-in-kotlin-45751a2bafb7 provide a solution, ie

@Suppress("UNCHECKED_CAST")
class SafeMutableLiveData<T>(value: T) : LiveData<T>(value) {

  override fun getValue(): T = super.getValue() as T
  public override fun setValue(value: T) = super.setValue(value)
  public override fun postValue(value: T) = super.postValue(value)
}

But that didn't support savedState.

How can we get a non-nullable LiveData that also has savedstate?

Answer 1

I have a solution that duplicate the data and don't look that elegant

@Suppress("UNCHECKED_CAST")
class SafeMutableLiveData<T: Any>(private val mutableLiveData: MutableLiveData<T>) :
    MutableLiveData<T>(mutableLiveData.value) {

    override fun getValue(): T = mutableLiveData.value as T
    override fun setValue(value: T) {
        super.setValue(value)
        mutableLiveData.value = value
    }
    override fun postValue(value: T) {
        super.postValue(value)
        mutableLiveData.postValue(value)
    }
}

The usage is as below

class MainViewModel(savedStateHandle: SavedStateHandle): ViewModel() {
    private val _liveData: SafeMutableLiveData<Int> =
        SafeMutableLiveData(savedStateHandle.getLiveData("Something", 0))

    val liveData: SafeMutableLiveData<Int> = _liveData

    fun triggerLiveData() {
        _liveData.value++
    }
}