Notify MutableLiveData when another MutableLiveData value changes

In my ViewModel I have the code shown below:

private val _sender = MutableLiveData<DialogListItem.ParticipantItem>()
val sender: LiveData<DialogListItem.ParticipantItem>
    get() = _sender

private val _receiverListItems: MutableLiveData<List<DialogListItem>> =
    CombinedLiveData<List<ParticipantTypeA>, List<ParticipantTypeB>, List<DialogListItem>>(
        _participantsA, _participantsB
    ) { participantsA, participantsB ->
        val sender = _sender.value

        ...
        ...
        
        return@CombinedLiveData ...
    }
val receiverListItems: LiveData<List<DialogListItem>>
    get() = _receiverListItems

When a new value is posted by doing _sender.postValue(selectedSender), _receiverListItems should be notified of that change. However, when I log the value of the _sender, I get null. How can I get _receiverListItems value dependent on _sender value?

Since you are using a custom implementation of LiveData, this answer might be limited in what it says due to that but in general, you want Transformation.switchMap.

private val _receiverListItems: LiveData<List<DialogListItem>> =
    Transformation.switchMap(sender).map { sender -> 
        CombinedLiveData<
            List<ParticipantTypeA>, List<ParticipantTypeB>, List<DialogListItem>
        >(
            _participantsA, 
            _participantsB
        ) { participantsA, participantsB ->
           ...
           ...        
        return@CombinedLiveData ...
    }
}