How to simply add another source to MediatorLiveData in kotlin? - kotlin

Question

I want to combine multiple data sources in a MediatorLiveData. Unfortunately, there are not many examples yet. So in my ViewModel I have
//all lists have been declared before
val playerList = MediatorLiveData<List<Player>>()
init {
playerList.addSource(footballPlayerList) { value ->
playerList.value = value
}
playerList.addSource(basketballPlayerList) { value ->
playerList.value = value
}
}
But apparently this will always override the current value of playerList. I mean I could build some hacky workarounds with helper variables like _playerList but maybe there is an easier solution?

Answer 1

Having done quite some research.. I found it out. Here is an example
fun blogpostBoilerplateExample(newUser: String): LiveData<UserDataResult> {
val liveData1 = userOnlineDataSource.getOnlineTime(newUser)
val liveData2 = userCheckinsDataSource.getCheckins(newUser)
val result = MediatorLiveData<UserDataResult>()
result.addSource(liveData1) { value ->
result.value = combineLatestData(liveData1, liveData2)
}
result.addSource(liveData2) { value ->
result.value = combineLatestData(liveData1, liveData2)
}
return result
}
The actual combination of data is done in a separate combineLatestData method like so
private fun combineLatestData(
onlineTimeResult: LiveData<Long>,
checkinsResult: LiveData<CheckinsResult>
): UserDataResult {
val onlineTime = onlineTimeResult.value
val checkins = checkinsResult.value
// Don't send a success until we have both results
if (onlineTime == null || checkins == null) {
return UserDataLoading()
}
// TODO: Check for errors and return UserDataError if any.
return UserDataSuccess(timeOnline = onlineTime, checkins = checkins)
}

Answer 2

Here is a simple example
class MergeMultipleLiveData : ViewModel() {
private val fictionMenu: MutableLiveData<Resource<RssMenu>> = MutableLiveData()
private val nonFictionMenu: MutableLiveData<Resource<RssMenu>> = MutableLiveData()
val allCategoryMenus: MediatorLiveData<Resource<RssMenu>> = MediatorLiveData()
init {
getFictionMenus()
getNonFictionMenus()
getAllCategoryMenus()
}
private fun getAllCategoryMenus() = viewModelScope.launch(Dispatchers.IO) {
allCategoryMenus.addSource(fictionMenu) { value ->
allCategoryMenus.value = value
}
allCategoryMenus.addSource(nonFictionMenu) { value ->
allCategoryMenus.value = value
}
}
private fun getFictionMenus() = viewModelScope.launch(Dispatchers.IO) {
fictionMenu.postValue( // todo )
}
private fun getNonFictionMenus() = viewModelScope.launch(Dispatchers.IO) {
nonFictionMenu.postValue( // todo )
}
}
And in your fragment you can observer as;
viewModel.allCategoryMenus.observe(viewLifecycleOwner) {
// todo
}