MutableLiveData value not updating confusion?

Question

Working on a project in Kotlin in AndroidStudio, fairly basic stuff, but MutableLiveData is not working like I expect it to. Am I doing something wrong or maybe I'm confused on how it works?

I'm using a viewModel to handle the LiveData and for testing purposes rn am just running some code in a fragment's onCreate method:

ViewModel

class UserViewModel : ViewModel() {

    var currentUser: User = DataSource.user

    private var _selectedSkillPoints = MutableLiveData<Int>(currentUser.selectedSkill.points) 
    val selectedSkillPoints: LiveData<Int> = _selectedSkillPoints
    ...
}

Fragment

override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)

     private var user: User = DataSource.user

     user.selectedSkill.points++

     MaterialAlertDialogBuilder(requireContext())
          .setMessage("user.selectedSkill.points = ${user.selectedSkill.points} \n sharedViewModel.selectedSkillPoints.value = ${sharedViewModel.selectedSkillPoints.value} ")
          .show()

}

I would expect that updating currentUser.points also updates the LiveData pointing to it. but the Alert shows that only currentUser.points is updated while userPoints stays the same at the initial value so an observer will never run. So what am I missing? How can I get this to work like I was expecting? Thanks

MutableLiveData cannot possibly know that some other property pointing at the same thing that it holds a reference to has changed. You must manually update MutableLiveData every time you want it to publish a new value. — Tenfour04, Dec 26 '21 at 22:10
I feel like having to manually update defeats the purpose of it being live... well guess I am misunderstanding a bit, but thanks for clearing this up — Rayge, Dec 26 '21 at 22:33
From the Fragment/Activity’s point of view, it is being updated automatically and can be subscribed to. That’s the point of LiveData. — Tenfour04, Dec 26 '21 at 23:15
**I would expect that updating currentUser.points also updates the LiveData pointing to it.** - Except the live data is not "pointing to it" - it's a LiveData of Ints. Ints are just values that get copied. There is no "pointing to" anything. — dominicoder, Dec 27 '21 at 00:14

Dissident Dev · Answer 1 · 2021-12-27T04:21:52.510

The only relationship between _selectedSkillPoints MutableLiveData object in the ViewModel and currentUser (referencing DataSource.user) is that the _selectedSkillPoints MutableLiveData object was only initialised with the value of currentUser.selectedSkill.points. This is just an initial value for the MutableLiveData object, but MutableLiveData is not observing currentUser nor Datasource.user -MutableLiveData does not observe whatever object you pass as initial value to its public constructor

The value observed is MutableLiveData.value (in your case sharedViewModel.selectedSkillPoints.value). So if you want to .observe() the MutableLiveData object and trigger a callback, you have to set the value of sharedViewModel.selectedSkillPoints.value with one of the following:

- sharedViewModel.selectedSkillPoints.value = newValue     //equivalent to setValue()
- sharedViewModel.selectedSkillPoints.setValue(newValue)   //from main thread
- sharedViewModel.selectedSkillPoints.postValue(newValue)  //from background thread

The problem arises because you are incrementing the points value in the Fragment; it would make more sense to do the increment in the ViewModel as part of an event called from the Fragment, and observing the value in the Fragment for a callback to update the displayed state. Something like:

ViewModel

class UserViewModel : ViewModel() {

    var currentUser: User = DataSource.user

    private var _selectedSkillPoints = MutableLiveData<Int>(currentUser.selectedSkill.points) 
    val selectedSkillPoints: LiveData<Int> = _selectedSkillPoints

    fun incrementSelectedSkillPoints {
        _selectedSkillPoints.value = _selectedSkillPoints.value + 1
    }
    ...
}

Fragment

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    sharedViewModel.selectedSkillPoints.observe(viewLifecycleOwner) { points ->
        MaterialAlertDialogBuilder(requireContext())
            .setMessage("sharedViewModel.selectedSkillPoints.value = $points")
            .show()
    }

MutableLiveData value not updating confusion?

1 Answers1