MutableStateFlow not working with MutableList

Question

Here is my MutableStateFlow value I try to work with:

val songList: MutableStateFlow<MutableList<Song>> = MutableStateFlow(arrayListOf())

I need to observe changes (after methods like add, removeAt etc.) on my MutableList above but can't achieve that. My guess is since only the elements of the list change instead of the list itself, collect method doesn't get fired.

How can I achieve that using StateFlow if possible? If not, what is the correct way to do it?

Note that I need initial value of arrayListOf(), so LiveData probably won't be enough for me.

Robichaud

Answer 1

• Helpful reading: Notify LiveData observers when nested properties change

• What is the issue:

  • StateFlow and LiveData won't get notified if the nested field gets changed.

  • Ex 1: Update specific item of a List (not add/remove).

  • Ex 2: Update specifc property of an object.

---

Solution:

1. Create helper extension function:

   fun <T> List<T>.mapButReplace(targetItem: T, newItem: T) = map {
       if (it == targetItem) {
           newItem
       } else {
           it
       }
   }
   

2. React on action (ex: click event):

    val newStudent = student.copy(isSelected = !student.isSelected)
   
    viewModel.updateStudents(student, newStudent)
   

3. Handle in ViewModel:

    fun updateStudents(currentStudent: Student, newStudent: Student) {
        val newList = _students.value.mapButReplace(currentStudent, newStudent)
   
        _students.value = newList
    }
   

Clow

Thank you for the answer. So, it turns out for me that there is no direct way when observing list items. – Robichaud

@Robichaud Compose has fixed this problem: https://mcmap.net/q/367601/-jetpack-compose-update-composable-when-list-changes. – Clow

Answer 2

In your view model:

val songList: MutableStateFlow<MutableList<Song>> = MutableStateFlow(emptyList()) // modified

fun add(song: Song) {
    songList.update {
        songList.value.toMutableList().apply { this.add(song) }
    }
}

in a composable:

val songs = yourViewModel.songList.collectAsStateWithLifecycle()

Duodiode

It would be good to declare the type as read-only List to avoid the possibility of accidentally mutating the list instead of copying it. val songList: MutableStateFlow<List<Song>> = ... – Chewning

Answer 3

For updating the MutableList inside the Stateflow, you have to map the item based on conditions like below, which I have accomplished, and it's performing nicely.

 _userList.value = _userList.value.map {
                if (it?.userName == updatedUser.userName) updatedUser
                else it
            }

Here _userList is a MutableStateFlow @Sam Sen's explanation assisted me a lot with my workaround solution.

Roomette