Sending Data between Fragments using Shared Viewmodel and SharedFlow

Question

I'm very new to Kotlin Flows. As the title suggests I basically have 2 Fragments that Shares a ViewModel. I want to send data between them using SharedFlow as a substitute for LiveData without retaining it's state.

Fragment A

class FragmentA: Fragment() {
    private lateinit var viewModelShared: SharedViewModel
    //Others//
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
         uper.onViewCreated(view, savedInstanceState)
         viewModelShared = ViewModelProvider(requireActivity())[SharedViewModel::class.java]
           
         someView.setOnClickListener{
             viewModelShared.sendData("Hello")}
             //Fragment Navigates From Fragment A to B using NavController
             navController.navigate(some_action_id)
         }
     }     
 }

Fragment B

class FragmentB: Fragment() {
    private lateinit var viewModelShared: SharedViewModel
    //Others//
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        viewModelShared = ViewModelProvider(requireActivity())[SharedViewModel::class.java]
               
        lifecycleScope.launchWhenCreated {
            viewModelMain.sharedFlow.collectLatest {
            //Data or the word 'Hello' sent from Fragment A not being Received Here
            }
        }
    }  
}

SharedViewModel

class SharedViewModel:ViewModel() {
    private val _sharedFlow= MutableSharedFlow<String>()
    val sharedFlow= _sharedFlow.asSharedFlow()
       
    fun sendData(data:String){
        viewModelScope.launch {
             _sharedFlow.emit(data)
        }
    }
}

score 3 · Accepted Answer · answered Dec 07 '21 at 15:37

3

Your FragmentB only collects when it is in started state or higher. This is correct because you don't want to be working with views when it's not visible and possibly doesn't currently have views.

However, since your SharedFlow has no replay history, this means there is nothing for FragmentB to collect when it is back on screen. Presumably it is off screen when FragmentA updates the flow.

So, your SharedFlow with no replay has nothing to do when it currently has no collectors, and the emitted value is thrown away. You need to have a replay of at least 1 for this to work.

private val _sharedFlow= MutableSharedFlow<String>(replay = 1)

You mentioned "without retaining its state", but this isn't possible without state if the two fragments are not on screen at the same time.

By the way, there is a simpler way to declare your shared ViewModel:

private val viewModelShared: SharedViewModel by activityViewModels()

answered Dec 07 '21 at 15:37

Tenfour04

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Thank you very much, it now works! When I said 'retaining its state', I meant having the value retained by 'n' times as long it's being observed. Sorry for the confusion about that. Kotlin Flows is really new to me, looks flexible but at a cost of the learning curve compared to LiveData haha. – Matthew Nantes Dec 07 '21 at 16:01
I got a side effect after using this approach (setting `replay` to 1), after the first emission, now I navigate to anywhere and come back, the `collect{}` gets called:https://stackoverflow.com/q/72006254/3466808. – Sam Chen Apr 26 '22 at 15:28
@SamChen, you could add `distinctUntilChanged()` to the Flow to prevent unnecessary refreshes. In some cases, I think you might be returning to a freshly recreated Fragment so it actually does need to recollect the latest value. – Tenfour04 Apr 26 '22 at 15:58

Sending Data between Fragments using Shared Viewmodel and SharedFlow

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