Using Flow to "talk" between service and fragment

Question

After more than 2 years, I am "updating" myself with android/kotlin changes, and boy, has it changed a lot.

Scenario

• I have a main activity with MyFragment and a MyFragmentViewModel
• I have a foreground service MyService
• I have a repository that has a Flow<MyState> which should be collected by both MyFragmentViewModel and MySerice Basically in the past, when I wanted to communicate between a not exported service and the main activity I've used LocalBroadCastReceiver which worked really well and removed the tight coupling between the two. Now that is deprecated so I thought why not have in the Repository a Flow that gets collected whenever it changes, so any client can react to changes.

Here is, for the sake of simplicity, some basic code related

enum class MyState{
  STATE_LOADING,
  STATE_NORMAL,
  ....
}

class MyRepository(){
   //for simplicity there is no private immutable _state for now
   val state:MutableStateFlow<MyState> = MutableStateFlow(MyState.STATE_NORMAL)
   
   fun updateState(newState: MyState){
       state.value = newState
   }

}

class MyFragmentViewModel @Inject constructor(
   private val myRepository: MyRepository
): ViewModel(){

   fun updateCurrentState(){
       myRepository.updateState(MyState.STATE_LOADING)
   }
}

@AndroidEntryPoint
class MyService:Service(){
  @Inject lateinitvar myRepository: MyRepository

  private val myJob = SupervisorJob()
  
  private val myServiceScope = CoroutineScope(Dispachers.IO+myJob)

  fun listenForState(){
     myServiceScope.launch{
        myRepository.state.collect{
             when(it)
               ....
         }
     }
  }
}

What happens is that on starting, the collect in MyService does get the initial value STATE_NORMAL but when from MyFragmentViewModel I update the MyRepository state, this value is not received by the service.

My questions:

• what am I doing wrong? Is something related to service scope/coroutines and how collect works?
• is this a good approach, architecturally speaking or are there better way to do it?

Answer 1

Your Services should never communicate with the Repository , since it should come under the UI Module and thus it must communicate to the ViewModel which further communicates to the Repository .

You can read my answer on MVVM pattern here :

Is this proper Android MVVM design?

. I have explaind the MVVM pattern here .

Also for your specific useCase , I recommend you to check this github - project :

https://github.com/mitchtabian/Bound-Services-with-MVVM

In the ReadMe section there is a link to a Youtube video which will explain you in depth about how to use Services with MVVM .

---

Also in your code , you have made use of enum classes which is not wrong , but since you are using you can make use of Sealed Classes , which is built on top of Enums and provides to maintain strict hierarchy .Your enum class in the form of Sealed Class will look in the following manner :

sealed class MyState{
   object State_Loading : MyState()
   object State_Normal : MyState()
}

---

And for you issue about not able to update the data , I suggest you to try

fun updateState(newState: MyState){
       state.emit( newState)
   }

If this does not work , you need to debug at every step from where the data passes using Log and know where is the error taking place