StateFlow 收集发出 NullPointerException

Question

My repository layer have a MutableStateFlow , collecting it in my ViewModel. I am getting this NPE on some user devices

Fatal Exception: java.lang.NullPointerException
       at a.b.c.ui.viewmodel.HomeViewModel$collectFlowState$$inlined$collect$1.emit(HomeViewModel.java:189)
       at a.b.c.ui.viewmodel.HomeViewModel$collectFlowState$$inlined$collect$1$1.invokeSuspend(HomeViewModel.java:12)
       at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(BaseContinuationImpl.java:33)
       at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTaskKt.java:176)
       at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTaskKt.java:111)
       at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.java:308)
       at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.java:318)
       at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.java:400)
       at kotlinx.coroutines.android.HandlerContext$scheduleResumeAfterDelay$$inlined$Runnable$1.run(HandlerContext.java:19)
       at android.os.Handler.handleCallback(Handler.java:883)
       at android.os.Handler.dispatchMessage(Handler.java:100)
       at android.os.Looper.loop(Looper.java:237)
       at android.app.ActivityThread.main(ActivityThread.java:7830)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1040)

MutableStateFlow is of non-null data, if the data is somehow null the app would have crashed earlier.

An example how I am using StateFlow on the repository (producer) layer:

data class ApiData(...)
private val INITIAL = ApiData(...)
private var someState = INITIAL

private val dataSF = MutableStateFlow(someState)

fun dataFlow() = dataSF

// called on remote api success, we poll for updated data (delta) from the server
fun onDataChangeAvailable(x: Int, y: Double) {
        someState = someState.copy(x = x, y= y)
        dataSF.value = someState
}

The ViewModel (consumer) side:

private val repository // constructor injected; repository is Application scoped
private val job = SupervisorJob()
private val uiScope = CoroutineScope(Dispatchers.Main + job)
// Viewmodel init block
init {
     uiScope.launch {
                repository.dataFlow().collect { // crash sometimes here.
                    // consume values
                }
        }
}

override fun onCleared() {
        job.cancel()
        super.onCleared()
    }

From the StateFlow doc

State flow never completes. A call to Flow.collect on a state flow never completes normally, and neither does a coroutine started by the Flow.launchIn function.

and the Flow doc recommends to catch exceptions like this

try {
    flow.collect { value ->
        println("Received $value")
    }
} catch (e: Exception) {
    println("The flow has thrown an exception: $e")
}

So is it recommended to swallow all exceptions from the collect of a StateFlow or only those thrown by the producer end? What is the cause of the NPE in general?

Answer 1

"If we just call cancel, it doesn't mean that the coroutine work will just stop."

I resolve this problem checking if the coroutine is active using ensureActive()

...
       job = uiScope.launch {
                ensureActive()
                repository.dataFlow().collect { // crash sometimes here.
                    // consume values
                }
        }

override fun onCleared() {
        job.cancel()
        super.onCleared()
    }

You can see detail of this solutin in How to cancel collect coroutine StateFlow?

Answer 2

I faced the same problem. Actually I was mapping the StateFlow to something else and collecting that. In the mapping process I was using !! notation. Removing that notation solved my problem.

Answer 3

I think this could solve your problem

private val dataSF = MutableStateFlow<Int?>(someState)

Answer 4

I faced the same problem but it was happening when recreates the fragment because Stateflow still has an old value so simply add a check before call flow.collect , check for flow contains init value something like this

if (viewModel.mutableStateFlow.value == YourInitValue) {
        lifecycleScope.launch {
            viewModel.mutableStateFlow.timeLineData.collect {
        
            }
        }
    }

because if stateflow didn't contain init value that means collect method called before