StateFlow收集发出NullPointerException异常 [英] StateFlow collect emit NullPointerException

问题描述

我的存储库层有一个MutableStateFlow，将其收集到我的ViewModel中。我在某些用户设备上收到此NPE

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为非空数据，如果数据不知为何为空，应用程序将在较早时间崩溃。

我如何在存储库(生产者)层上使用StateFlow的示例：

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
}

ViewModel(消费者)端：

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()
    }

来自StateFlow doc

状态流从未完成。对状态流的Flow.Collect调用永远不会正常完成，由Flow.LaunchIn函数启动的协程也不会正常完成。

和Flow doc建议捕获如下异常

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

那么，建议接受StateFlow的collect中的所有异常，还是只接受生产者端抛出的异常？NPE的一般原因是什么？

推荐答案

我遇到了同样的问题。实际上，我正在将StateFlow映射到其他对象，并收集该对象。 在映射过程中，我使用了!!符号！ 删除该符号解决了我的问题。

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