Android:使用viewModelscope和withContext时出现不可能的NullPointerExcept

Question

我的问题是，我得到了一个不可能的NullPointerException。当我在不使用elvis-operator的情况下从price variable访问emailEntity数据时，我的price variable将为null，而我将获得NullPointerException。

现在问题来了：当我在price variable中使用elvis-operator并在函数中访问emailEntity数据时，我不会得到NullPointerException，并且价格设置正确。我做错了什么？

基本代码

class EmailViewModel @ViewModelInject constructor() : ViewModel() {
// This is the value I access from my price variable and the function
private val emailEntity = MutableLiveData<EmailEntity?>()
// Setting the value of my emailEntity here
init {
// I have to use viewModelScope because "getCalibratePrice and getRepairPrice" are suspend functions
viewModelScope.launch {
withContext(Dispatchers.IO) {
when(subject.value.toString()) {
"Toast" -> emailEntity.postValue(emailRepository.getCalibratePrice())
else -> emailEntity.postValue(emailRepository.getRepairPrice())
}
}
}
}
}

问题代码

// NullPointerException
val price = MutableLiveData(emailEntity.value?.basePrice!!)
fun checkIfPriceIsInitialized() {
Timber.d("Emailprice is ${emailEntity.value.basePrice}")
}

工作代码

// NO NullPointerException but value is now always 0F
val price = MutableLiveData(emailEntity.value?.basePrice ?: 0F)
// EmailEntity price is correctly set here!!!
fun checkIfPriceIsInitialized() {
Timber.d("Emailprice is ${emailEntity.value.basePrice}")
}

StackTrace

java.lang.NullPointerException
at com.example.app.framework.ui.viewmodel.EmailViewModel.<init>(EmailViewModel.kt:164)
at com.example.app.framework.ui.viewmodel.EmailViewModel_AssistedFactory.create(EmailViewModel_AssistedFactory.java:58)
at com.example.app.framework.ui.viewmodel.EmailViewModel_AssistedFactory.create(EmailViewModel_AssistedFactory.java:20)
at androidx.hilt.lifecycle.HiltViewModelFactory.create(HiltViewModelFactory.java:76)
at androidx.lifecycle.AbstractSavedStateViewModelFactory.create(AbstractSavedStateViewModelFactory.java:69)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:185)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
at androidx.lifecycle.ViewModelLazy.getValue(ViewModelProvider.kt:54)
at androidx.lifecycle.ViewModelLazy.getValue(ViewModelProvider.kt:41)
at com.example.app.framework.ui.view.fragments.home.calibrateAndRepair.CalibrateRepairMessageFragment.getViewModel(Unknown Source:2)
at com.example.app.framework.ui.view.fragments.home.calibrateAndRepair.CalibrateRepairMessageFragment.getViewModel(CalibrateRepairMessageFragment.kt:26)
at com.example.app.framework.ui.view.basefragments.BaseFragment.onCreateView(BaseFragment.kt:30)
at com.example.app.framework.ui.view.basefragments.EmailFragment.onCreateView(EmailFragment.kt:54)
at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2699)
at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:320)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1199)
at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2236)
at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:2009)
at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1965)
at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1861)
at androidx.fragment.app.FragmentManager$4.run(FragmentManager.java:413)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

CCD_ 11指向->val price = MutableLiveData(emailEntity.value?.basePrice!!)

请记住，我是从零开始参加科特林郊游的。因此，我不知道100%它是如何真正工作的

编辑

这是我的存储库：

interface EmailRepository {
fun sendEmail(email: Email): Flow<EmailStatus<Unit>>
suspend fun getCalibratePrice(): Flow<EmailEntity?>
suspend fun getRepairPrice(): Flow<EmailEntity?>
}

这就是我的实现：

class EmailRepositoryImpl @Inject constructor(
private val db: FirebaseFirestore
) : EmailRepository {
override suspend fun getCalibratePrice(): Flow<EmailEntity?> = flow {
val result = db.collection("emailprice").document("Kalibrieren").get().await()
val emailEntity = result.toObject<EmailEntity?>()
emit(emailEntity)
}.catch {
Timber.d("Error on getCalibrate Price")
}.flowOn(Dispatchers.Main)
override suspend fun getRepairPrice(): Flow<EmailEntity?> = flow {
val collection = db.collection("emailprice").document("Reparieren").get().await()
val emailEntity = collection.toObject<EmailEntity?>()
emit(emailEntity)
}.catch {
Timber.d("Error on getRepairPrice")
}.flowOn(Dispatchers.Main)
}

另一种选择是在末尾使用.single()，并将返回类型从Flow<EmailEntity?>更改为EmailEntity

编辑2

private var emailEntity: EmailEntity = EmailEntity("", 50F)
init {
viewModelScope.launch {
when(subject.value.toString()) {
context.getString(R.string.home_calibrate_card_headline) -> emailRepository.getCalibratePrice().collect {
emailEntity = it ?: EmailEntity("Error", 100F)
}
else -> emailRepository.getRepairPrice().collect {
emailEntity = it ?: EmailEntity("Error Zwei", 150F)
}
}
}
}
// Price is 50 and does not change..
val price = MutableLiveData(emailEntity.basePrice)

Answer 1

您的协同程序正在运行异步代码。当您的EmailViewModel被实例化时，协同程序还没有完成运行，所以在这一点上，LiveData的值仍然为null。在协程完成运行之前，您必须尝试立即从主线程中检索值。

通常使用LiveData，您几乎从不直接检索值。相反，您应该通过回调来观察LiveData，这样当它获得值时，您就可以做出反应，而挂起函数和协同程序不会立即发生这种情况。

顺便说一句，你应该只从主线程更新LiveData，这是你在协同程序中无法做到的。假设您的挂起函数正确地委托给后台线程(如果它们来自使用Room库的您(，那么您应该从协程中删除包装withContext块。一个适当组合的挂起函数总是可以从主调度程序安全地调用，并将在内部根据需要委托给后台调度程序。