Android：使用 viewModelscope 和 withContext 时不可能出现 NullPointerException
Android: Impossible NullPointerException when using viewModelscope and withContext
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 访问我的 emailEntity 数据时，我的 price variable 变为空并且我得到 NullPointerException.
现在问题来了：当我在我的 price variable 处使用 elvis-operator 并在一个函数中访问我的 emailEntity 数据时，我 没有得到 a 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}")
}
堆栈跟踪
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.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)
EmailViewModel.<init>(EmailViewModel.kt:164) 指向 -> val price = MutableLiveData(emailEntity.value?.basePrice!!)
请记住，我是从头开始使用 kotlin 协程的。因此我 100% 不知道它是如何工作的
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)
}
另一种方法是在最后使用 .single() 并将 return 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)
您的协程是 运行 异步代码。当你的 EmailViewModel 被实例化时，协同程序还没有完成 运行，所以在那个时候，LiveData 的值仍然是空的。在协程完成 运行.
之前，您必须尝试立即从主线程检索值
通常使用 LiveData，您几乎从不直接检索值。相反，您应该使用回调观察 LiveData，以便您可以在它获得值 时做出反应 ，这不会立即发生在挂起函数和协程中。
顺便说一下，您应该只从主线程更新 LiveData，而您没有使用协程执行此操作。假设您的挂起函数正确地委托给后台线程，如果它们来自您使用 Room 库的情况就是这种情况，您应该从协程中删除包装 withContext 块。始终可以从 Main Dispatcher 安全地调用一个正确组合的挂起函数，并将在内部根据需要委托给后台调度程序。
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 访问我的 emailEntity 数据时，我的 price variable 变为空并且我得到 NullPointerException.
现在问题来了：当我在我的 price variable 处使用 elvis-operator 并在一个函数中访问我的 emailEntity 数据时，我 没有得到 a 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}")
}
堆栈跟踪
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.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)
EmailViewModel.<init>(EmailViewModel.kt:164) 指向 -> val price = MutableLiveData(emailEntity.value?.basePrice!!)
请记住，我是从头开始使用 kotlin 协程的。因此我 100% 不知道它是如何工作的
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)
}
另一种方法是在最后使用 .single() 并将 return 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)
您的协程是 运行 异步代码。当你的 EmailViewModel 被实例化时，协同程序还没有完成 运行，所以在那个时候，LiveData 的值仍然是空的。在协程完成 运行.
通常使用 LiveData，您几乎从不直接检索值。相反，您应该使用回调观察 LiveData，以便您可以在它获得值 时做出反应 ，这不会立即发生在挂起函数和协程中。
顺便说一下，您应该只从主线程更新 LiveData，而您没有使用协程执行此操作。假设您的挂起函数正确地委托给后台线程，如果它们来自您使用 Room 库的情况就是这种情况，您应该从协程中删除包装 withContext 块。始终可以从 Main Dispatcher 安全地调用一个正确组合的挂起函数，并将在内部根据需要委托给后台调度程序。