Android：使用viewModelscope和withContext

如何解决Android：使用viewModelscope和withContext

我的问题是，我得到了一个不可能的NullPointerException。当我不使用emailEntity而从price variable访问elvis-operator数据时，我的price variable为空，而我得到的是NullPointerException。

现在出现了问题：当我在elvis-operator上使用price variable并访问函数中的emailEntity数据时，我没有得到 {1}}，并且价格已正确设置。我在做什么错了？

基本代码

NullPointerException

问题代码

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())
                    }
              }
          }
      }
}

工作代码

   // NullPointerException
   val price = MutableLiveData(emailEntity.value?.basePrice!!)

  fun checkIfPriceIsInitialized() {
    Timber.d("Emailprice is ${emailEntity.value.basePrice}")
  }

StackTrace

   // 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}")
  }

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) 指向-> EmailViewModel.<init>(EmailViewModel.kt:164)

请记住，我是从头开始使用kotlin协程的。因此，我不知道100％这一切如何真正发挥作用

编辑

这是我的存储库：

val price = MutableLiveData(emailEntity.value?.basePrice!!)

这是我的实现：

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

另一种方法是在末尾使用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) } 并将返回类型从.single()更改为Flow<EmailEntity?>

编辑2

EmailEntity

解决方法

您的协程正在运行异步代码。到您的EmailViewModel实例化时，协程尚未完成运行，因此，此时LiveData的值仍为空。在协程完成运行之前，您必须尝试立即从主线程检索该值。

通常使用LiveData，几乎永远不会直接检索值。取而代之的是，您应该通过回调观察LiveData，以便在获得值时做出反应，这不会在暂停函数和协程中立即发生。

顺便说一句，您应该只从主线程更新LiveData，而这与协程无关。假设您的暂挂函数正确地委派给了后台线程（如果使用Room库从您那里获得了这种情况），则应从协程中删除包装的withContext块。可以始终从主分派器安全地调用正确组成的暂停函数，并将在内部将其委托给后台分派器。