如何在 Android 上的协程中使用 LiveData

Question

In my application I want use Room database into MVVM architecture and for this I used LiveData with Couroutines .
I write below codes and I want check data, if data is empty then show empty layout.
I write below codes, but after run application show me Error and application has crashed!
Dao codes:

@Dao
interface DataDao {

    @Query("SELECT * FROM $my_table ORDER BY id DESC")
    fun getAllData(): LiveData<MutableList<DataEntity>>
}

Repository codes:

class MainRepository @Inject constructor(private val dao: DataDao) {

   fun allData() = dao.getAllData()
}

ViewModel codes:

@HiltViewModel
class MainViewModel @Inject constructor(private val repository: MainRepository) : ViewModel() {

    val dataList = repository.allData()
    val isEmpty = MutableLiveData<Boolean>()

    fun checkData() = viewModelScope.launch(Dispatchers.Main) {
        if (repository.allData().value!!.isEmpty()) {
            isEmpty.postValue(true)
        } else {
            isEmpty.postValue(false)
        }
    }
}

Show me this error in logcat :

java.lang.NullPointerException
        at com.myapp.viewmodel.MainViewModel$checkData$1.invokeSuspend(MainViewModel.kt:22)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
        Suppressed: kotlinx.coroutines.DiagnosticCoroutineContextException: [StandaloneCoroutine{Cancelling}@35a0def, Dispatchers.Main]

Show error for this line: repository.allNotes().value.!.isEmpty()

How can I fix it?

Answer 1

First of all I think you don't need to launch a coroutine in MainViewModel.checkData() method, because there is no suspend functions invoked. And regarding the error, please check the value in LiveData on null :

fun checkData() {
    val data = repository.allData().value
    if (data == null || data.isEmpty()) {
        isEmpty.postValue(true)
    } else {
        isEmpty.postValue(false)
    }
}