观察onCreate片段时调用的LiveData

Question

class EmergencyFragment : Fragment() {

   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)
       emergencyFragmentViewModel = ViewModelProviders.of(this).get(EmergencyFragmentViewModel::class.java)
   }

在onViewCreated中，我设置了Observer对象，以便在LiveData中所做的任何更改都可以在这里得到更改通知：

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
   super.onViewCreated(view, savedInstanceState)
   emergencyFragmentViewModel.isEmergencyEventMediaLDSuccess.observe(viewLifecycleOwner, Observer {
       Log.d("sendEmergencyEvent", "isEmergencyEventMediaLDSuccess observer called")
       }
   })
}

在ViewModel类中，我将LiveData参数设置为：

EmergencyFragmentViewModel: ViewModel() {
   var isEmergencyEventMediaLDSuccess: LiveData<Boolean> = MutableLiveData()
       private set
   private val observerEventMedia = Observer<Boolean> { (isEmergencyEventMediaLDSuccess as MutableLiveData).value = it}

在init中，我设置了一个观察者：

init {
   EmergencyFragmentModel.isEmergencyEventMediaLDSuccessModel.observeForever(observerEventMedia)
}

override fun onCleared() {
   super.onCleared()
   EmergencyFragmentModel.isEmergencyEventMediaLDSuccessModel.removeObserver(observerEventMedia)
}

class EmergencyFragmentModel {
   companion object{
       val isEmergencyEventMediaLDSuccessModel: LiveData<Boolean> = MutableLiveData()

override fun onResponse(call: Call<Int>, response: Response<Int>) {
   if(response.isSuccessful) {
       (isEmergencyEventLDModelSuccess as MutableLiveData).postValue(true)
       Log.d("succeed", "sendEmergencyEvent success: ${response.body().toString()}")
   }

override fun onStop() {
        super.onStop()
        emergencyFragmentViewModel.isEmergencyEventMediaLDSuccess.removeObservers(viewLifecycleOwner)
        emergencyFragmentViewModel.isEmergencyEventMediaLDSuccess.removeObserver(observeEmergencyEventLDSuccess)
    }

Answer 1

@Pawel是对的。LiveData存储该值，每次观察它（在本例中是在您的onViewCreated中），它都会发出存储的最后一个值。

也许您想要像SingleLiveEvent这样的东西，在有人阅读它之后清理它的值。

所以当你来回的时候，它不会发出最后一个值（一旦它被清理了）。