LiveData没有观察到具有共享视图模型的子viewpage片段

Question

LiveData不观察子viewpage片段，子片段有一个共享的视图模型来访问数据。

这里FragmentA和FragmentB是viewpager的一部分，它们都共享一个视图模型SharedViewModel。

public class SharedViewModel extends AndroidViewModel { //in Java
private final MutableLiveData<Data> mLiveData = new MutableLiveData<>();
public LiveData<Data> getLiveData() {
return mLiveData;
}
//for updating data through LiveData, using post as and when I get the response from DataSource as shown below.
mLiveData.postValue(response); //getting the response on debugging
}

class FragmentA : Fragment()  { //in Kotlin
override fun onCreate(@Nullable savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
mViewModel = ViewModelProvider(this).get(SharedViewModel::class.java)
}
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View {
....
mViewModel.liveData.observe(viewLifecycleOwner, {
//no call coming in this block so unable to update view
})
}
}

class FragmentB : Fragment()  { //in Kotlin
override fun onCreate(@Nullable savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
mViewModel = ViewModelProvider(this).get(SharedViewModel::class.java)
}
}

需要一些帮助，为什么实时数据不能观察到变化。

提前感谢。

Answer 1

您的ViewModel未被共享。你已经将其命名为sharedViewmodel，但你通过传递唯一所有者来获得它的实例的方式它也将是唯一实例.

共享视图模型的正确方法是

class SharedViewModel : ViewModel() {
private val myLiveData = MutableLiveData<Data>()
fun getMyLiveDta():LiveData {
return myLiveData
}
}

现在在第一个片段

class MasterFragment : Fragment() {
private lateinit var itemSelector: Selector
// Get instance of viewmodel in fragment like this
private val model: SharedViewModel by activityViewModels()
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)

}
}

在第二个片段中，像这样

class DetailFragment : Fragment() {
private val model: SharedViewModel by activityViewModels()
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
model.selected.observe(viewLifecycleOwner, Observer<Item> { item ->
// Update the UI
})
}
}