BaseFragment 中的 LiveData Consumer 未从 BaseViewModel 接收更新

Question

我有一个看起来像这样的 BaseFragment.kt

open class BaseFragment: Fragment() {

private lateinit var viewModel: BaseViewModel

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    viewModel = ViewModelProvider(this).get(BaseViewModel::class.java)
    observeNavigationCommands()
}

/**
 * Method that observes Navigation commands triggered by BaseViewHolder
 * This allows us to navigate from a viewHolder using the MVVM pattern
 */
private fun observeNavigationCommands() {
    viewModel.navigationCommands.observe(viewLifecycleOwner, EventObserver {
        Timber.e("received nav command $it")
        when(it) {
            is NavigationCommand.To -> findNavController().navigate(it.destinationId)
            is NavigationCommand.Back -> findNavController().popBackStack()
            is NavigationCommand.BackTo -> TODO()
            NavigationCommand.ToRoot -> TODO()
        }
    })
}

}

...还有一个看起来像这样的 BaseViewModel.kt

open class BaseViewModel: ViewModel() {

val navigationCommands = MutableLiveData<Event<NavigationCommand>>()

/**
 * Navigate to a specific fragment using Id
 */
fun navigate(id: Int) {
    Timber.e("trigger navigation event $id")
    // navigationCommands.postValue(NavigationCommand.To(id))
    navigationCommands.value = Event(NavigationCommand.To(id))
}

/**
 * Pop backStack
 */
fun goBack() {
    navigationCommands.value = Event(NavigationCommand.Back)
}

}

NavigationCommand 类看起来像

sealed class NavigationCommand {

data class To(val destinationId: Int) : NavigationCommand()
data class BackTo(val destinationId: Int): NavigationCommand()

object Back: NavigationCommand()
object ToRoot: NavigationCommand()

}

现在在我的其他扩展 BaseViewModel 的视图模型中，我希望能够调用 navigate(R.id.action_fragmentA_to_fragmentB) 但问题是 observeNavigationCommands() 中的消费者永远不会收到 NavigationCommands

.....

但如果我复制observeNavigationCommands() 的内容并将其放在我当前的片段（扩展BaseFragment 的片段）中，消费者会收到更新

我错过了什么？请帮忙

Answer 1

我是否理解正确，对于扩展 BaseFragment 的片段，您想要附加扩展 BaseViewModel 的 viewModel，但这不适用于 liveData？

如果是这样，请查看我创建的这个简单的工作项目以反映您的情况：

https://github.com/phamtdat/OpenViewModelDemo

重点是使viewModel 可覆盖并在扩展BaseFragment 的片段中覆盖它。