Android 导航无法与共享视图模型 mvvm 一起正常工作

Question

我有两个共享相同 viewModel 的片段

ShoeListFragment 显示带有 fab 的 sheos 列表和 ShoeDetailsFragment 让用户在 fab 单击时添加新鞋

在ShoeDetailsFragment中用户可以点击添加或取消。添加后，鞋子将添加并显示在 ShoeListFragment

如果用户点击取消，我们会将用户导航到 ShoeListFragment。

首先运行我尝试在 fab 添加新鞋点击它被添加并显示。在第二个运行中，我尝试了取消按钮，然后导航到了 ShoeListFragment。

但是，如果在同一个运行中，我尝试添加，然后导航到 ShoeListFragment 我点击 fab，它被点击并且 ShoeDetailsFragment 被调用，但是布局不可见！

我的意思是 ShoeListFragment 布局仅可见。

ShoeListFragment

class ShoeListFragment : Fragment() {

lateinit var viewModel : ShoeListViewModel
lateinit var binding : FragmentShoeListBinding
lateinit var parentLayout: LinearLayout

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {

    binding = DataBindingUtil.inflate(inflater , R.layout.fragment_shoe_list , container , false)

    binding.fabAddShoe.setOnClickListener({
        Timber.i("fab clicked")
        findNavController().navigate(ShoeListFragmentDirections.actionShoesFragmentToShoeDetailFragment())
    })

    viewModel = ViewModelProvider(requireActivity()).get(ShoeListViewModel::class.java)

    Timber.i(viewModel.mShoeList.size.toString())

    viewModel.shoeList.observe(viewLifecycleOwner , Observer { shoeList ->
        Timber.i("List shoe observer")
        displayList(shoeList)
    })

    parentLayout = binding.llParent
    setHasOptionsMenu(true)

    // Inflate the layout for this fragment
    return binding.root
}


override fun onCreateOptionsMenu(menu: Menu, inflater: MenuInflater) {
    Timber.i("onCreateOptionsMenu")
    inflater.inflate(R.menu.main_menu, menu)
    super.onCreateOptionsMenu(menu, inflater)
}

// Menu item id must match the destination id to navigate there
override fun onOptionsItemSelected(item: MenuItem): Boolean {
    Timber.i("onOptionsItemSelected")
    return NavigationUI.onNavDestinationSelected(item, requireView().findNavController())
            || super.onOptionsItemSelected(item)
}
}

ShoeDetailFragment

class ShoeDetailFragment : Fragment() {

lateinit var binding : FragmentShoeDetailBinding
lateinit var viewModel : ShoeListViewModel

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {

    // View binding
    binding = DataBindingUtil.inflate(inflater,R.layout.fragment_shoe_detail , container , false)

    // Data binding - shared view model
    viewModel = ViewModelProvider(requireActivity()).get(ShoeListViewModel::class.java)
    binding.shoeListViewModel = viewModel

    binding.shoeModel = Shoe("" , 0.0 , "" , "")

    //Observers
    viewModel.isCancelAdd.observe(viewLifecycleOwner , Observer { isCancel ->
        Timber.i("Fragment Cancel shoe")
        findNavController().navigate(ShoeDetailFragmentDirections.actionShoeDetailFragmentToShoesFragment())
    })

    viewModel.isAddShoe.observe(viewLifecycleOwner , Observer { isAdd ->
       findNavController().navigate(ShoeDetailFragmentDirections.actionShoeDetailFragmentToShoesFragment())
    })

    // Inflate the layout for this fragment
    return binding.root
}

共享视图模型

class ShoeListViewModel : ViewModel() {

/* Should we make shoeList observing mShoeList instead of doing
shoeList.value = mShoeList in once we add new shoe?
*/

private var _shoeList = MutableLiveData<MutableList<Shoe>>()
val shoeList : LiveData<MutableList<Shoe>>
    get() = _shoeList

private var _isCancelAdd = MutableLiveData<Boolean>()
val isCancelAdd : LiveData<Boolean>
    get() = _isCancelAdd

private var _isAddShoe = MutableLiveData<Boolean> ()
val isAddShoe : LiveData<Boolean>
    get() = _isAddShoe

var mShoeList : MutableList<Shoe>



init {
    mShoeList = mutableListOf(
        Shoe("Filla" , 40.0 , "Filla" , "Filla shoes") ,
        Shoe("Addidad" , 30.0 , "Addidas" , "Addidas shoes")
    )
    _shoeList.value = mShoeList
}

fun addShoe (shoe : Shoe) {
    _isAddShoe.value = true
    mShoeList.add(shoe)
    _shoeList.value = mShoeList
    Timber.i("In Add shoe")
}

fun cancelAddShoe () {
    _isCancelAdd.value = true
    Timber.i("In Cancel shoe")
}

我试过调试它，第二次这条线在 shoeDetailsFragment

中执行

 binding = DataBindingUtil.inflate(inflater,R.layout.fragment_shoe_detail , container , false)

直到达到

 return binding.root

然后这个被调用了

binding = DataBindingUtil.inflate(inflater , R.layout.fragment_shoe_list , container , false)

这就是我认为鞋子细节片段不可见的原因。但为什么会发生这种行为？

Answer 1

问题是，您的 isCancelAdd 值将被缓存，因此在第二次导航后您将立即获得事件，然后您将从详细信息导航回来。

通常对于活动，您有两种选择：

使用 SingleLiveEvent 模式或 Consumable，以确保不会多次使用事件。有关详细信息，请参阅此博客：https://proandroiddev.com/singleliveevent-to-help-you-work-with-livedata-and-events-5ac519989c70
使用SharedFlows。我会接受这个，因为这是现在推荐的方式，并且 LiveDatas 即将被弃用

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