java - 使用导航组件多次触发 LiveData 观察者

Question

场景:我有两个名为 FirstFragment 的 fragment 和 UnitFragment .我来自 FirstFragment至UnitFragment选择一个单位返回FirstFragmet使用 navController.popBackStack();并将单位数据发送至FirstFragment这是观察单位数据。
这是我的onViewCreated的 FirstFragment :

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);

    if (viewModel == null) { // Lazy Initialization
        ApiService apiService = ApiServiceProvider.getInstance();
        AddNewWareViewModelFactory addNewWareViewModelFactory = new AddNewWareViewModelFactory(apiService);
        viewModel = new ViewModelProvider(this, addNewWareViewModelFactory).get(AddWareViewModel.class);
    }

    Log.i(TAG, "OnViewCreated -----> Called");
    viewModel.callNewWare(parentCode);
    viewModel.getNewWareResponse().observe(getViewLifecycleOwner(),
            resObject -> Log.i(TAG, "API Response LiveData Count -----> " + count++)); // Started From Zero

    NavHostFragment navHostFragment = (NavHostFragment) requireActivity()
            .getSupportFragmentManager()
            .findFragmentById(R.id.container);

    binding.button.setOnClickListener(v -> {
        if (navHostFragment != null) {
            NavController navController = navHostFragment.getNavController();
            navController.navigate(FirstFragmentDirections.actionFirstFragmentToUnitFragment());
        }
    });


    if (navHostFragment != null) {
        NavController navController = navHostFragment.getNavController();
        NavBackStackEntry navBackStackEntry = navController.getCurrentBackStackEntry();
        if (navBackStackEntry != null) {
            SavedStateHandle savedStateHandle = navBackStackEntry.getSavedStateHandle();
            MutableLiveData<Unit> unitLiveData = savedStateHandle.getLiveData("unit_data");
            unitLiveData.observe(getViewLifecycleOwner(), unit -> binding.tvUnit.setText(unit.getTitle()));
        }
    }
}

这是 LogCat 结果:

--- Go to FirstFragment for first time ---
I/FirstFragment: OnViewCreated -----> Called
I/FirstFragment: API Response LiveData Count -----> 0
--- Button clicked to go to UnitFragment to select a unit ---
I/UnitFragment: Selected Unit -----> Meter
--- Come back to FirstFragment ---
I/FirstFragment: OnViewCreated -----> Called
I/FirstFragment: API Response LiveData Count -----> 1
I/FirstFragment: API Response LiveData Count -----> 2

正如您在 LogCat 结果中看到的，每次我单击按钮并转到 UnitFragment并返回 FirstFragment onViewCreated将再次调用，API LiveDataObserver 将被触发两次!!!
我知道 onViewCreated 会再次调用，因为导航组件替换了 fragment 而不是添加它们。但我不知道为什么 LiveData 观察者会被触发两次。
我读了 this post但他似乎忽略了导航组件。
我需要一个解决方案...

避免调用 onViewCreated再次编码。

避免再次触发 LiveData 观察者。

Answer 1

不幸的是，这不是您问题的答案:

I need a solution to ...

Avoid calling onViewCreated codes again.

Avoid triggering LiveData observer again.

我试图解释导航及其行为或纠正一些误解。这些问题有不同的原因和 Avoid calling onViewCreated codes again.是一种迂回的方式。

I know onViewCreated will call again because Navigation Component replaces the fragments instead of adding them.

如您所知，将 fragment 添加到后台堆栈时进行替换，只需将旧 fragment 从 fragmentManager 中分离出来.表示旧 fragment 的 查看 会破坏。
当您从后台堆栈中弹出 UnitFragment 时，将创建其 View 。
所以不要在 onViewCreated 中调用任何 API。因为它可能会调用多次(在配置更改、破坏 fragment 等中)
最好使用 onCreate用于初始化非 View 相关的组件(ViewModel + API 调用)。它减少了 Lazy(!?) Initialization查看。

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    ApiService apiService = ApiServiceProvider.getInstance();
    AddNewWareViewModelFactory addNewWareViewModelFactory = new AddNewWareViewModelFactory(apiService);
    viewModel = new ViewModelProvider(this /*owner*/, addNewWareViewModelFactory).get(AddWareViewModel.class);

    viewModel.callNewWare(parentCode);
}

并开始在 onViewCreated 中观察它们.此外，您应该使用 unit_data当你得到它时，来自 navBackStackEntry。

if (navHostFragment != null) {
    NavController navController = navHostFragment.getNavController();
    NavBackStackEntry navBackStackEntry = navController.getCurrentBackStackEntry();
    if (navBackStackEntry != null) {
        SavedStateHandle savedStateHandle = navBackStackEntry.getSavedStateHandle();
        MutableLiveData<Unit> unitLiveData = savedStateHandle.getLiveData("unit_data");
        unitLiveData.observe(getViewLifecycleOwner(), unit -> {
            savedStateHandle.remove("unit_data");       // add this line
            return binding.tvUnit.setText(unit.getTitle());
        });
    }
}