使用导航组件多次触发LiveData observer

Question

场景：我有两个片段，分别名为FirstFragment和UnitFragment。我从firstfragment转到unitfragment选择一个单元，使用navcontroller.popbackstack()返回到firstfragmet，并将单元数据发送到firstfragment，后者正在观察单元数据。

@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的结果：

--- 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，每次单击按钮并转到unitfragment并返回到firstfragment时，onviewcreated将再次调用，API LiveDataObserver将被触发两次！！！

我知道onViewCreated将再次调用，因为导航组件替换了片段，而不是添加它们。但我不知道为什么LiveData observer被触发了两次。

null




Answer 1

不幸的是，这不是你的问题的答案：

我需要解决...

避免再次调用onViewCreated代码。

最好使用oncreate来初始化与视图无关的组件(ViewModel+API调用）。它减少了lazy（！？）初始化检查。

@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，当您获得navBackStackEntry时，应该使用它的unit_data。

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());
        });
    }
}