java - 如何在 fragment 中使用 ViewModel？

Question

我正在使用 MVVM 架构。我有一个 Activity 和几个 fragment ，我想在 Activity 中的API中发出请求，然后使用ViewModel，感谢获得的数据，将它们显示在 fragment 中。我该怎么做？我当前不起作用的解决方案:
Activity :

viewModelRoutesFragment = new ViewModelProvider(this).get(ViewModelRoutesFragment.class);
viewModelRoutesFragment.init();

分段:

viewModelRoutesFragment = new ViewModelProvider(this).get(ViewModelRoutesFragment.class);
viewModelRoutesFragment.getRoutes().observe(getActivity(), new Observer<List<RoutesResponse>>() {
    @Override
    public void onChanged(List<RoutesResponse> routes) {

                //Show data
    }
});

存储库:

public class RemoteRepository {

private ApiRequest apiRequest;
private MutableLiveData<List<RoutesResponse>> routes = new MutableLiveData<>();

public RemoteRepository() {
    apiRequest = RetrofitRequest.getInstance().create(ApiRequest.class);
}

public MutableLiveData<List<RoutesResponse>> getRoutes() {

    apiRequest.getRoutes()
            .enqueue(new Callback<List<RoutesResponse>>() {
                @Override
                public void onResponse(Call<List<RoutesResponse>> call, Response<List<RoutesResponse>> response) {

                    if (response.isSuccessful())
                        routes.setValue(response.body());
                }

                @Override
                public void onFailure(Call<List<RoutesResponse>> call, Throwable t) {
                    Log.i("Failure", "Fail!");
                }
            });

        return routes;
    }
}

View 模型:

public class ViewModelRoutesFragment extends AndroidViewModel {

private RemoteRepository remoteRepository;
private LiveData<List<RoutesResponse>> routes;

public ViewModelRoutesFragment(@NonNull Application application) {
    super(application);
}

public void init() {
    remoteRepository = new RemoteRepository();
    routes = remoteRepository.getRoutes();
}

public LiveData<List<RoutesResponse>> getRoutes() {
        return routes;
    }
}

当前出现空错误。我怎样才能正确避免它？

java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.lifecycle.LiveData.observe(androidx.lifecycle.LifecycleOwner, androidx.lifecycle.Observer)' on a null object reference

Answer 1

基本上，我们试图在 Activity 和 fragment 之间共享 View 模型。
所以在 Activity 创建期间，我们必须创建 View 模型的实例

viewModelRoutesFragment = new ViewModelProvider(requireActivity()).get(ViewModelRoutesFragment.class);
viewModelRoutesFragment.init();

在 fragment 中，我们还需要在 onViewCreated() 中重用 ViewModelRoutesFragment
获取 ViewModel 的实例并观察实时数据

viewModelRoutesFragment = new ViewModelProvider(requireActivity()).get(ViewModelRoutesFragment.class);
viewModelRoutesFragment.getRoutes().observe(getActivity(), new Observer<List<RoutesResponse>>() {
    @Override
    public void onChanged(List<RoutesResponse> routes) {
       // updation of UI
    }
});