如何在片段中使用ViewModel?

我使用的是MVVM架构。我有一个活动和几个片段,我想在活动中的API中发出一个请求,然后使用ViewModel,感谢获得的数据,将它们显示在片段中。我该怎么做呢?我当前的解决方案不起作用:

活动:

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

片段:

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;
    }
}

视图模型:

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;
    }
}

当前收到空错误。我如何才能正确地避免它?

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

解决方案

基本上,我们尝试跨活动和片段共享视图模型。 因此,在活动创建期间,我们必须创建视图模型的实例

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
    }
});

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