如何将数据从活动或片段传递到视图模型？

Question

I am creating a project in which i am using themoviedb api to fetch data. For this I am using MVVM architecture. From MovieListFragment , I want to pass category such as popular, top_rated to MovieViewModel for filtering the movies accordingly , but there i am getting null value.

Here is MovieListFragment class :-

public class MovieListFragment extends Fragment {

    private MovieViewModel movieViewModel;
    private static final String BASE_URL = "https://api.themoviedb.org";
    private static final String API_KEY = "API_KEY";
    private String category;

    private GridView gridView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_movie_list, container, false);
        gridView = rootView.findViewById(R.id.images_grid_view);

        movieViewModel = ViewModelProviders.of(this).get(MovieViewModel.class);
        movieViewModel.setCategory(category);  // from here i am passing value
        movieViewModel.getMoviesRepository().observe(getActivity(), new Observer<MovieResults>() {  // error line no :- 76
                @Override
                public void onChanged(MovieResults movieResults) {
                    final List<MovieResults.ResultsBean> listOfMovies = movieResults.getResults();  // error line no :- 79

                    MovieListAdapter mAdapter = new MovieListAdapter(getContext(), listOfMovies);
                    gridView.setAdapter(mAdapter);
                    gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                        @Override
                        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                            mCallback.onImageSelected(position, listOfMovies);
                        }
                    });
                }
            });
        return rootView;
    }

    public void setCategory(String category) {
        this.category = category;
    }
}

Here is MovieViewModel class :-

public class MovieViewModel extends AndroidViewModel {
    private String category;
    private static final String API_KEY = "api_key";
    private MovieRepository repository;
    private MutableLiveData<MovieResults> listOfMovies;

    public MovieViewModel(@NonNull Application application) {
        super(application);
        Log.d("MovieViewModel", category); // but here i am getting null value
        listOfMovies = repository.getListOfMovies(category, API_KEY);
    }

    public MutableLiveData<MovieResults> getMoviesRepository() {
        return listOfMovies;
    }

    public void setCategory(String category) {
        this.category = category;
    }
}

I am getting error like this :-

java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.mountblue.moviesapp.entity.MovieResults.getResults()' on a null object reference
        at com.mountblue.moviesapp.fragment.MovieListFragment$2.onChanged(MovieListFragment.java:79)
        at com.mountblue.moviesapp.fragment.MovieListFragment$2.onChanged(MovieListFragment.java:76)

Answer 1

If you are getting null value for the string category in line movieViewModel.setCategory(category); , it means the field category of MovieListFragment is null.

Try checking if you have invoked the setCategory(String category) method on the fragment .

If that's not the case, please elaborate on the error. Eg on which line it occurs etc.

Answer 2

Since the Category is read by the constructor of MovieViewModel before the setter, it is always null.

What about category as method arguments?

public class MovieViewModel extends AndroidViewModel {

    private static final String API_KEY = "api_key";
    private MovieRepository repository;
    private MutableLiveData<MovieResults> listOfMovies = new MutableLiveData<MovieResults>();

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

    public MutableLiveData<MovieResults> getMoviesRepository(category) {
        loadData(category);
        return listOfMovies;
    }

    private void loadData(category) {
        // Do an asynchronous operation to fetch MovieResults.
        repository.getListOfMovies(category, API_KEY);

        ...
        // Receive asynchronous result in callback

        // Post the result after getting the asynchronous result.
        listOfMovies.postValue(response)
    }
}

For a more detailed implementation, need a repository class.

Documents: https://developer.android.com/topic/libraries/architecture/viewmodel

Answer 3

You have to create viewmodel factory to pass data to viewmodel from activity or fragment. You can refer this at here .

Answer 4

In MovieListFragment class:

First delete this line: movieViewModel.setCategory(category);

And pass "category" here: movieViewModel.getMoviesRepository(category) like code below:

movieViewModel.getMoviesRepository(category).observe(getActivity(), new Observer<MovieResults>() {  // error line no :- 76
            @Override
            public void onChanged(MovieResults movieResults) {
                final List<MovieResults.ResultsBean> listOfMovies = movieResults.getResults();  // error line no :- 79

                MovieListAdapter mAdapter = new MovieListAdapter(getContext(), listOfMovies);
                gridView.setAdapter(mAdapter);
                gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                    @Override
                    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                        mCallback.onImageSelected(position, listOfMovies);
                    }
                });
            }
        });

In MovieViewModel class:

Move this line inside getMoviesRepository(String category) listOfMovies = repository.getListOfMovies(category, API_KEY);

And just remove category variable and setCategory() method from MovieViewModel class like this:

public class MovieViewModel extends AndroidViewModel {
    
    private static final String API_KEY = "api_key";
    private MovieRepository repository;
    private MutableLiveData<MovieResults> listOfMovies;

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

    public MutableLiveData<MovieResults> getMoviesRepository(String category) {
        Log.d("MovieViewModel", category); // but here i am getting null value
        listOfMovies = repository.getListOfMovies(category, API_KEY);
        return listOfMovies;
    }

}