如何将数据从活动或片段传递到视图模型？

Question

我正在创建一个项目，在该项目中我使用 themoviedb api 来获取数据。为此，我使用 MVVM 架构。从 MovieListFragment ，我想将诸如流行、top_rated 之类的类别传递给 MovieViewModel 以相应地过滤电影，但我得到的是空值。

这里是 MovieListFragment 类：-

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;
    }
}

这是 MovieViewModel 类：-

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;
    }
}

我收到这样的错误：-

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

由于Category是在setter之前被MovieViewModel的构造函数读取的，所以一直为null。

类别作为方法参数呢？

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)
    }
}

如需更详细的实现，需要一个存储库类。

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

Answer 2

您必须创建视图模型工厂才能将数据从活动或片段传递到视图模型。您可以通过here 参考。

Answer 3

如果您在movieViewModel.setCategory(category); 行中获得字符串category 的null 值，则表示MovieListFragment 的字段category 为空。

尝试检查您是否在fragment 上调用了setCategory(String category) 方法。

如果不是这样，请详细说明错误。例如。它发生在哪一行等。

Answer 4

在 MovieListFragment 类中：

先删除这一行：movieViewModel.setCategory(category);

并在此处传递“类别”：movieViewModel.getMoviesRepository(category) 如下代码：

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);
                    }
                });
            }
        });

在 MovieViewModel 类中：

将这一行移到 getMoviesRepository(String category) listOfMovies = repository.getListOfMovies(category, API_KEY);

然后像这样从MovieViewModel 类中删除类别变量和setCategory() 方法：

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;
    }

}