ViewModel/LiveData/Observer设计

Question

我的应用程序获取顶级评级或最受欢迎的电影从themoviedb.org.

我正在尝试用LiveData实现ViewModel来切换这两个按钮。在MainViewModel中。java，我有这个：

    //private static long String TAG = MainViewModel.class.getSimpleName();
    public LiveData<List<MovieRoom>> movies;
    private AppDatabase database;


    /*
        Use constructor to initialize all data that UI needs to populate
     */
    public MainViewModel(@NonNull Application application) {
        super(application);

        // how do I know this was initialized correctly?
        database = AppDatabase.getInstance(this.getApplication());
        //database = AppDatabase.getInstance(application);
        //movies is null after calling ViewModelProvider constructor

        // what are the values in movies?
        // _movieDao = null, mDatabase = null
        movies = database.movieDao().getAllMovies();
    }


    // Loads most popular movies
    public void loadData() {
        //MainViewModel.FetchMovieTask movies = new MainViewModel.FetchMovieTask();
        //movies.execute("popular");
        // Assign to movies
    }

    public void getAllMovies() {
        movies = database.movieDao().getAllMovies();
    }

    public void getPopularMovies() {
        movies = database.movieDao().getPopularMovies();
    }

    public void getTopRatedMovies() {
        movies = database.movieDao().getTopRatedMovies();
    }

    public void getFavoriteMovies() {
        movies = database.movieDao().loadFavoriteMovies(true);
    }

}

在我的主要活动中。java，按钮有一个onclick监听器，我将观察者设置为：

        b_pop.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //updateMovies("popular");
                mViewModel.getPopularMovies();
            }
        });

mViewModel.movies.observe(this, new Observer<List<MovieRoom>>() {
            @Override
            public void onChanged(@Nullable List<MovieRoom> movieEntries) {
                //Log.d(TAG, "Receiving database update from LiveData");
                // mMovieAdapter.mMovies = 0
                mMovieAdapter.clear();
                // movieEntries = 0
                // mMovieAdapter mMovies = 0
                mMovieAdapter.setList(movieEntries);
                mMovieAdapter.notifyDataSetChanged();
            }
        });

问题是，当您将ViewModel的“电影”变量重新分配给最高等级或最受欢迎的电影列表时，它不会触发onChanged（）。最高等级的电影列表与最受欢迎的电影列表相同，只是顺序不同。所以onChanged（）永远不会被触发，我将无法将其设置为适配器。

我正在从0学习ViewModel、LiveData和observer，所以不知道如何重新设计它，使其能够工作？

Answer 1

使用LiveData上的value将新获取的数据的值传递给ViewModel：

public void getAllMovies() {
        movies.setValue(database.movieDao().getAllMovies().getValue())
}

此外，您还需要可变的LiveData

public MutableLiveData<List<MovieRoom>> movies;