未调用 LiveData observe（）

问：

我的观察者回调没有被调用，你能指出我做错了什么吗？我的应用程序非常简单，因为我只是在测试这个架构

MainActivity 代码

viewModel = ViewModelProviders.of(this).get(UpcomingMoviesViewModel.class);
viewModel.getUpcomingMovies().observe(this, upcomingMovies -> {
        // NOT CALLED !!!
        for(UpcomingMovies movie : upcomingMovies) {
            Log.d("Upcoming", "Movie - " + movie.movie.title);
        }
    });

View模型代码

private LiveData<List<UpcomingMovies>> mUpcomingMovies;
private MovieRepository movieRepository;

public UpcomingMoviesViewModel(@NonNull Application application ) {
    super(application);
    movieRepository = MovieRepository.getsInstance(application); 
}

public LiveData<List<UpcomingMovies>> getUpcomingMovies() {
    mUpcomingMovies = movieRepository.getUpcomingMovies();
    return mUpcomingMovies;
}

还有我的存储库方法

public LiveData<List<UpcomingMovies>> getUpcomingMovies(){
    if(movieDao.loadUpcomingMovies().getValue() == null) {
        webService.getUpcomingMovies(api_key).
                enqueue(new Callback<MovieResults>() {

                    @Override
                    public void onResponse(Call<MovieResults> call, Response<MovieResults> response) {
                        MovieResults results = response.body();
                        if(results == null) return;
                        new insertAsyncTask(movieDao).execute(results.movies.toArray(new Movie[results.movies.size()]));
                    }

                    @Override
                    public void onFailure(Call<MovieResults> call, Throwable t) {
                        Log.d("Retrofit error", t.getMessage());
                    }
                });
    }
    return movieDao.loadUpcomingMovies();
}

private static class insertAsyncTask extends AsyncTask<Movie, Void, Void> {

    private final MovieDao mAsyncTask;

    insertAsyncTask(MovieDao dao){
        mAsyncTask = dao;
    }

    @Override
    protected Void doInBackground(Movie... movies) {
        long [] results = mAsyncTask.insertMovies(Arrays.asList(movies));
        long [] results2 = mAsyncTask.insertUpcomingMovies(Arrays.asList(movies));
        return null;
    }
}

两者都返回 20 个数字，我将其归因于我收到 20 部电影的事实。 我在这里做错了什么？感谢您抽出宝贵时间接受采访resultsresults2