即使我知道对象存在，我的查询也会从数据库返回null时出现问题。我在这里已经阅读了几种解决方案并尝试了它，但是它没有用。所以这就是我想要做的，我有一个按钮，如果用户按下它，则在将数据库添加为收藏的电影之前，它会检查数据库；当用户再次点击该按钮时，它会检查该电影是否已存在于数据库中，如果是，则将其从数据库中删除。请这是我的代码...
//this is from my detail class to display the details of the movie selected
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.detail_activity);
playTrailer = findViewById(R.id.playTrailer);
playTrailer.setImageResource(R.drawable.play_icon);
starButton = findViewById(R.id.floatingActionButton);
movieViewModel = ViewModelProviders.of(this).get(MovieViewModel.class);
populateUI();
}
//here I receive an intent from the main activity then display all the movie information on the detail ACTIVITY. populates the UI
public void populateUI(){
imageView = findViewById(R.id.imageView3);
movieTitle = findViewById(R.id.textView2);
moviePlot = findViewById(R.id.moviePlot2);
ratings = findViewById(R.id.ratings);
releaseDate = findViewById(R.id.releaseDate2);
rateStar = findViewById(R.id.ratingBar);
contentReview = findViewById(R.id.reviewContent);
final Movie movieData = getIntent().getParcelableExtra("MOVIE_DATA");
movieID = movieData.getMovieId();
setTitle(movieData.getMovieTitle());
movieTitle.setText(movieData.getMovieTitle());
moviePlot.setText(movieData.getMoviePlot());
ratings.setText(movieData.getMovieRating());
releaseDate.setText(movieData.getReleaseDate());
rateStar.setRating(Float.parseFloat(movieData.getMovieRating()));
if(movieData.getBackDrop().isEmpty() || movieData.getBackDrop().equals("null")){
imageUrl.append("original/").append(movieData.getMovieImage());
Picasso.with(getApplicationContext()).
load(imageUrl.toString())
.error(R.drawable.image1)
.placeholder(R.drawable.image1)
.into(imageView);
return;
}
imageUrl.append("original/").append(movieData.getBackDrop());
Picasso.with(getApplicationContext()).load(imageUrl.toString()).into(imageView);
TrailerAsyncTask asyncTask1 = new TrailerAsyncTask();
ReviewAsyncTask asyncTask2 = new ReviewAsyncTask();
asyncTask1.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,trailerUrl);
asyncTask2.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, reviewsUrl);
playTrailer = findViewById(R.id.playTrailer);
trailerTextView = findViewById(R.id.playTrailer2);
Log.v("change","the movie id before the click is: "+movieID);
starButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Movie temp2 = movieViewModel.getMovieID(movieID);
if(temp2 == null){
Log.v("change", " movie is null");
movieViewModel.insert(movieData);
} else if (temp2.getMovieId().equals(movieData.getMovieId())){
Log.v("change", ": "+movieData.getMovieTitle()+ " was deleted from database");
movieViewModel.delete(movieID);
}
}
});
}
// here is my movie repository which provided data to the view model which will then send data to the detail activity UI
public class MovieRepository {
private MovieDao mMovieDao;
private LiveData<List<Movie>> mAllMovies;
private static Movie temp;
public MovieRepository(Application application){
MovieRoomDatabase db = MovieRoomDatabase.getDatabase(application);
mMovieDao = db.movieDao();
mAllMovies = mMovieDao.getAllMovie();
Log.v("change", "database constructor called");
}
public Movie getMovie(String movie){
new getMovieIDAsyncTask(mMovieDao).execute(movie);
return temp;
}
public LiveData<List<Movie>> getmAllMovies(){
return mAllMovies;
}
public void insert(Movie movie){
new insertMovieAsyncTask(mMovieDao).execute(movie);
}
public void delete(String movie){
new deleteMovieAsyncTask(mMovieDao).execute(movie);
}
public static class getMovieIDAsyncTask extends AsyncTask<String,Void, Movie>{
private MovieDao mMovieIDAsyncTask;
getMovieIDAsyncTask(MovieDao dao){
mMovieIDAsyncTask = dao;
}
@Override
protected Movie doInBackground(String... movies) {
temp = mMovieIDAsyncTask.getMovie(movies[0]);
return temp;
}
}
public static class insertMovieAsyncTask extends AsyncTask<Movie,Void,Void>{
private MovieDao mMovieAsyncTaskDao;
insertMovieAsyncTask(MovieDao dao){
mMovieAsyncTaskDao = dao;
}
@Override
protected Void doInBackground(final Movie... movies) {
mMovieAsyncTaskDao.insert(movies[0]);
return null;
}
}
public static class deleteMovieAsyncTask extends AsyncTask<String,Void,Void>{
private MovieDao mMovieAsyncTaskDao;
deleteMovieAsyncTask(MovieDao dao){
mMovieAsyncTaskDao = dao;
}
@Override
protected Void doInBackground(String... movies) {
mMovieAsyncTaskDao.deleteMovie(movies[0]);
return null;
}
}
}
// here is my Doa
@Dao
public interface MovieDao {
@Insert(onConflict = OnConflictStrategy.REPLACE)
void insert(Movie movie);
@Delete
void delete(Movie movie);
@Query("SELECT * from movie_database ORDER BY title ASC")
LiveData<List<Movie>> getAllMovie();
@Query("SELECT * FROM MOVIE_DATABASE WHERE movieID = :movieID")
Movie getMovie(String movieID);
@Query("DELETE FROM MOVIE_DATABASE WHERE movieID = :movieID")
void deleteMovie(String movieID);
}
//here is my view model
public class MovieViewModel extends AndroidViewModel{
private MovieRepository movieRepository;
private MovieRepositoryNetwork movieRepositoryNetwork;
private LiveData<List<Movie>> mAllMovies;
private LiveData<List<Movie>> dataFromNetwork;
public MovieViewModel(@NonNull Application application) {
super(application);
// Log.v("change"," view model constructor is called");
movieRepository = new MovieRepository(application);
movieRepositoryNetwork = new MovieRepositoryNetwork();
}
public LiveData<List<Movie>> getFavoritesFromDatabase(){
// Log.v("change","favorite movies is called from view model");
dataFromNetwork = movieRepository.getmAllMovies();
return dataFromNetwork;
}
public void getPopularMovies(){
// Log.v("change","popular movies is called from view model");
dataFromNetwork = movieRepositoryNetwork.getPopularMoviesFromNetwork();
}
public void getHighRatedMovies(){
// Log.v("change","hih rated movies is called from view model");
dataFromNetwork = movieRepositoryNetwork.getHighRatedFromNetwork();
}
public LiveData<List<Movie>> getDataFromNetwork(){
return dataFromNetwork;
}
public void insert(Movie movie){
movieRepository.insert(movie);
}
public void delete(String movie){
movieRepository.delete(movie);
}
public Movie getMovieID(String movieID){
return movieRepository.getMovie(movieID);
}
}
// here's also a link to my GitHub page https://github.com/Ben-Rapha/android-movie-app-project