Android：通過Room數據庫獲取具有特定代碼的商品計數

Question

我實質上是在嘗試創建某種搜索功能。 我正在使用Room數據庫，並且每個元素都有用戶提交的特定3位代碼。 但是，當我嘗試獲取具有特定3位代碼的元素數並將其顯示在“主要活動”中的Toast消息中時，結果總是為0。

表的每個實體都是我創建的Word類的對象，如下所示：

@Entity(tableName = "word_table")
public class Word {
    @PrimaryKey(autoGenerate = true)
    @NonNull
    private int uid;

    @ColumnInfo(name = "word_code")
    private int wordCode;

    @ColumnInfo(name = "word")
    private String word;

    public Word(int uid, int wordCode, String word){
        this.uid = uid;
        this.wordCode = wordCode;
        this.word = word;
    }
    public int getUid(){
        return uid;
    }
    public int getWordCode(){
        return wordCode;
    }
    public String getWord(){
        return word;
    }
}

我的DAO類如下所示：

@Dao
public interface WordDao {
    @Insert
    void insert(Word word);


    @Query("SELECT COUNT(*) FROM word_table WHERE word = :selectedCode")
    LiveData<Integer> getWordWithCode(int selectedCode);


}

我擴展RoomDatabase的類如下所示：

@Database(entities = {Word.class}, version = 1, exportSchema = false)
public abstract class WordRoomDatabase extends RoomDatabase {
    public abstract WordDao wordDao();

    private static WordRoomDatabase INSTANCE;
    public static WordRoomDatabase getDatabase(final Context context){
        if (INSTANCE == null){
            synchronized (WordRoomDatabase.class){
                if (INSTANCE==null){
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(), WordRoomDatabase.class, "word_database")
                    .fallbackToDestructiveMigration()
                            .addCallback(sDatabasePopulateCallback)
                    .build();
                }
            }
        }
        return INSTANCE;
    }
    private static RoomDatabase.Callback sDatabasePopulateCallback = new RoomDatabase.Callback(){
        @Override
        public void onCreate(@NonNull SupportSQLiteDatabase db) {
            super.onCreate(db);
            new PopulateDatabaseAsync(INSTANCE).execute();
        }
    };
    private static class PopulateDatabaseAsync extends AsyncTask<Void, Void, Void> {
        private WordDao mWordDao;
        PopulateDatabaseAsync(WordRoomDatabase db){
            mWordDao = db.wordDao();
        }
        int[] discCodes = {232, 432, 523,233,242,554,221};
        String[] discNames = {"First Word", "Second Word", "Third Word", "Fourth Word", "Fifth Word", "Sixth word", "Seventh Word"};
        @Override
        protected Void doInBackground(Void... voids) {
            for (int i = 0;i < discCodes.length; i++){
                Word newWord = new Word(0, discCodes[i], discNames[i]);
                mWordDao.insert(newWord);
            }
            return null;
        }
    }
}

我正在使用discCodes和discNames數組來填充列表作為示例。

這是我的存儲庫的外觀：

public class WordRepository {
    private WordDao mWordDao;
    private LiveData<Integer> mCount;

    int thiscount;

    WordRepository(Application application){
        WordRoomDatabase db = WordRoomDatabase.getDatabase(application);
        mWordDao = db.wordDao();
    }

    public void insert(Word word){
        new insertWordAsync(mWordDao).execute(word);
    }
    private static class insertWordAsync extends AsyncTask<Word, Void, Void>{
        private WordDao mDao;
        insertWordAsync(WordDao dao){
            mDao = dao;
        }
        @Override
        protected Void doInBackground(Word... words) {
            mDao.insert(words[0]);
            return null;
        }
    }

    public LiveData<Integer> getCount(int givenCode){
        return mWordDao.getWordWithCode(givenCode);
    }

}

這是ViewModel類：

public class WordViewModel extends AndroidViewModel {
    private WordRepository mRepo;

    public WordViewModel(@NonNull Application application) {
        super(application);
        mRepo = new WordRepository(application);

    }
    public void insert(Word word){
        mRepo.insert(word);
    }


    public LiveData<Integer> getCount(int givenCode){
        return mRepo.getCount(givenCode);
    }
}

最后是MainActivity類：

public class MainActivity extends AppCompatActivity {
    private WordViewModel mWordViewModel;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mWordViewModel = ViewModelProviders.of(this).get(WordViewModel.class);


        mWordViewModel.getCount(242).observe(this, new Observer<Integer>() {
            @Override
            public void onChanged(Integer integer) {
                String toShow = Integer.toString(integer);
                Toast.makeText(getApplicationContext(), toShow, Toast.LENGTH_SHORT).show();
            }
        });
    }
}

現在，在此特定示例中，我將代碼242作為搜索的基礎。 我首先嘗試使用int而不是LiveData的返回類型，這意味着我的DAO查詢將如下所示：

@Query("SELECT COUNT(*) FROM word_table WHERE word = :selectedCode")
    int getWordWithCode(int selectedCode);

經過一些搜索后，我發現使用LiveData和Observer是更可取的。 但是，當我運行該應用程序時，而不是Toast消息中得到1，而是得到0。

我是android開發的新手，所以我為凌亂的代碼和粘貼所​​有這些代碼表示歉意。 我一直在通過Google Codelabs教程學習有關Room的知識，因此，如果對Room有了更好的了解可以解決我的問題，請隨時向我建議任何學習資源。 提前致謝！

Answer 1

您實際上是在搜索word而不是wordCode

在WordDao嘗試此代碼

@Query("SELECT COUNT(*) FROM word_table WHERE word_code = :selectedCode")
LiveData<Integer> getWordWithCode