Android - Room Repository 和 AsyncTask，如何使用查询从表中取回数据

Question

如何使用 AsyncTask 从 Room 实体获取数据？我一直在查看有关 AsyncTask 的答案，但我不明白。

这是我的道：

@Dao
public interface BudgetDao {

    @Insert
    public void insert (Budget budget);

    @Update
    public void update (Budget budget);

    @Query("SELECT id FROM budget_table WHERE category = :category AND date = :date")
    int getId(String category, String date);

    @Query("SELECT * FROM budget_table")
    LiveData<List<Budget>> getAllBudgets();

    @Query("SELECT category FROM budget_table")
    List<String> getAllCategories();

}

如何让存储库中的 AsyncTask 返回字符串列表？现在代码是public void getAllCategories() {new getAllCatsAsyncTask(mBudgetDao).execute();当我把void改成List时，报错提示我不能返回字符串列表

我的仓库：

public class BudgetRepository {

    private BudgetDao mBudgetDao;
    private LiveData<List<Budget>> mAllBudgets;

    BudgetRepository(Application application) {
        BudgetRoomDatabase db = BudgetRoomDatabase.getDatabase(application);
        mBudgetDao = db.budgetDao();
        mAllBudgets = mBudgetDao.getAllBudgets();
    }

    LiveData<List<Budget>> getAllBudgets() {return mAllBudgets;}
    public void insert (Budget budget) {new insertAsyncTask(mBudgetDao).execute(budget);}
    public void getAllCategories() {new getAllCatsAsyncTask(mBudgetDao).execute();}
    //How do I return a list of strings?


    private static class insertAsyncTask extends AsyncTask<Budget, Void, Void> {
        //code
    }

    private class getAllCatsAsyncTask extends AsyncTask<Void, Void, List<String>> {
        private BudgetDao mAsyncTaskDao;

        getAllCatsAsyncTask(BudgetDao dao) {mAsyncTaskDao = dao;}

        @Override
        protected List<String> doInBackground(Void... voids) {
            return mAsyncTaskDao.getAllCategories();
        }

    }

} 

在我的 ViewModel 中：

public class BudgetViewModel extends AndroidViewModel {

    private BudgetRepository mRepository;
    private LiveData<List<Budget>> mAllBudgets;

    public BudgetViewModel(@NonNull Application application) {
        super(application);
        mRepository = new BudgetRepository(application);
        mAllBudgets = mRepository.getAllBudgets();
    }

    LiveData<List<Budget>> getAllBudgets() {return mAllBudgets;}

    public void insert(Budget budget) {mRepository.insert(budget);}

    public List<String> getAllCats() { return mRepository.getAllCategories();} 
    //Android Studios show incompatible types (Void vs List<String>)

现在，在我的 ViewModel 中，mRepository.getAllCategories 不返回字符串列表。

我必须在我的 AsyncTask 中调用 onPostExecute 吗？如何链接来自onPostExecute 的结果，以便getAllCategories 可以在我的AsyncTask 中返回字符串列表？或者有没有更好的方法从我的 Dao 中调用查询？

Answer 1

试试这个代码..

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    DisplayData displayData=new DisplayData(studentDatabase);
    displayData.execute();
 }
/**
 * this method display student.
 */
private void DisplayStudent() {
    mStudentList.addAll(studentDatabase.studentDao().getStudent());
}


  private class DisplayData extends AsyncTask<Void,Void,Void>
{
    private StudentDatabase studentDatabase;

    public DisplayData(StudentDatabase studentDatabase) {
        this.studentDatabase = studentDatabase;
    }

    @Override
    protected Void doInBackground(Void... params) {
        DisplayStudent();
        return null;
    }
    @Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
        mDialog.dismiss();
        mStu_adapter=new StudentAdapter(mStudentList,getApplicationContext());
        mStu_adapter.notifyDataSetChanged();
        mRvStudent.setAdapter(mStu_adapter);
        DividerItemDecoration divider=new DividerItemDecoration(getApplicationContext(),LinearLayout.VERTICAL);
        divider.setDrawable(ContextCompat.getDrawable(getApplicationContext(),R.drawable.divider));
        mRvStudent.addItemDecoration(divider);
   }
}

如果您想在 ma​​inThread 上允许，请设置此行。

      db = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, "database-name").allowMainThreadQueries().build();