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<>> getAllBudgets();

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

}

如何让我的存储库中的 AsyncTask 返回字符串列表？现在代码是 public void getAllCategories() {new getAllCatsAsyncTask(mBudgetDao).execute(); 当我将 void 更改为 List 时，错误显示我无法返回字符串列表

我的仓库:

public class BudgetRepository {

    private BudgetDao mBudgetDao;
    private LiveData<>> mAllBudgets;

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

    LiveData<>> 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 {
        //code
    }

    private class getAllCatsAsyncTask extends AsyncTask> {
        private BudgetDao mAsyncTaskDao;

        getAllCatsAsyncTask(BudgetDao dao) {mAsyncTaskDao = dao;}

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

    }

} 

在我的 ViewModel 中:

public class BudgetViewModel extends AndroidViewModel {

    private BudgetRepository mRepository;
    private LiveData<>> mAllBudgets;

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

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

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

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

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 onPostExecute 的结果，以便 getAllCategories 可以在我的 AsyncTask 中返回字符串列表？或者是否有更好的方法从我的 Dao 中调用查询？

câu trả lời hay nhất

试试这段代码..

@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
{
    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);
   }
}

如果你想在 mainThread 上允许，那么设置这一行。

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

关于Android - Room Repository 和 AsyncTask，如何使用查询从表中取回数据，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/53514909/