我正在尝试使用 id 查询保存在房间本地数据库中的一些数据。我已经从一个序列化对象接收到 id，并将它传递给我的 View 模型类，该类调用存储库以使用 dao 执行查询。
asynctask 用于使事物异步并从 db 返回请求的值。我创建了一个接口(interface)并在异步任务中对其进行了初始化，因此我可以从 db 返回结果，但它因 NPE 而失败。
在 Activity 中，我调用了存储库并传递了序列化数据，例如:
public class ViewSessionActivity extends AppCompatActivity implements View.OnClickListener{
private static final String TAG = ViewSessionActivity.class.getSimpleName();
private Toolbar toolbar;
private Sessions sessions; // serialized object from adapter
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_view_session);
init();
setSupportActionBar(toolbar);
}
private void init() {
// skipped fvid calls
sessions = (Sessions) getIntent().getSerializableExtra(Constants.SESSION_DETAIL_KEY);
Log.d(TAG, "Prog id:\t" + sessions.prog_sessionId);
//error occurs here which leads to async task call in repository
SessionsRepository.getRepository(getApplication()).getSessionByID(sessions.prog_sessionId);
}
在存储库中:
public class SessionsRepository {
private static final String TAG = SessionsRepository.class.getSimpleName();
private SessionsDAO dao;
private static SessionsRepository repository = null;
private LiveData<List<Sessions>> allSessions;
private MutableLiveData<Boolean> loadingState;
private Context context;
private final SportsDatabase database;
public SessionsRepository(Application application) {
database = SportsDatabase.getInstance(application);
dao = database.sessionsDAO();
allSessions = dao.getAllSessions();
loadingState = new MutableLiveData<>();
context = application.getApplicationContext();
}
public static SessionsRepository getRepository(Application context){
if (repository == null){
repository = new SessionsRepository(context);
}
return repository;
}
public void fetchSessions() {
String coachId = new PrefsUtils(context).getCoachId();
Call<SessionDetails> call = RestClient.getRestInstance().getSessionsService().fetchSessions(coachId);
call.enqueue(new Callback<SessionDetails>() {
@Override
public void onResponse(Call<SessionDetails> call, Response<SessionDetails> response) {
if (response.isSuccessful()) {
loadingState.postValue(false); // remove progress
SessionDetails details = response.body();
List<Sessions> sessions = details.getSessions();
// Log.d(TAG, "N/w sesh size:\t" + sessions.size());
saveSessions(sessions);
}
}
@Override
public void onFailure(Call<SessionDetails> call, Throwable t) {
loadingState.postValue(false);
Toast.makeText(context, "Error Fetching Sessions", Toast.LENGTH_SHORT).show();
}
});
}
private void saveSessions(List<Sessions> sessions) {
new SaveSessionsTask(dao).execute(sessions);
}
// this method which calls the async task
public void getSessionByID(String id){
new FindSessionTask(dao, context).execute(id);
}
public LiveData<List<Sessions>> getSavedSessions() {
return allSessions;
}
public class SaveSessionsTask extends AsyncTask<List<Sessions>, Void, Void> {
private SessionsDAO dao;
public SaveSessionsTask(SessionsDAO dao) {
this.dao = dao;
}
@Override
protected Void doInBackground(List<Sessions>... lists) {
dao.addSessions(lists[0]);
return null;
}
}
public class FindSessionTask extends AsyncTask<String, Void, Sessions>{
private SessionsDAO dao;
private OnSessionResultCallback callback;
public FindSessionTask(SessionsDAO dao, Context context) {
this.dao = dao;
this.callback = (OnSessionResultCallback) context;
}
@Override
protected Sessions doInBackground(String... strings) {
Sessions sessions = dao.getSessionById(strings[0]);
return sessions;
}
@Override
protected void onPostExecute(Sessions sessions) {
super.onPostExecute(sessions);
Log.d(SessionsRepository.TAG, "Session name:\t" + sessions.session_name);
}
}
// need help in returning the session found from db in onpostexecute method
}
道类:
@Dao
public interface SessionsDAO {
@Insert
void addSessions(List<Sessions> sessions);
@Query("select * from sessions")
LiveData<List<Sessions>> getAllSessions();
@Query("select * from sessions where prog_sessionId = :id")
Sessions getSessionById(String id); // here
}
和接口(interface)回调以在后执行方法上接收异步任务中的数据:
public interface OnSessionResultCallback {
void onSessionFound(Sessions sessions);
}
logcat 错误:
Caused by: java.lang.ClassCastException: sashi.in.ecosports.extras.App cannot be cast to sashi.in.ecosports.interfaces.OnSessionResultCallback
at sashi.in.ecosports.rest.db.repositories.SessionsRepository$FindSessionTask.<init>(SessionsRepository.java:110)
at sashi.in.ecosports.rest.db.repositories.SessionsRepository.getSessionByID(SessionsRepository.java:82)
at sashi.in.ecosports.ui.activities.ViewSessionActivity.init(ViewSessionActivity.java:63)
at sashi.in.ecosports.ui.activities.ViewSessionActivity.onCreate(ViewSessionActivity.java:38)
at android.app.Activity.performCreate(Activity.java:6671)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
....
我原本希望注销找到的 session 名称，但却遇到了 NPE。有人可以帮我执行房间查询异步，使用实体字段作为搜索字符串从列表中返回整个实体。 View 模型如何接收数据，我的接口(interface)是否必要？谢谢。
最佳答案
由于您在尝试打印 session.session_name 时遇到 NPE，这意味着您返回的 session 对象为空，这可能会发生:
查询错误(因为您的选择查询没问题，我猜您传递了错误的 ID)
当您尚未插入任何具有该 ID 的记录时(在这种情况下，您必须调试插入过程并检查数据库以查看插入是否正确)
要在运行时检查数据库内容，您可以轻松使用 Stetho .
关于房间的更新查询，您可以使用内置的 @Update 注释，它通过主键查找传递的对象并更新所有列或使用 @Query 用于更新特定列的注释，例如 @Query("UPDATE sessions SET session_name = :newName WHERE prog_sessionId = :id")，它使用 session ID 更新 session_name 列id 的。下面是一个示例代码:
@Dao
public interface SessionsDAO {
@Query("SELECT * FROM sessions") // Retrieve all columns saved in sessions table
LiveData<List<Sessions>> getAllSavedSessions();
@Query("SELECT * FROM sessions WHERE prog_sessionId = :id") // Retrieve a single session with all columns saved in sessions table
Sessions findSessionWithId(String id);
@Query("SELECT session_name FROM sessions WHERE prog_sessionId = :id") // Retrieve a single session name saved in sessions table
String getSessionNameWithId(String id);
@Update // Update all non-primary columns of a session
void updateSession(Sessions session);
@Query("UPDATE sessions SET session_name = :newName") // Update session_name column of all saved sessions
void updateAllSessionNamesTo(String newName);
@Query("UPDATE sessions SET session_name = :newName WHERE prog_sessionId = :id") // Update session_name column of a session located with its id
void updateSessionNameTo(String newName, String id);
@Query("UPDATE sessions SET session_name = :newName AND session_date = :newDate WHERE prog_sessionId = :id") // Update session_name and session_date column of a session located with its id
void updateSessionNameAndDateTo(String newName, long newDate, String id);
}
如果您需要在工作线程上运行查询，您可以使用 java 执行器，这是一个检索 session 名称的简单示例:
@Dao
public abstract class SessionsDAO {
void getSessionNameWithId(final String id, final NameLoadedListener onFinishListener) {
Executors.newSingleThreadExecuter().submit(new Runnable() {
@Override
public void run() {
String name = getSessionNameWithId(id);
onFinishListener.onNameLoaded(name);
}
});
}
@Query("SELECT session_name FROM sessions WHERE prog_sessionId = :id") // Retrieve a single session name saved in sessions table
abstract String getSessionNameWithId(String id);
public interface NameLoadedListener {
void onNameLoaded(String name);
}
}
下面是一个通过检索实体并在最后返回更新的实体来更新 session 名称的示例。
@Dao
public abstract class SessionsDAO {
void updateSessionNameWithId(final String id, final String newName, final SessionUpdateListener onFinishListener) {
Executors.newSingleThreadExecuter().submit(new Runnable() {
@Override
public void run() {
updateSessionName(id, newName, onFinishListener);
}
});
}
@Transaction
void updateSessionName(String id, String newName, SessionUpdateListener onFinishListener) {
Sessions session = findSessionWithId(id);
session.setName(newName);
updateSession(session);
onFinishListener.onSessionUpdated(session);
}
@Query("SELECT * FROM sessions WHERE prog_sessionId = :id") // Retrieve a single session with all columns saved in sessions table
Sessions findSessionWithId(String id);
@Update // Update all non-primary columns of a session
void updateSession(Sessions session);
public interface SessionUpdateListener {
void onSessionUpdated(Sessions session);
}
}
希望你能收到备忘录。
关于Android 房间查询在 Repository 空指针异常中使用 AsyncTask，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/55347191/