我正在尝试使用ID查询保存在会议室本地数据库中的某些数据。我已经从序列化对象接收到ID，并将其传递给我的视图模型类，该视图模型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。我已经制作了一个接口，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NPE失败。
在活动中，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);
}
以及在存储库中：
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
}
dao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
}
和接口回调，以通过后执行方法在异步任务中接收数据：
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会话名称，但得到了NPE。有人可以帮我执行异步房间查询吗，使用实体字段作为搜索字符串从列表中返回整个实体。视图模型如何接收数据，我的界面是否必要？谢谢。
答案 0 :(得分：2)
由于尝试打印session.session_name时会得到NPE，这意味着返回的会话对象为null，这可能会发生：
查询不正确（因为您选择的查询很好，我猜您传递的是错误的ID）
尚未插入具有该ID的任何记录时（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，可以轻松使用description。
关于带空间的更新查询，您还可以使用内置的@Update注释，该注释通过其主键查找传递的对象并更新所有列，或者使用@Query注释更新特定的列（ s）像@Query("UPDATE sessions SET session_name = :newName WHERE prog_sessionId = :id")，它用会话ID为session_name更新id列。下面是示例代码：
@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);
}
如果需要在辅助线程上运行查询，可以使用Java执行程序，这是一个检索会话名称的简单示例：
@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);
}
}
这是通过检索实体并最后返回更新的实体来更新会话名称的示例。
@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);
}
}
希望您能得到备忘录。