[英]Using AsyncTask in Activity fails
我有一個應用程序,我想在其中存儲項目列表,並確定數據庫可以做到。 我發現了新的Room API,並嘗試使用它,盡管在使其正常工作時遇到了一些麻煩。 我有一個后台服務,應該將條目寫入數據庫。 我讀到建議使用單例模式,但似乎無法正常工作。 當我嘗試檢索MainActivity中的所有條目時,返回的列表始終為空,表明我從一開始就無法保存它們。
單例數據庫類
@Database(entities = {TemperatureReading.class}, version = 1)
public abstract class DatabaseSingleton extends RoomDatabase {
private static DatabaseSingleton INSTANCE;
public abstract TemperatureReadingDao temperatureReadingDao();
public static DatabaseSingleton getAppDatabase(Context context) {
if (INSTANCE == null) {
INSTANCE =
Room.databaseBuilder(context.getApplicationContext(), DatabaseSingleton.class, "fireTempDatabase")
.build();
}
return INSTANCE;
}
public static void destroyInstance() {
INSTANCE = null;
}
}
實體
@Entity
public class TemperatureReading {
@PrimaryKey(autoGenerate = true)
private int uid;
@ColumnInfo(name = "dateTime")
private long dateTime;
@ColumnInfo(name = "location")
private String readingLocation;
@ColumnInfo(name = "value")
private float value;
public long getDateTime() {
return dateTime;
}
public void setDateTime(long dateTime) {
this.dateTime = dateTime;
}
public String getReadingLocation() {
return readingLocation;
}
public void setReadingLocation(String readingLocation) {
this.readingLocation = readingLocation;
}
public float getValue() {
return value;
}
public void setValue(float value) {
this.value = value;
}
public int getUid() {
return uid;
}
public void setUid(int uid) {
this.uid = uid;
}
}
實體DAO
@Dao
public interface TemperatureReadingDao {
@Query("SELECT * FROM temperatureReading")
List<TemperatureReading> getAll();
@Query("SELECT * FROM temperatureReading ORDER BY uid desc limit 1")
TemperatureReading getLatest();
@Insert
void insertAll(TemperatureReading... temperatureReading);
@Update
void update(TemperatureReading... temperatureReading);
@Delete
void delete(TemperatureReading temperatureReading);
}
保存到數據庫的后台服務
private void saveTempDatabase(float tmpMessageAsFloat, long tmpMessageDateTime) {
Log.d(TAG, "saveTempDatabase");
TemperatureReading tr = new TemperatureReading();
tr.setDateTime(tmpMessageDateTime);
tr.setReadingLocation("XXX"); //TODO
tr.setValue(tmpMessageAsFloat);
DatabaseSingleton.getAppDatabase(getApplicationContext()).temperatureReadingDao().insertAll(tr);
}
MainActivity是從中讀取db的對象,使用Async任務,因此不會阻止UI私有void updateTemperature(){Log.d(TAG,“ updateTemperature”);
new AsyncTask<Void, Void, Integer>() {
@Override
protected Integer doInBackground(Void... params) {
List<TemperatureReading> tr = DatabaseSingleton.getAppDatabase(MainActivity.this).temperatureReadingDao().getAll(); //List is always empty, no matter how many times I have called the saveTempDatabase() method in the service class.
return 0;
}
@Override
protected void onPostExecute(Integer agentsCount) {
}
}.execute();
}
也許與上下文有關?
編輯:
剛剛嘗試在構建數據庫時添加.allowMainThreadQueries()
,現在它可以工作了。 因此,由於某種原因,我的異步任務無法正常工作?
您的AsyncTask似乎是錯誤的。 您應該在doInBackground中返回要處理的列表,然后在onPostExecute中使用它。 為什么總是返回0?
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