Using AsyncTask in Activity fails

Question

I have an application in which I wanted to store a list of items and I figured a database would do. I found out about the new Room API and tried using it, though I'm having some trouble getting it working. I have a background service which is supposed to write entries to the database. I read that using the singleton pattern was recomended, but I can't seem to get it working. When I try to retrieve all entries in my MainActivity, the list I get back is always empty, indicating that I wasn't able to save them from the start.

Singleton db class

@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;
    }
}

Entity

@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;
    }
}

EntityDAO

@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);
}

Background service which saves to db

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 were db is read from, uses Async task so it doesn't block UI private 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();
}

Maybe it has to do with the context somehow?

EDIT:

Just tried adding .allowMainThreadQueries() when building the database and now it works. So for some reason my Async task isn't working?

Answer 1

Your AsyncTask seems to be wrong. You should return the list you would like to handle in doInBackground and then expect it in onPostExecute. Why do you always return 0?