Question

我有一个应用程序，我想存储一个项目列表，我认为数据库可以做。我发现了新的Room API并尝试使用它，尽管我在使用它时遇到了一些麻烦。我有一个后台服务，应该写入数据库的条目。我读到了使用单例模式的建议，但我似乎无法让它工作。当我尝试检索MainActivity中的所有条目时，我返回的列表始终为空，表示我无法从一开始就保存它们。

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;
    }
}

实体

@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);
}

后台服务，保存到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是从db读取的，使用Async任务所以它不会阻止UI private void updateTemperature（）{ Log.d（TAG，＆＃34; updateTemperature＆＃34;）;

    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();
}

也许它与某种情况有关？

编辑：

刚刚在构建数据库时尝试添加.allowMainThreadQueries()，现在它可以运行了。所以出于某种原因我的异步任务不起作用？

Answer 1

您的AsyncTask似乎有误。你应该在doInBackground中返回你想要处理的列表，然后在onPostExecute中预期它。为什么总是返回0？

在Activity中使用AsyncTask失败

1 个答案: