如何在使用LiveData时从房间检索自动生成的ID？

Question

我正在创建一个Android应用程序，它将存储价值列表，例如。特定用户定义名称下不同时间的温度值。我打算使用SQLite来存储值，我读到使用Room将为它提供一个ORM层，所以我使用了它。但是后来我遇到了一个异常，基本上说我不能从主线程打开数据库连接，所以我尝试使用LiveData进行插入和检索。现在我有两张桌子。我只是想展示它们的结构，而不是在句法上准确：

**PLACE_DETAILS**
place_id integer auto_increment
place_name string
latitude double
longitude double

**TEMPERATURE_DETAILS**
temperature_id integer auto_increment
place_id foreign key references place_details(place_id)
time_of_record timestamp
temperature float

最初，我考虑不强制外键关系，只在插入PLACE_DETAILS对象时检索生成的键，就像Hibernate所做的那样，并在进一步插入TEMPERATURE_DETAILS表时使用它。但是，从这个问题来看：

我发现DAO方法本身需要返回一个表示生成ID的长值。

@Insert
long insertPlaceDetails(PlaceDetails placeDetails);

但是，在ViewModel中运行的AsyncTask需要重写返回Void的doInBackground（）方法。

public class PlaceDetailsViewModel extends AndroidViewModel {

private final LiveData<List<PlaceDetails>> placeDetailsList;
private PlaceDatabase placeDatabase;

public PlaceDetailsViewModel(@NonNull Application application) {
    super(application);

    placeDatabase = PlaceDatabase.getDatabase(this.getApplication());
    placeDetailsList = placeDatabase.daoAccess().fetchAllPlaceDetails();
}

public LiveData<List<PlaceDetails>> getPlaceDetailsList() {
    return placeDetailsList;
}

public void addPlace(final PlaceDetails placeDetails) {
    Log.d("Adding", "Place: " + placeDetails.getPlaceName());
    new addAsyncTask(placeDatabase).execute(placeDetails);
}

private static class addAsyncTask extends AsyncTask<PlaceDetails, Void, Void> {

    private PlaceDatabase placeDatabase;

    addAsyncTask(PlaceDatabase placeDatabase) {
        this.placeDatabase = placeDatabase;
    }

    @Override
    protected Void doInBackground(PlaceDetails... placeDetails) {
        placeDatabase.daoAccess().insertPlaceDetails(placeDetails[0]);
        return null;
    }
}
}

那么，如何在代码中实际检索生成的ID呢？另外，如果LiveData不能提供这个值，并且关系是要解决的问题，那么如何根据PLACE_DETAILS表中自动生成的外键在TEMPERATURE_DETAILS表中插入值呢？web上的所有教程都给出了手动给定id的示例。

public class PlaceDetailsViewModel extends AndroidViewModel {

private final LiveData<List<PlaceDetails>> placeDetailsList;
private PlaceDatabase placeDatabase;
private long insertId = -1;

public long getInsertId() {
    return insertId;
}

public PlaceDetailsViewModel(@NonNull Application application) {
    super(application);

    placeDatabase = PlaceDatabase.getDatabase(this.getApplication());
    placeDetailsList = placeDatabase.daoAccess().fetchAllPlaceDetails();
}

public LiveData<List<PlaceDetails>> getPlaceDetailsList() {
    return placeDetailsList;
}

public void onPlaceInserted(long id) {
    insertId = id;
}

public void addPlace(final PlaceDetails placeDetails) {
    Log.d("Adding", "Place: " + placeDetails.getPlaceName());
    new addAsyncTask(placeDatabase).execute(placeDetails);
}

private class addAsyncTask extends AsyncTask<PlaceDetails, Void, Void> {

    private PlaceDatabase placeDatabase;

    addAsyncTask(PlaceDatabase placeDatabase) {
        this.placeDatabase = placeDatabase;
    }

    @Override
    protected Void doInBackground(PlaceDetails... placeDetails) {
        insertId = placeDatabase.daoAccess().insertPlaceDetails(placeDetails[0]);
        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        super.onPostExecute(result);
        onPlaceInserted(insertId);
    }
}
}

Answer 1

我认为可以通过在OnPostExecute中使用回调函数来获取ID：

private static class addAsyncTask extends AsyncTask<PlaceDetails, Void, Void> {

    private PlaceDatabase placeDatabase;
    private PlaceDetails place;
    private int insertId = -1;

    addAsyncTask(PlaceDatabase placeDatabase) {
        this.placeDatabase = placeDatabase;
    }

    @Override
    protected Void doInBackground(PlaceDetails... placeDetails) {
        place = placeDetails[0];
        insertId = placeDatabase.daoAccess().insertPlaceDetails(place);
        return null;
    }

     @Override
     protected void onPostExecute(Void result) {
          super.onPostExecute(result);
          onPlaceInserted(insertId, place);
     }
}

void onPlaceInserted(int id, PlaceDetails place) {
    // you get ID here
}