如何在房间数据库类中获取或观察实时数据

Question

I have created an entity along with corresponding dao. In dao class, i have a following method:

@Query("select * from Book")
LiveData<List<Book>> getAll();

I have observed this live data in activities and fragments. Now i wanted to get the book list when database opened its connection. I want this operation inside room database class. Actually inside onOpen() method.

public abstract class AppDatabase extends RoomDatabase {
...
    public static synchronized AppDatabase getInstance(Context context) {
        if (instance == null) {
            instance = Room.databaseBuilder(context.getApplicationContext(),
                    AppDatabase.class, DATABASE_NAME)
                    .fallbackToDestructiveMigration()
                    .addCallback(roomCallback)
                    .build();
        }
        return instance;
    }
...
    private static RoomDatabase.Callback roomCallback = new 
        RoomDatabase.Callback() {
        @Override
        public void onCreate(@NonNull SupportSQLiteDatabase db) {
            super.onCreate(db);
            new PopulateDbAsyncTask(instance).execute();
        }

        @Override
        public void onOpen(@NonNull SupportSQLiteDatabase db) {
            super.onOpen(db);
            // get book list information and do some task
        }
    };
}

I am following the android mvvm architecture pattern. I have some questions at this point. Is it possible to get the value or observe the livedata(book) inside onOpen() method?. I know I could do this by writing another method in dao class which doesn't return livedata, but will it be a good practice?

Answer 1

You can get the liveData by calling the dao.getAll(). But in general it's not recommended. You will be getting the liveData from the model. Check out this sample for recommended way of handling the data and the use of liveData. https://github.com/googlesamples/android-architecture-components/tree/master/BasicSample