问如何在房间数据库类中获取或观察livedata

EN

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

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
        }
    };
}