合並兩個具有相同觀察者類型的LiveData對象

Question

我有兩個Room實體，這兩個實體均來自相同的自定義基類。

@Entity
public class BaseEntity {}

@Entity
public class EntityA extends BaseEntity {
    ...
}

@Entity
public class EntityB extends BaseEntity {
    ...
}

這兩個派生類都有對應的Dao接口。

@Dao
public interface BaseDao {}

@Dao
public interface DaoA extends BaseDao {
    @Query("SELECT * FROM EntityA")
    public LiveData<List<EntityA>> getAll();
}

@Dao
public interface DaoB extends BaseDao {
    @Query("SELECT * FROM EntityB")
    public LiveData<List<EntityB>> getAll();
}

兩個表中的數據足夠多樣化，可以分別存儲它們，但是我的數據訪問方法是相同的。 因此，我想使用一個Repository類來同時從兩個表中返回條目。

public class Repository {
    private List<BaseDao> daos;
    private LiveData<List<BaseEntity>> entities;

    public Repository(Application application) {
        final EntityDatabase database = EntityDatabase.getInstance(application);
        daos = new ArrayList();
        daos.add(database.daoA());
        daos.add(database.daoB());
        entities = /** Combine entities from all daos into one LiveData object */;
    }

    public LiveData<List<BaseEntity>> getEntities() {
        return entities;
    }
}

有什么方法可以將daoA.getAll（）和daoB.getAll（）的結果合並到單個LiveData<List<BaseEntity>>對象中？

Answer 1

我想出了使用MediatorLiveData的解決方案。

public class Repository {
    private DaoA daoA;
    private DaoB daoB;

    public Repository(Application application) {
        final EntityDatabase database = EntityDatabase.getInstance(application);
        daos = new ArrayList();
        daoA = database.daoA();
        daoB = database.daoB();
    }

    public LiveData<List<BaseEntity>> getEntities() {
        return mergeDataSources(
            daoA.getAll(), 
            daoB.getAll());
    }

    private static LiveData<List<BaseEntity>> mergeDataSources(LiveData... sources) {
        MediatorLiveData<List<BaseEntity>> mergedSources = new MediatorLiveData();
        for (LiveData source : sources) {
            merged.addSource(source, mergedSources::setValue);
        }
        return mergedSources;
    }
}