合并两个具有相同观察者类型的 LiveData 对象

Question

我有两个 Room 实体，它们都派生自同一个自定义基类。

@Entity
public class BaseEntity {}

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

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

两个派生类都有对应的 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();
}

两个表中的数据多样性足以将它们分开存储，但是我的数据访问方法是相同的。因此，我想使用单个 Repository 类同时返回两个表中的条目。

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;
    }
}

有没有办法可以将 daoA.getAll() 和 daoB.getAll() 的结果组合成一个LiveData<List<BaseEntity>>对象？

Answer 1

我想出了一个使用 MediatorLiveData 的解决方案。

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;
    }
}