mvvm+rxjava2+retrofit+room实现先从缓存取数据然后再从服务端获取数据覆盖的详细代码

以下是一个简单的示例代码，演示了如何使用MVVM + RxJava2 + Retrofit + Room实现从缓存中获取数据，然后再从服务端获取数据并覆盖缓存。

1. 定义数据模型

@Entity
public class User {
    @PrimaryKey
    public int id;

    public String name;

    public String email;

    // getters and setters
}

2. 定义数据访问对象（DAO）

@Dao
public interface UserDao {
    @Query("SELECT * FROM User WHERE id = :id")
    Maybe<User> getUserById(int id);

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    Completable insert(User user);
}

3. 定义数据仓库

public class UserRepository {
    private final UserDao userDao;
    private final ApiService apiService;

    public UserRepository(UserDao userDao, ApiService apiService) {
        this.userDao = userDao;
        this.apiService = apiService;
    }

    public Maybe<User> getUserById(int id) {
        // 从本地数据库获取数据
        Maybe<User> local = userDao.getUserById(id);

        // 从服务端获取数据并更新本地数据库
        Maybe<User> remote = apiService.getUserById(id)
                .doOnSuccess(userDao::insert)
                .onErrorResumeNext(throwable -> local);

        // 返回先从本地获取数据，如果本地没有数据再从服务端获取数据的 Maybe 对象
        return Maybe.concat(local, remote)
                .filter(user -> user != null)
                .firstElement();
    }
}

4. 在ViewModel中使用数据仓库

public class UserViewModel extends ViewModel {
    private final UserRepository userRepository;

    public UserViewModel(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public LiveData<User> getUserById(int id) {
        MutableLiveData<User> userLiveData = new MutableLiveData<>();
        userRepository.getUserById(id)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(userLiveData::setValue, throwable -> {});
        return userLiveData;
    }
}

5. 在Activity中使用ViewModel

public class MainActivity extends AppCompatActivity {
    private UserViewModel userViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        UserDatabase userDatabase = UserDatabase.getInstance(this);
        UserDao userDao = userDatabase.userDao();
        ApiService apiService = ApiClient.getInstance().getApiService();
        UserRepository userRepository = new UserRepository(userDao, apiService);
        userViewModel = new ViewModelProvider(this, new ViewModelFactory(userRepository))
                .get(UserViewModel.class);

        userViewModel.getUserById(1).observe(this, user -> {
            // 更新UI
        });
    }
}

以上代码仅供参考，具体实现可能会因为业务需求而有所不同。