java - 为自定义查询实现ViewModel和存储库

我一直在网上浏览示例，但是找不到有关如何为该查询实现ViewModel和Repository的任何示例。

    @Query("SELECT * FROM user_table WHERE id = :userId")
    LiveData<User> getUser(int userId);

我对此的尝试看起来像这样

     public class UserRepository {
        private UserDao userDao;
        private LiveData<List<User>> allUsers;
        private LiveData<User> user;
        private int userId;

        public UserRepository(Application application) {
            UserDatabase database = UserDatabase.getInstance(application);
            userDao = database.userDao();
            allUsers = userDao.getAllUsers();
            user = userDao.getUserById(userId);

        }
        public LiveData<List<User>> getAllUsers() {
            return allUsers;
        }

        public LiveData<User> getUser(int userId){
            return user;
        }
    }

    public class UserViewModel extends AndroidViewModel {

        private UserRepository userRepository;
        private LiveData<List<User>> allUsers;
        private LiveData<User> user;
        private int userId;

        public UserViewModel(@NonNull Application application) {
            super(application);
            userRepository = new UserRepository(application);
            allUsers = ((UserRepository)userRepository).getAllUsers();
            user = ((UserRepository)userRepository).getUser(userId);
        }

        public LiveData<List<User>> getAllUsers() {
            return allUsers;
        }

        public LiveData<User> getUser(int userId){
            return user;
        }
    }

尝试将登录的用户详细信息设置到导航标题后，这将导致空Pointer Exception。

    View hView =  navigationView.inflateHeaderView(R.layout.nav_header_main);
    navHeaderUsername = hView.findViewById(R.id.nav_header_username);

    Intent intent = getIntent();
      if (intent.hasExtra(LoginActivity.EXTRA_ID)){

            int id = intent.getIntExtra(LoginActivity.EXTRA_ID, -1);
            if (id == -1) {
                Toast.makeText(this, "Login Error!!", Toast.LENGTH_SHORT).show();
                return;
            }


            navHeaderUsername.setText(Objects.requireNonNull(userViewModel.getUser(id).getValue()).getName());
       }

最佳答案

我也经历了这个阶段。该文档对此不力。他们没有明确说明这一点，但是在检索LiveData对象之后，又需要执行额外的步骤。

通过调用以下方法获取LiveData对象后：

LiveData<User> user = userViewModel.getUser(id);

然后，您需要执行以下操作：

user.observe(this, new Observer<User>() {
        @Override
        public void onChanged(@Nullable User u) {
            // Do your stuff here
        }
    });

当数据可用于处理工作时，将调用onChanged(..)方法。

希望这可以帮助！！