Android在使用流映射到livedata的视图模型中执行空检查

class UserViewModel(private val userRepository: UserRepository) : ViewModel() {

    private val _userLiveData = MutableLiveData<User>()
    val userLiveData: LiveData<User> get() = _userLiveData

    fun fetchUser(userId: String) {
        viewModelScope.launch {
            userRepository.getUserById(userId).collect { user ->
                _userLiveData.value = user
            }
        }
    }
}

class UserViewModel(private val userRepository: UserRepository) : ViewModel() {

    private val _userLiveData = MutableLiveData<User?>()
    val userLiveData: LiveData<User?> get() = _userLiveData

    fun fetchUser(userId: String) {
        viewModelScope.launch {
            userRepository.getUserById(userId).collect { user ->
                if (user != null) {
                    _userLiveData.value = user
                } else {
                    // 处理空值情况，例如显示错误信息或默认值
                    _userLiveData.value = User.getDefaultInstance()
                }
            }
        }
    }
}