如何在MVVM架构中使用实时数据

TLDR：如何使用LiveData正确实施MVVM体系结构？

我有一个片段类，它观察viewModel公开的livedata：

viewModel.loginResultLiveData.observe
class LoginFragment : Fragment() {

    private lateinit var binding: FragmentLoginBinding

    private val viewModel by fragmentScopedViewModel { injector.loginViewModel }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        binding = FragmentLoginBinding.inflate(inflater, container, false)

        val username = binding.loginInputField.toString()
        val password = binding.passwordInputField.toString()

        binding.loginSignInButton.setOnClickListener {  viewModel.login(
            username,
            password
        ) }

        viewModel.loginResultLiveData.observe(viewLifecycleOwner){
            when(it){
                is LoginResult.Success -> doSmth()
            }
        }

        return binding.root
    }
}

视图模型类仅要求映射的livedata对象。

class LoginViewModel @Inject internal constructor(
    private val loginUseCase: LoginUseCase
) : ViewModel() {
    lateinit var loginResultLiveData: MutableLiveData<LoginResult>

    fun login(username: String, password: String) {
        loginResultLiveData = loginUseCase.login(username, password)
    }
}
模型使用用例来映射原始格式的结果，并且还会映射错误：

class LoginUseCase @Inject internal constructor(
    private val authRepository: AuthEmailRepository
) {
    var loginResultLiveData = MutableLiveData<LoginResult>()

    fun login(userName: String, password: String): MutableLiveData<LoginResult> {
        authRepository.login(userName, password)
            .addOnCompleteListener {
                if (it.isSuccessful) {
                    loginResultLiveData.postValue(LoginResult.Success)
                } else {
                    loginResultLiveData.postValue(LoginResult.Fail(it.exception.toString()))
                }
            }
        return loginResultLiveData
    }
}

问题在于，只有在loginSignInButton单击之后，该模型才会创建一个liveData对象。但是在设置onClickListener之后，我立即开始观察该对象。同样，每次单击按钮时，都会创建的新实例viewModel.loginResultLiveData，这没有任何意义。

  binding.loginSignInButton.setOnClickListener {  viewModel.login(
        username,
        password
    ) }

    viewModel.loginResultLiveData.observe(viewLifecycleOwner){
        when(it){
            is LoginResult.Success -> doSmth()
        }
    }

在这种情况下，如何使用LiveData正确实现MVVM体系结构？

我也可以将现在拥有的逻辑LoginUseCase移到ModelView中，然后再进行类似的操作，从而避免了前面描述的问题。但是，然后我无法将映射/错误处理委托给用例。

class LoginViewModel @Inject internal constructor(
    private val loginUseCase: LoginUseCase
) : ViewModel() {
    val loginResult: MutableLiveData<LoginResult> = MutableLiveData()

    fun login(username: String, password: String) = loginUseCase.login(username, password)
        .addOnCompleteListener {
            if (it.isSuccessful) {
                loginResult.postValue(LoginResult.Success)
            } else {
                loginResult.postValue(LoginResult.Fail(it.exception.toString()))
            }
        }
}