TLDR:如何使用LiveData正确实现MVVM体系结构?
我有一个片段类,它观察暴露了viewModel的livedata:
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,以原始格式映射结果,也将映射错误:
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,仅在单击loginSignInButton
之后,模型才会创建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,都会创建一个新的viewModel.loginResultLiveData
实例,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?
我也可以将现在在LoginUseCase
中拥有的逻辑移到ModelView,然后再有类似的东西,这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()))
}
}
}
您正在尝试观察一个仅在onClickListener
之后初始化的可变LiveData,因此您将无法使用它,而且还有一个lateinit
属性,仅当您调用登录方法时才会初始化,该属性将抛出例外。
为了解决您的问题,您可以使用MediatorLiveData
来观察您的其他实时数据,并将结果传递回片段观察器。
您可以尝试以下操作:
class LoginViewModel @Inject internal constructor(
private val loginUseCase: LoginUseCase
) : ViewModel() {
private var _loginResultLiveData = MediatorLiveData<LoginResult>()
val loginResultLiveData: LiveData<LoginResult> = _loginResultLiveData
fun login(username: String, password: String) {
val loginUseCaseLiveData = loginUseCase.login(username, password)
_loginResultLiveData.addSource(loginUseCaseLiveData) {
_loginResultLiveData.value = it
}
}
}