实时数据在更改片段时再次赋予旧值

Question

我正在跨多个片段使用来自共享 ViewModel 的实时数据。我有一个登录片段，它接受用户的 phone 号码和密码，然后用户按下登录按钮，我为此调用 API，现在如果登录失败，我会祝酒"Sign In failed"，现在如果用户转到 "ForgotPassword" 屏幕，该屏幕也使用与 "SignInFragment" 相同的视图模型，然后从忘记密码屏幕按回，就会出现登录片段，但它再次显示 toast "Sign In failed" 但未调用 API，它从先前注册的观察者那里获取数据，那么有什么办法可以解决这个问题吗？

SignInFragment.kt

class SignInFragment : Fragment() {

    private lateinit var binding: FragmentSignInBinding

    //Shared view model across two fragments
    private val onBoardViewModel by activityViewModels<OnBoardViewModel>()

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        binding = DataBindingUtil.inflate(
            inflater,
            R.layout.fragment_sign_in,
            container,
            false
        )
        return binding.root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        onBoardViewModel.signInResponse.observe(viewLifecycleOwner) { response ->
            //This is calling again after coming back from new fragment it.
            showToast("Sign In Failed")
        }
    }

    override fun onClick(v: View?) {
        when (v?.id!!) {
            R.id.forgotPasswordTV -> {
                findNavController().navigate(SignInFragmentDirections.actionSignInFragmentToForgotPasswordFragment())
            }
            R.id.signInTV -> {
                val phoneNumber = binding.phoneNumberET.text
                val password = binding.passwordET.text
                val signInRequestModel = SignInRequestModel(
                    phoneNumber.toString(),
                    password.toString(),
                    ""
                )

                //Calling API for the sign-in
                onBoardViewModel.callSignInAPI(signInRequestModel)
            }
        }
    }
}

忘记密码片段

class ForgotPasswordFragment : Fragment() {

    private lateinit var binding: FragmentForgotPasswordBinding

    //Shared view model across two fragments
    private val onBoardViewModel by activityViewModels<OnBoardViewModel>()

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        binding = DataBindingUtil.inflate(
            inflater,
            R.layout.fragment_forgot_password,
            container,
            false
        )
        return binding.root
    }
}

OnBoardViewModel

class OnBoardViewModel : ViewModel() {

    private var repository: OnBoardRepository = OnBoardRepository.getInstance()

    private val signInRequestLiveData = MutableLiveData<SignInRequestModel>()

    //Observing this data in sign in fragment
    val signInResponse: LiveData<APIResource<SignInResponse>> =
        signInRequestLiveData.switchMap {
            repository.callSignInAPI(it)
        }

    //Calling this function from sign in fragment
    fun callSignInAPI(signInRequestModel: SignInRequestModel) {
        signInRequestLiveData.value = signInRequestModel
    }

    override fun onCleared() {
        super.onCleared()
        repository.clearRepo()
    }
}

我尝试将此代码移到 onActivityCreated 中，但从新片段返回后仍会调用它。

 onBoardViewModel.signInResponse.observe(viewLifecycleOwner) { response ->
            showToast("Sign In Failed")
        }

Answer 1

在 OnBoardViewModel class 中使用 SingleLiveEvent class 而不是 LiveData 将解决您的问题：

val signInResponse: SingleLiveEvent <APIResource<SignInResponse>>.

class SingleLiveEvent<T> : MutableLiveData<T>() {

    private val pending = AtomicBoolean(false)

    override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
        super.observe(owner, Observer<T> { t ->
            if (pending.compareAndSet(true, false)) {
                observer.onChanged(t)
            }
        })
    }

    override fun setValue(t: T?) {
        pending.set(true)
        super.setValue(t)
    }

    fun call() {
        postValue(null)
    }

}

这是一个生命周期感知的可观察对象，在订阅后只发送新的更新。 LiveData 只有在明确调用 setValue() 或 call() 时才会调用可观察对象。

Answer 2

我会提供一种重置您的实时数据的方法。给它一个可为空的类型。您的观察者在获得空值时可以忽略它。当您收到登录数据时调用此函数，因此您也不会在屏幕旋转时重复消息。

class OnBoardViewModel : ViewModel() {

    // ...

    fun consumeSignInResponse() {
        signInRequestLiveData.value = null
    }

}

onBoardViewModel.signInResponse.observe(viewLifecycleOwner) { response ->
    if (response != null) {
        showToast("Sign In Failed")
        onBoardViewModel.consumeSignInResponse()
    }
}

Answer 3

对于 Kotlin 用户，@Sergey 的回答也可以使用如下委托来实现

class SingleLiveEvent<T> : MutableLiveData<T>() {
    
    var curUser: Boolean by Delegates.vetoable(false) { property, oldValue, newValue ->
        newValue != oldValue
    }

    override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
        super.observe(owner, Observer<T> { t ->
            if (curUser) {
                observer.onChanged(t)
                curUser = false
            }
        })
    }

    override fun setValue(t: T?) {
      
        curUser = true
        super.setValue(t)
    }

    fun call() {
        postValue(null)
    }

}

实时数据在更改片段时再次赋予旧值

Live data giving old value again while changing fragment

android

kotlin

android-livedata

android-viewmodel

android-jetpack