LiveData无法观察到变化

Question

我正在从ViewModel中的Dialog片段更新LiveData值，但无法获取片段中的值。

视图模型：

class OtpViewModel(private val otpUseCase: OtpUseCase, analyticsModel: IAnalyticsModel) : BaseViewModel(analyticsModel) {
    override val globalNavModel = GlobalNavModel(titleId = R.string.otp_contact_title, hasGlobalNavBar = false)

    private val _contactListLiveData = MutableLiveData<List<Contact>>()
    val contactListLiveData: LiveData<List<Contact>>
        get() = _contactListLiveData

    private lateinit var cachedContactList: LiveData<List<Contact>>
    private val contactListObserver = Observer<List<Contact>> {
        _contactListLiveData.value = it
    }



    private lateinit var cachedResendOtpResponse: LiveData<LogonModel>
    private val resendOTPResponseObserver = Observer<LogonModel> {
        _resendOTPResponse.value = it
    }

    private var _resendOTPResponse = MutableLiveData<LogonModel>()
    val resendOTPResponseLiveData: LiveData<LogonModel>
        get() = _resendOTPResponse

    var userSelectedIndex : Int = 0 //First otp contact selected by default

    val selectedContact : LiveData<Contact>
        get() = MutableLiveData(contactListLiveData.value?.get(userSelectedIndex))

    override fun onCleared() {
        if (::cachedContactList.isInitialized) {
            cachedContactList.removeObserver(contactListObserver)
        }

        if (::cachedOtpResponse.isInitialized) {
            cachedOtpResponse.removeObserver(otpResponseObserver)
        }

        super.onCleared()
    }

    fun updateIndex(pos: Int){
        userSelectedIndex = pos
    }

    fun onChangeDeliveryMethod() {
        navigate(
            OtpVerificationHelpCodeSentBottomSheetFragmentDirections
                .actionOtpContactVerificationBottomSheetToOtpChooseContactFragment()
        )
    }

    fun onClickContactCancel() {
        navigateBackTo(R.id.logonFragment, true)
    }

    fun retrieveContactList() {
        cachedContactList = otpUseCase.fetchContactList()
        cachedContactList.observeForever(contactListObserver)
    }



    fun resendOTP(contactId : String){
        navigateBack()
        cachedResendOtpResponse = otpUseCase.resendOTP(contactId)
        cachedResendOtpResponse.observeForever(resendOTPResponseObserver)

    }
}

BaseViewModel：

abstract class BaseViewModel(val analyticsModel: IAnalyticsModel) : ViewModel() {
    protected val _navigationCommands: SingleLiveEvent<NavigationCommand> = SingleLiveEvent()
    val navigationCommands: LiveData<NavigationCommand> = _navigationCommands

    abstract val globalNavModel: GlobalNavModel


    /**
     * Posts a navigation event to the navigationsCommands LiveData observable for retrieval by the view
     */
    fun navigate(directions: NavDirections) {
        _navigationCommands.postValue(NavigationCommand.ToDirections(directions))
    }

    fun navigate(destinationId: Int) {
        _navigationCommands.postValue(NavigationCommand.ToDestinationId(destinationId))
    }

    fun navigateBack() {
        _navigationCommands.postValue(NavigationCommand.Back)
    }

    fun navigateBackTo(destinationId: Int, isInclusive: Boolean) {
        _navigationCommands.postValue(NavigationCommand.BackTo(destinationId, isInclusive))
    }

    open fun init() {
        // DEFAULT IMPLEMENTATION - override to initialize your view model
    }


    /**
     * Called from base fragment when the view has been created.
     */
    fun onViewCreated() {
        analyticsModel.onNewState(getAnalyticsPathCrumb())
    }

    /**
     * gets the Path for the current page to be used for the trackstate call
     *
     * Override this method if you need to modify the path
     *
     * the page id for the track state call will be calculated in the following manner
     * 1) analyticsPageId
     * 2) titleId
     * 3) the page title string
     */
    protected fun getAnalyticsPathCrumb() : AnalyticsBreadCrumb {

        return analyticsBreadCrumb {
            pathElements {
                if (globalNavModel.analyticsPageId != null) {
                    waPath {
                        path = PathElement(globalNavModel.analyticsPageId as Int)
                    }
                } else if (globalNavModel.titleId != null) {
                    waPath {
                        path = PathElement(globalNavModel.titleId as Int)
                    }
                } else {
                    waPath {
                        path = PathElement(globalNavModel.title ?: "")
                    }
                }
            }
        }
    }
}

对话框片段：

class OtpVerificationHelpCodeSentBottomSheetFragment : BaseBottomSheetDialogFragment(){

    private lateinit var rootView: View
    lateinit var binding: BottomSheetFragmentOtpVerificationHelpCodeSentBinding

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {

        viewModel = getViewModel<OtpViewModel>()

        binding = DataBindingUtil.inflate(inflater, R.layout.bottom_sheet_fragment_otp_verification_help_code_sent, container, false)

        rootView = binding.root

        return rootView
    }

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


        val otpViewModel = (viewModel as OtpViewModel)
        binding.viewmodel = otpViewModel

        otpViewModel.resendOTPResponseLiveData.observe(viewLifecycleOwner, Observer {

            it?.let { resendOtpResponse ->
                if(resendOtpResponse.statusCode.equals("000")){
                    //valid status code
                    requireActivity().toastMessageOtp(getString(R.string.otp_code_verification_sent))
                }else{
                    //show the error model
                    //it?.errorModel?.let { it1 -> handleDiasNetworkError(it1) }
                }
            }

        })
    }
}

我从DialogFragment的xml文件中调用viewmodel的resendOTP（contactId:String）方法：

 <TextView
            android:id="@+id/verification_help_code_sent_resend_code"
            style="@style/TruTextView.SubText2.BottomActions"
            android:layout_height="@dimen/spaceXl"
            android:gravity="center_vertical"
            android:text="@string/verification_help_resend_code"
            android:onClick="@{() -> viewmodel.resendOTP(Integer.toString(viewmodel.userSelectedIndex))}"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/top_guideline" />

现在，每当我尝试从片段中调用resendOTP响应LiveData时，它都不会被调用：

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        Log.d("OtpVerify" , "OnViewCreatedCalled")
        viewModel.onViewCreated()
        val otpViewModel = (viewModel as OtpViewModel)

        binding.lifecycleOwner = this
        binding.viewmodel = otpViewModel
        binding.toAuthenticated = OtpVerifyFragmentDirections.actionOtpVerifyFragmentToAuthenticatedActivity()
        binding.toVerificationBtmSheet = OtpVerifyFragmentDirections.actionOtpVerifyFragmentToOtpContactVerificationCodeSentBottomSheet()


        otpViewModel.resendOTPResponseLiveData.observe(viewLifecycleOwner, Observer {
            if(it?.statusCode.equals("000")){
                //valid status code
                requireActivity().toastMessageOtp(getString(R.string.otp_code_verification_sent))
            }else{
                //show the error model
                it?.errorModel?.let { it1 -> handleDiasNetworkError(it1) }
            }
        })

    }

所以我在这里做错了什么。

编辑

基本上我需要对话片段中的clicklistener（重新发送按钮单击），并且需要在片段中读取它。所以我使用了SharedViewModel的概念。

因此，我在ViewModel中进行了必要的更改：

private val selected = MutableLiveData<LogonModel>()

 fun select(logonModel: LogonModel) {
        selected.value = logonModel
    }

    fun getSelected(): LiveData<LogonModel> {
        return selected
    }

在DialogFragment中：

 otpViewModel.resendOTPResponseLiveData.observe(viewLifecycleOwner, Observer{

           otpViewModel.select(it);

        })

在我想读取值的片段中：

otpViewModel.getSelected().observe(viewLifecycleOwner, Observer {

            Log.d("OtpVerify" , "ResendCalled")
            // Update the UI.
            if(it?.statusCode.equals("000")){
                //valid status code
                requireActivity().toastMessageOtp(getString(R.string.otp_code_verification_sent))
            }else{
                //show the error model
                it?.errorModel?.let { it1 -> handleDiasNetworkError(it1) }
            }
        })

但它仍然不起作用。

编辑：

片段的ViewModel源：

viewModel = getSharedViewModel<OtpViewModel>(from = {
            Navigation.findNavController(container as View).getViewModelStoreOwner(R.id.two_step_authentication_graph)
        })

dialogfragment的ViewModel源：

viewModel = getViewModel<OtpViewModel>()

Answer 1

问题是您实际上没有在片段和对话框之间共享ViewModel。要共享ViewModel的实例，必须从相同的ViewModelStore中检索它们。

您用于检索ViewModels的语法似乎来自第三方框架。我觉得可能是Koin。

如果是这种情况，请注意，在Koin中，getViewModel从片段自己的ViewModelStore检索ViewModel。因此，您正在从自己的ViewModelStore检索DialogFragment中的ViewModel。另一方面，在片段中，您使用getSharedViewModel来检索它，您可以在其中指定它应该从哪个ViewModelStore中检索ViewModel。因此，您正在从两个不同的ViewModelStores检索ViewModel，然后获得两个不同的ViewModel。与其中一个交互不会影响另一个，因为它们不是同一个实例。

要解决这个问题，您应该从同一个ViewModelStore中检索片段和对话片段中的ViewModel。例如，您可以在两者中使用getSharedViewModel，也许在每个地方手动指定相同的ViewModelStore，或者甚至不指定Koin将默认为他们的活动的一个。

您甚至可以在您的片段中使用getViewModel，然后将其自己的特定ViewModelStore传递给DialogFrament，然后您可以在其中使用getSharedViewModel，指定传递的片段的ViewModelStore。

Answer 2

几个月前，作为Jetpack库和静态编程语言的新手，如果我理解正确的话，我遇到了类似的问题。

我认为这里的问题是，您正在使用viewModels检索ViewModel，这意味着您返回的ViewModel将仅限于当前片段上下文。。。如果您想在应用程序的多个部分之间共享视图模型，那么它们必须是活动范围的。

例如：

//this will only work for the current fragment, using this declaration here and anywhere else and observing changes wont work, the observer will never fire, except if the method is called within the same fragment that this is declared
private val viewModel: AddPatientViewModel by viewModels {
    InjectorUtils.provideAddPatientViewModelFactory(requireContext())
}

//this will work for the ANY fragment in the current activies scope, using this code and observing anywhere else should work, the observer will fire, except if the method is called fro another activity
private val patientViewModel: PatientViewModel by activityViewModels {
    InjectorUtils.providePatientViewModelFactory(requireContext())
}

请注意，我的类型为AddPatientViewModel的视图模型仅通过视图模型XXX作用于当前片段上下文，对该特定视图模型所做的任何更改等仅会传播到我的当前片段中。

其中，作为类型为病人视窗模型的病人视窗模型通过活动视窗模型将范围限定为活动上下文。这意味着只要两个片段属于同一个活动，并且您通过活动视窗模型获得视窗模型...您应该能够观察到在全局范围内对视窗模型所做的任何更改（全局意味着声明它的同一活动中的任何片段）。

考虑到以上所有内容，如果您的viewModel正确地限定了您的活动范围，并且在这两个片段中，您可以使用actiityViewModels的检索viewModel，并通过XXX. postValue（YYY）或XXX. value=YYY更新观察到的值，您应该能够在同一活动上下文中的任何地方观察到对ViewModel所做的任何更改。

希望这是有道理的，现在已经很晚了，我在睡觉前看到了这个问题！