LiveData值回傳null

當我想在 viewModelScope 之外接收它時，為什么我的外部不可變 LiveData 值回傳 null？

正如您在 logcat 中看到的那樣，當我想在 viewModelScope 中接收它或第二次呼叫登錄函式時沒有問題。

這是我的視圖模型

class LoginViewModel(private val mainRepository: MainRepository) : ViewModel() {

    private val _loginResponse = MutableLiveData<LoginResponse>()

    val loginResponse: LiveData<LoginResponse> = _loginResponse

    fun login(post: LoginBody) {
        viewModelScope.launch {
            try {
                _loginResponse.value = mainRepository.loginPost(post = post)
                Log.d(
                    LOGCAT,
                    "Login response : ${loginResponse.value?.data?.answerCount}"
                )
            } catch (e: Exception) {
                Log.d(LOGCAT, "Login error: $e")
            }
        }
        loginResponseOutsideOfTheViewModelScope()
    }

    fun loginResponseOutsideOfTheViewModelScope() {
        Log.d(LOGCAT, "LoginResponseOutsideOfTheViewModelScope : ${loginResponse.value?.data?.answerCount}")
    }
}

class LoginViewModelFactory(val app: GameApplication) :
    ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        val retrofitService = app.retrofit
        val context = app.applicationContext
        val mainRepository = MainRepository(
            retrofitService = retrofitService,
            context = context
        )
        val viewModel = LoginViewModel(mainRepository)
        return viewModel as T
    }
}

這是我呼叫登錄功能的片段。

class FirstFragment : Fragment() {

private var _binding: FragmentFirstBinding? = null

private val binding
    get() = _binding!!

private val viewModel: LoginViewModel by activityViewModels {
    LoginViewModelFactory(
        (activity?.application as GameApplication)
    )
}

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    // Inflate the layout for this fragment
    _binding = FragmentFirstBinding.inflate(inflater)

    binding.lifecycleOwner = this

    binding.firstFragment = this

    return binding.root
}

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


    val myPost = LoginBody(
        deviceId = Settings.Secure.getString(
            requireContext().contentResolver, Settings.Secure.ANDROID_ID
        ),
        country = Locale.getDefault().country,
        language = Locale.getDefault().language,
        platform = Build.VERSION.SDK_INT.toString(),
        version = BuildConfig.VERSION_NAME,
        deviceModel = Build.MODEL
    )
    viewModel.login(myPost)
    }
}

邏輯貓

2021-10-21 16:32:10.676 1520-1520/com.example.a4p1w D/DEBUG: LoginResponseOutsideOfTheViewModelScope : null
2021-10-21 16:32:12.354 1520-1520/com.example.a4p1w D/DEBUG: Login response : 12

uj5u.com熱心網友回復：

在login函式中，你有launch一個異步協程（它LiveData在記錄之前獲取一個值并設定它），然后你記錄 LiveData 的當前值。該協程需要時間，因此“當前值”日志陳述句首先運行（這就是它們按該順序列印的原因）。

您LiveData最初沒有值（您沒有將一個值傳遞給建構式）：

private val _loginResponse = MutableLiveData<LoginResponse>()

所以loginResponse.value回傳null，直到您為其設定一個值（就像在您的協程中一樣）。這就是為什么當協程完成時它作業正常，之后每次閱讀它

標籤：安卓 科特林 虚拟机 kotlin 协程 android-livedata

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