LiveData不会发出所需值

Question

我创建了这样的实时数据：

val authTokenLiveData: LiveData<String?> = liveData {
    emit(accountManager.myAuthToken())
}

我正在观察它：

accountViewModel.authTokenLiveData.observe(this) {
    toast("Token is $it")
    if(it == null) goToLogin()
}

[起初效果很好，我烘烤了令牌，但是注销后删除令牌，使得令牌为null，根据观察者，我不应该登录。

[当令牌变为authTokenLiveData时如何确保null发出null，以便观察者允许我进入登录屏幕？

Answer 1

   // LiveData object is usually stored within a ViewModel 
    val currentName: MutableLiveData<String> by lazy {
        MutableLiveData<String>()
    }


   // updating the live data object 
   // using setValue(T) or postValue(T)
   button.setOnClickListener {
          val anotherName = "John Doe"
          model.currentName.setValue(anotherName)
      }

  // Observe LiveData Object
  // Use the 'by viewModels()' Kotlin property delegate
  // from the activity-ktx artifact
private val model: NameViewModel by viewModels()

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    // Other code to setup the activity...

    // Create the observer which updates the UI.
    val nameObserver = Observer<String> { newName ->
        // Update the UI, in this case, a TextView.
        nameTextView.text = newName
    }

    // Observe the LiveData, passing in this activity as the LifecycleOwner and the observer.
    model.currentName.observe(this, nameObserver)
}

Answer 2

在LiveData中创建ViewModel，例如：

val authToken: MutableLiveData<String?> by lazy {
    MutableLiveData<String?>()
}

像[[0]一样观察LiveData：

Activity

像[[0]一样观察

viewModel.authToken.observe(viewLifecycleOwner, Observer {
    toast("Token is $it")
    if(it == null) goToLogin()
})

：

LiveData

将Fragment传递到

viewModel.authToken.observe(viewLifecycleOwner, Observer {
    toast("Token is $it")
    if(it == null) goToLogin()
})

。然后将数据从ViewModel发送到AccountManager，例如：

LiveData

您将获得最新数据到您的观察者区。