为什么当我导航回片段时会调用onChanged？

Question

我有一个ViewModel处理我的业务逻辑，我正在使用Koin将它注入到我的活动和每个片段中。然而，在我从片段A-片段B导航并导航回片段A之后，我的观察者再次被触发。为什么会发生这种情况？当我返回时，如何阻止这种onChanged被触发？

我尝试将'this'和'view LifecycleOwner'设置为LiveData的LifecycleOwner。

我还尝试将observable移动到onCreate、onActivityCreated和onViewCreated

class MyViewModel : ViewModel() {

    private val _myData = MutableLiveData<Data>()
    val myData = LiveData<Data>()
        get() = _myData

    fun doSomething() {
        ... // some code
        _myData.postValue(myResult)
}

class Activity : BaseActivity() {

    private val viewModel by viewModel<MyViewModel>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)
        setSupportActionBar(main_toolbar)

        subscribeUI()
    }

    private fun subscribeUI() {
        myViewModel.isLoading.observe(this, Observer {
            toggleProgress(it)
        })
    }
}

class FragmentA : BaseFragment() {

    private val viewModel by sharedViewModel<MyViewModel>()

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        subscribeUI()
    }

    private fun subscribeUI() {
        viewModel.myData.observe(viewLifecycleOwner, Observer {
            val direction =
                FragmentADirections.actionAtoB()
            mainNavController?.navigate(direction)
        })
    }
}

class FragmentB : BaseFragment() {

    private val authViewModel by sharedViewModel<LoginViewModel>()

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        subscribeUI()
    }

    private fun subscribeUI() {
        viewModel.otherData.observe(viewLifecycleOwner, Observer {
            // Do something else...
        })
    }
}

Answer 1

这是使用MutableLiveData时的预期行为。我认为你的问题与在哪里添加或删除订阅者无关。

MutableLiveData保存设置它的最后一个值。当我们回到前一个片段时，我们的LiveData观察再次收到现有值的通知。这是为了保留片段的状态，这正是LiveData的确切目的。

谷歌自己已经解决了这个问题，并提供了一种方法来克服这种行为，那就是使用事件包装器。

null

null

/**
* Used as a wrapper for data that is exposed via a LiveData that represents an event.
*/
open class Event<out T>(private val content: T) {

 var hasBeenHandled = false
     private set // Allow external read but not write

 /**
  * Returns the content and prevents its use again.
  */
 fun getContentIfNotHandled(): T? {
     return if (hasBeenHandled) {
         null
     } else {
         hasBeenHandled = true
         content
     }
 }

 /**
  * Returns the content, even if it's already been handled.
  */
 fun peekContent(): T = content
}

class EventObserver<T>(private val onEventUnhandledContent: (T) -> Unit) : Observer<Event<T>> {
 override fun onChanged(event: Event<T>?) {
     event?.getContentIfNotHandled()?.let { value ->
         onEventUnhandledContent(value)
     }
 }
}

// Declare live data object
val testLiveData: MutableLiveData<Event<Boolean>
             by lazy{ MutableLiveData<Event<Boolean>>() }

testLiveData.postValue(Event(true))

viewModel?.testLiveData?.observe(this, EventObserver { result ->
 // Your actions
}