NavigationUI.navigateUp什么都不做

如何解决NavigationUI.navigateUp什么都不做

我有一个活动，其中有一个片段，其中引用了导航图组件：

活动XML：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <androidx.fragment.app.FragmentContainerView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_fragment"
        app:defaultNavHost="true"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:id="@+id/fragment_main" />
</LinearLayout>

导航图：

<navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/nav_fragment"
    app:startDestination="@id/countryPickerFragment">

    <fragment
        android:id="@+id/countryPickerFragment"
        android:name="me.sparker0i.ottcontent.view.composer.countrypicker.CountryPickerFragment"
        android:label="Pick a Country"
        tools:layout="@layout/fragment_country_picker">
        <action android:id="@+id/countryPickerFragmentToPlatformPickerFragment"
            app:destination="@+id/platformPickerFragment">
            <argument
                android:name="countryValue"
                app:argType="string" />
        </action>
    </fragment>
    <fragment
        android:id="@+id/platformPickerFragment"
        android:name="me.sparker0i.ottcontent.view.composer.platformpicker.PlatformPickerFragment"
        android:label="Pick a Platform"
        tools:layout="@layout/fragment_platform_picker">
        <argument
            android:name="countryValue"
            app:argType="string" />
    </fragment>
</navigation>

活动代码：

class MainActivity : AppCompatActivity() {
    private lateinit var navController: NavController

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

        val navHostFragment = supportFragmentManager.findFragmentById(R.id.fragment_main) as NavHostFragment
        navController = navHostFragment.navController

        NavigationUI.setupActionBarWithNavController(this,navController)
    }

    override fun onSupportNavigateUp(): Boolean {
        return NavigationUI.navigateUp(navController,null)
    }
}

在打开的第一个片段（CountryPickerFragment）中，我有一段代码观察MutableLiveData内部ViewModel的值。观察到该值后，它将打开第二个片段，使用导航库进行导航。 CountryPickerFragment的相关代码如下：

class CountryPickerFragment : Fragment(),KodeinAware,CoroutineScope {
    override val kodein by closestKodein()

    private lateinit var job: Job

    override val coroutineContext: CoroutineContext
        get() = job + Dispatchers.Main

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        job = Job()
    }

    override fun onDestroy() {
        super.onDestroy()
        job.cancel()
    }

    private var mutableCountries = MutableLiveData<List<Country>>()

    private val viewModelFactory: ContentViewModelFactory by instance()
    private lateinit var viewModel: ContentViewModel
    private lateinit var selectionTracker: SelectionTracker<Long?>

    override fun onCreateView(
        inflater: LayoutInflater,container: ViewGroup?,savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_country_picker,container,false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        viewModel = ViewModelProvider(this,viewModelFactory).get(ContentViewModel::class.java)

        viewModel.countryValue.observeForever{value ->
            val action = CountryPickerFragmentDirections.countryPickerFragmentToPlatformPickerFragment(value)
            requireView().findNavController().navigate(action)
        }
    }
}

这也是第二个片段：

class PlatformPickerFragment : Fragment(),CoroutineScope {
    override val kodein by closestKodein()

    private lateinit var job: Job

    override val coroutineContext: CoroutineContext
        get() = job + Dispatchers.Main

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        job = Job()
    }

    override fun onDestroy() {
        super.onDestroy()
        job.cancel()
    }

    val args: PlatformPickerFragmentArgs by navArgs()

    private val viewModelFactory: ContentViewModelFactory by instance()
    private lateinit var viewModel: ContentViewModel

    override fun onCreateView(
        inflater: LayoutInflater,savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_platform_picker,viewModelFactory).get(ContentViewModel::class.java)
    }

    override fun onViewCreated(view: View,savedInstanceState: Bundle?) {
        val countryValue = args.countryValue
        Log.i("Country Value",countryValue)
    }
}

虽然这段代码确实在同一活动中打开了第二个片段，并且还显示了Logcat内的Country Value，但它在操作栏上还带有一个“后退”按钮。当我尝试按下该后退按钮时，它不会返回上一个片段，而是在Logcat中放置了另一个Country Value。按下手机导航栏上的后退按钮也可以做到这一点。

如何从新产生的片段（CountryPickerFragment）返回上一个片段（PlatformPickerFragment）？

PS。如果有人想看一下代码，可以去here