android - 向上按钮出现在导航中的起始目的地片段上

Question

我一直在开发一个应用程序，并且遇到了这个问题，我试图在演示应用程序中重新创建。请看一下这个演示应用程序的简单代码。

列表片段.kt

class ListFragment : Fragment() {
override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    val binding = DataBindingUtil.inflate<FragmentListBinding>(
        inflater, R.layout.fragment_list, container, false)

    binding.buttonGoToDetail.setOnClickListener {
        findNavController().navigate(ListFragmentDirections.actionNavigateToDetailFragment())
    }

    return binding.root
}

细节片段.kt

class DetailFragment : Fragment() {

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val binding = DataBindingUtil.inflate<FragmentDetailBinding>(
            inflater, R.layout.fragment_detail, container, false)

        return binding.root
    }
}

MainActivity.kt

class MainActivity : AppCompatActivity() {

    private lateinit var appBarConfiguration: AppBarConfiguration
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val navHostFragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment) as NavHostFragment
        val navController = navHostFragment.navController

        appBarConfiguration = AppBarConfiguration(navController.graph)
        setupActionBarWithNavController(navController, appBarConfiguration)
    }

    override fun onSupportNavigateUp(): Boolean {
        val navController = findNavController(R.id.nav_host_fragment)
        return navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp()
    }
}

片段列表.xml

<layout
    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">
    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".ListFragment">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:text="@string/description_list_fragment"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            android:layout_marginTop="30dp"/>

        <Button
            android:id="@+id/buttonGoToDetail"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/detail_page_button_label"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            android:layout_marginBottom="30dp"/>

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

片段详细信息.xml

<layout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">
    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".DetailiFragment">

        <!-- TODO: Update blank fragment layout -->
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:text="@string/description_detail_fragment" />

    </FrameLayout>
</layout>

activity_main.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"
    tools:context=".MainActivity">

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/nav_graph">
    </fragment>

</LinearLayout>

nav_graph.xml

<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_graph"
    app:startDestination="@id/ListFragment">

    <fragment
        android:id="@+id/ListFragment"
        android:name="com.example.navigationtest1.ListFragment"
        android:label="list_fragment"
        tools:layout="@layout/fragment_list">
        <action
            android:id="@+id/action_navigateToDetailFragment"
            app:destination="@+id/DetailFragment" />
    </fragment>

    <fragment
        android:id="@+id/DetailFragment"
        android:name="com.example.navigationtest1.DetailFragment"
        android:label="detail_fragment"
        tools:layout="@layout/fragment_detail">
    </fragment>
</navigation>

问题

现在，当我运行应用程序时，它甚至会在 startDestination 片段（即 ListFragment）上显示向上按钮。我不知道为什么。据我从文档和 codelab 教程中了解到的，它不应该在 startDestination 片段上显示 Up 按钮。但是，在我的例子中，Up 按钮出现在 ListFragment 和 DetailFragment 上。不过，我必须说，Up 按钮在 ListFragment 页面上没有按应有的方式执行任何操作。

任何帮助解决这个问题都非常感谢。