我已经创建了一个小项目与2个片段，它的工作流程。我们有一个登录屏幕和密码屏幕。
登录片段

class LoginFragment : Fragment() {

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

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val toolbar = view.findViewById<androidx.appcompat.widget.Toolbar>(R.id.loginToolbar)
        toolbar.navigationIcon = null
        // Set the Toolbar as your activity's ActionBar
        (requireActivity() as AppCompatActivity).setSupportActionBar(toolbar)

        // Find this Fragment's NavController
        val navController = NavHostFragment.findNavController(this);

        // And set up the ActionBar
        NavigationUI.setupActionBarWithNavController(
            requireActivity() as AppCompatActivity,
            navController
        )

        val b = view.findViewById<MaterialButton>(R.id.loginButton)
        b.setOnClickListener {
            this.findNavController().navigate(R.id.passwordsFragment)
        }
    }
}

密码片段

class PasswordsFragment : Fragment() {

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

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val toolbar = view.findViewById<androidx.appcompat.widget.Toolbar>(R.id.myToolbar)
        // Set the Toolbar as your activity's ActionBar
        (requireActivity() as AppCompatActivity).setSupportActionBar(toolbar)

        // Find this Fragment's NavController
        val navController = NavHostFragment.findNavController(this);

        // And set up the ActionBar
        NavigationUI.setupActionBarWithNavController(
            requireActivity() as AppCompatActivity,
            navController
        )
        toolbar.setNavigationOnClickListener {
            navController.navigateUp()
        }
    }
}

LoginFragment XML中的工具栏

<androidx.appcompat.widget.Toolbar
        android:id="@+id/loginToolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@color/black"
        app:layout_constraintTop_toTopOf="parent" />

PasswordFragment XML中的工具栏

<androidx.appcompat.widget.Toolbar
    android:id="@+id/myToolbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@color/black"
    app:layout_constraintTop_toTopOf="parent" />

主要活动XML

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/constraintLayout3"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:elevation="6dp"
    android:fillViewport="true"
    android:orientation="vertical">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/my_nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_graph" />

</androidx.constraintlayout.widget.ConstraintLayout>

我从loginFragment导航到PasswordFragment，然后按向上按钮，转到上一个片段：）
如果有用就告诉我

private lateinit var appBarConfiguration: AppBarConfiguration
//onCreate
val navHostFragment =
        supportFragmentManager.findFragmentById(R.id.fragment4) as NavHostFragment
    val navController = navHostFragment.navController
    appBarConfiguration = AppBarConfiguration(navController.graph, drawerLayout)
    setupActionBarWithNavController(navController, appBarConfiguration)
    navDrawView.setupWithNavController(navController)
    bottomAppBar.background = null
    bottomAppBar.setupWithNavController(navController)
//automatic NavigateUp setup
override fun onSupportNavigateUp(): Boolean {
    val navController = findNavController(R.id.fragment4)
    return navController.navigateUp(appBarConfiguration)
            || super.onSupportNavigateUp()
}

你能在你的主活动中试试这个设置吗，它对我很有效，我把bottomNav navdrawer和appbar一起使用。
如果要使用自定义工具栏

//in fragment or activity
private fun handleToolbarChanges() {
    with(binding.toolbarDataAnalysis) {
        setNavigationOnClickListener { onBackPressed() }
        title = "your title here"
    }
}
//in xml file
<androidx.appcompat.widget.Toolbar
    android:id="@+id/toolbarDataAnalysis"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@color/green"
    app:navigationIcon="@drawable/ic_baseline_arrow_back_24"
    app:titleTextColor="@color/white" />

现在，我们可以使用popBackStack（）为第二个片段的返回值创建观察器
假设我们有第一片段和第二片段
在firstFragment中，观察者使用以下代码：-

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    val navController = findNavController()
    navController.currentBackStackEntry?.savedStateHandle?.getLiveData<String>("isDelete")
        ?.observe(viewLifecycleOwner) {
            if(it.lowercase().compareTo("true")==0)
            {
                //write code here
            }
            else
            {
                //write code here
            }
        }
}

在secondFragment中使用以下代码

val navController = findNavController()
    navController.previousBackStackEntry?.savedStateHandle?.set("isDelete", "true/false")
    navController.popBackStack()