如何使用导航组件保留片段？

Question

我正在尝试使用最新的导航组件库创建一个基于侧边导航抽屉的应用程序（

'androidx.navigation:navigation-fragment-ktx:2.5.3'

和

'androidx.navigation:navigation-ui-ktx:2.5.3'

） 我尝试遵循创建导航抽屉的基本模板，但每次单击菜单项时，它都会破坏当前片段并创建一个新片段。同样，当按下返回时，我们返回到开始片段，但是开始片段也被重新创建，而不是原来的状态。 所以我的问题是是否有可能在不导致重复销毁/创建循环的情况下保留片段？

我知道片段的“状态”可以使用活动视图模型或保存实例状态来保留，但在某些时候，只使用单例片段而不是重新创建它会更有益。

代码：

//navgraph sidenav advance.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"
    app:startDestination="@+id/nav_home">

    <navigation android:id="@+id/nav_home" app:startDestination="@+id/frag_home">
        <fragment
            android:id="@+id/frag_home"
            android:name="work.curioustools.fragmentpracticalsamples.side_navigation.HomeFragment"
            android:label="@string/menu_home"
            tools:layout="@layout/fragment_side_navigation" />
    </navigation>

    <navigation android:id="@+id/nav_gallery" app:startDestination="@+id/frag_gallary">
        <fragment
            android:id="@+id/frag_gallary"
            android:name="work.curioustools.fragmentpracticalsamples.side_navigation.GalleryFragment"
            android:label="@string/menu_gallery"
            tools:layout="@layout/fragment_side_navigation" />

    </navigation>

    <navigation android:id="@+id/nav_slideshow" app:startDestination="@+id/frag_slideshow">
        <fragment
            android:id="@+id/frag_slideshow"
            android:name="work.curioustools.fragmentpracticalsamples.side_navigation.SlideshowFragment"
            android:label="@string/menu_slideshow"
            tools:layout="@layout/fragment_side_navigation" />
    </navigation>
    
</navigation>

activity sidenav default.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout 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/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"

    tools:ignore="ContentDescription"
    tools:openDrawer="start">

    <androidx.coordinatorlayout.widget.CoordinatorLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <com.google.android.material.appbar.AppBarLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:theme="@style/ThemeOverlay.MaterialComponents.Dark.ActionBar">
            <androidx.appcompat.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="?attr/actionBarSize"
                android:background="?attr/colorPrimary"
                tools:navigationIcon="@drawable/ic_sidenav"
                app:title="@string/app_name"
                app:popupTheme="@style/ThemeOverlay.MaterialComponents.Light" />

        </com.google.android.material.appbar.AppBarLayout>

        <androidx.constraintlayout.widget.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layout_behavior="@string/appbar_scrolling_view_behavior">

            <androidx.fragment.app.FragmentContainerView
                android:id="@+id/fragment_navhost_default"
                android:name="androidx.navigation.fragment.NavHostFragment"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                app:defaultNavHost="true"
                app:layout_constraintLeft_toLeftOf="parent"
                app:layout_constraintRight_toRightOf="parent"
                app:layout_constraintTop_toTopOf="parent"
                app:navGraph="@navigation/navgraph_sidenav_advance" />
        </androidx.constraintlayout.widget.ConstraintLayout>

        <com.google.android.material.floatingactionbutton.FloatingActionButton
            android:id="@+id/fab"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="bottom|end"
            android:layout_margin="16dp"
            app:srcCompat="@android:drawable/ic_dialog_email"
             />


    </androidx.coordinatorlayout.widget.CoordinatorLayout>

    <com.google.android.material.navigation.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        tools:layout_width="100dp"
        app:menu="@menu/menu_side_navigation" />
</androidx.drawerlayout.widget.DrawerLayout>

// activity : SideNavigationDrawerDefaultActivity

class SideNavigationDrawerDefaultActivity : AppCompatActivity() {
    private  val binding by lazy {  ActivitySideNavigationDefaultBinding.inflate(layoutInflater) }
    private  val appBarConfiguration by lazy { AppBarConfiguration(setOf(R.id.nav_home, R.id.nav_gallery, R.id.nav_slideshow), binding.drawerLayout) }
    private  val navController by lazy {
        val navHost = supportFragmentManager.findFragmentById(R.id.fragment_navhost_default) as NavHostFragment
        navHost.navController
    }
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)

        //setupActionBar
        setSupportActionBar(binding.toolbar)
        setupActionBarWithNavController(navController, appBarConfiguration)

        //setup side drawer
        binding.navView.setupWithNavController(navController)
        binding.fab.setOnClickListener { snack("hello",it) }
    }

    override fun onSupportNavigateUp() = navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp()


}