学习完这一节后Fragment出现了重叠，而且我将其他Fragment设置为启动Fragment后也是这样。
FixFragmentNavigator代码如下：

private const val TAG = "FixFragmentNavigator"
@Navigator.Name("fixfragment")
class FixFragmentNavigator(
    private val context: Context,
    private val manager: FragmentManager,
    private val containerId: Int
) : FragmentNavigator(context, manager, containerId) {


    override fun navigate(
        destination: Destination,
        args: Bundle?,
        navOptions: NavOptions?,
        navigatorExtras: Navigator.Extras?
    ): NavDestination? {
        if (manager.isStateSaved) {
            Log.i(TAG, "Ignoring navigate() call: FragmentManager has already" + " saved its state")
            return null
        }
        var className = destination.className
        if (className[0] == '.') {
            className = context.packageName + className
        }
        //由于我们需要重用FragmentManager，所以不需要每次跳转都创建
        /* val frag = instantiateFragment(
             context, manager,
             className, args
         )
         frag.arguments = args*/
        val ft = manager.beginTransaction()

        var enterAnim = navOptions?.enterAnim ?: -1
        var exitAnim = navOptions?.exitAnim ?: -1
        var popEnterAnim = navOptions?.popEnterAnim ?: -1
        var popExitAnim = navOptions?.popExitAnim ?: -1
        if (enterAnim != -1 || exitAnim != -1 || popEnterAnim != -1 || popExitAnim != -1) {
            enterAnim = if (enterAnim != -1) enterAnim else 0
            exitAnim = if (exitAnim != -1) exitAnim else 0
            popEnterAnim = if (popEnterAnim != -1) popEnterAnim else 0
            popExitAnim = if (popExitAnim != -1) popExitAnim else 0
            ft.setCustomAnimations(enterAnim, exitAnim, popEnterAnim, popExitAnim)
        }

        //获取当前活跃的Fragment，并将其隐藏
        val fragment = manager.primaryNavigationFragment
        fragment?.let { ft.hide(it) }

        val tag = destination.id.toString()
        var destinationFrg = manager.findFragmentByTag(tag)
        when (destinationFrg) {
            null -> {
                destinationFrg = instantiateFragment(
                    context, manager,
                    className, args
                )
                destinationFrg.arguments = args
                ft.add(containerId, destinationFrg, tag)
            }
            else -> ft.show(destinationFrg)
        }
 //        ft.replace(containerId, frag)
         ft.setPrimaryNavigationFragment(destinationFrg)

        @IdRes val destId = destination.id
        var mBackStack: ArrayDeque<Int> = ArrayDeque()
        //通过反射获取Fragment回退栈mBackStack
        try {
            val field = FragmentNavigator::class.java.getDeclaredField("mBackStack")
            field.isAccessible = true
            mBackStack = field.get(this) as ArrayDeque<Int>
        } catch (e: NoSuchFieldException) {
            e.printStackTrace()
        } catch (e: IllegalAccessException) {
            e.printStackTrace()
        }

        val initialNavigation = mBackStack.isEmpty()
        val isSingleTopReplacement = (navOptions != null && !initialNavigation
                && navOptions.shouldLaunchSingleTop()
                && mBackStack.peekLast() == destId)

        val isAdded: Boolean
        if (initialNavigation) {
            isAdded = true
        } else if (isSingleTopReplacement) {
            // Single Top means we only want one instance on the back stack
            if (mBackStack.size > 1) {
                // If the Fragment to be replaced is on the FragmentManager's
                // back stack, a simple replace() isn't enough so we
                // remove it from the back stack and put our replacement
                // on the back stack in its place
                manager.popBackStack(
                    generateBackStackName(mBackStack.size, mBackStack.peekLast()),
                    FragmentManager.POP_BACK_STACK_INCLUSIVE
                )
                ft.addToBackStack(generateBackStackName(mBackStack.size, destId))
            }
            isAdded = false
        } else {
            ft.addToBackStack(generateBackStackName(mBackStack.size + 1, destId))
            isAdded = true
        }
        if (navigatorExtras is Extras) {
            val extras = navigatorExtras as Extras?
            for ((key, value) in extras!!.sharedElements) {
                ft.addSharedElement(key, value)
            }
        }
        ft.setReorderingAllowed(true)
        ft.commit()
        // The commit succeeded, update our view of the world
        return if (isAdded) {
            mBackStack.add(destId)
            destination
        } else {
            null
        }
    }

    private fun generateBackStackName(backStackIndex: Int, destid: Int): String {
        return "$backStackIndex-$destid"
    }
}