How to change the navigation host in android architecture navigation?

Question

so I am trying to learn about using Navigation graph in Android, I am not sure what I am doing, so maybe you have better way.

let say there are 2 sections in my app:

1. Section one is called Main section, which has bottom navigation view and toolbar

2. Section two is called Authentication Section (for login page, sign up) , it doesn't has bottom navigation view or toolbar

so because these 2 sections has different 'frame' (one has bottom navigation and toolbar, the other doesn't) so I make 2 activities. MainActivity and AuthenticationActivity that will host the fragment

here is the xml of MainActivity:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

    <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:background="?attr/colorPrimary"
            android:theme="?attr/actionBarTheme"
            android:minHeight="?attr/actionBarSize"
            app:layout_constraintTop_toTopOf="parent" app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintEnd_toEndOf="parent"/>


    <fragment
            android:id="@+id/nav_host_fragment"
            android:layout_width="0dp"
            android:layout_height="0dp"
            app:layout_constraintBottom_toTopOf="@+id/bottom_nav"
            app:layout_constraintTop_toBottomOf="@+id/toolbar"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            android:name="androidx.navigation.fragment.NavHostFragment"
            app:navGraph="@navigation/navigation_graph"
            app:defaultNavHost="true"
    />


    <android.support.design.widget.BottomNavigationView
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintBottom_toBottomOf="parent"
            android:background="@color/colorPrimary"
            app:itemIconTint="@color/color_bottom_view_navigation"
            app:itemTextColor="@color/color_bottom_view_navigation"
            app:menu="@menu/menu_bottom_view"
            app:labelVisibilityMode="labeled"
            android:id="@+id/bottom_nav"/>

</android.support.constraint.ConstraintLayout>

and here is the xml of AuthenticationActivity:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".Activity.LoginActivity"
        android:background="@android:color/white">

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


</androidx.constraintlayout.widget.ConstraintLayout>

so in the HomeFragment class, I want to make a logic. if the user has logged in before then stay in the MainActivity (in home fragment), otherwise I want my app to move to AuthenticationActivity to show the LoginFragment

but how to do that ?

I have no idea how to set that in the navigation graph xml (how to set the destination and action), and I also have no idea how to set up the code in the kotlin file to move to loginFragment from HomeFragment

I previously thought that in the main navigation graph I make the nested graph like this

and when I click that nested graph it seems I can't choose the host.

in the HomeFragment kotlin file I am confused how to make action to that authenticationActivity (to LoginFragment), I need to make logic like this:

if (userHasLoggedIn) {
     Navigation.findNavController(it).navigate(loginDestination)
}

但是主机仍在使用MainActivity，这意味着它具有底视图和工具栏。在“登录”中，我不需要底部导航视图和工具栏。

老实说，我不确定是否可以使用2种活动将其分开，也许您有更好的方法？