IssueTracker

popUpTo will be able to do that: https://stackoverflow.com/questions/50336112/how-to-disable-up-in-navigation-for-some-fragment-with-the-new-navigation-archit

At current now，there is two ways to do the above。

One：
In the NavHostFragment's Activity, override the onBackPress() method,adn if the current NavDestination is MainFragment then just finish the Activity.

@Override
    public void onBackPressed() {
       NavDestination navDestination = mNavController.getCurrentDestination();
        if (navDestination != null
                && navDestination.getId() == R.id.mainFragment) {
            finish();
            return;
        }
        super.onBackPressed();
    }

Two:
Set the to MainFragment action in Navigation_graph app:popUpTo="@id/nav_graph" and app:popUpToInclusive="true"

<?xml version="1.0" encoding="utf-8"?>
<navigation
    android:id="@+id/nav_graph"
    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/splashFragment">

    <fragment
        android:id="@+id/splashFragment"
        android:name="com.ihuman.storydemo.fragment.splash.SplashFragment"
        android:label="fragment_splash"
        tools:layout="@layout/fragment_splash">
        <action
            android:id="@+id/action_splashFragment_to_mainFragment"
            app:destination="@id/mainFragment"
            app:enterAnim="@anim/anim_right_in"
            app:exitAnim="@anim/anim_left_out"
            app:popEnterAnim="@anim/anim_left_in"
            app:popExitAnim="@anim/anim_right_out"
            app:popUpTo="@id/nav_graph"
            app:popUpToInclusive="true"/>
        <action
            android:id="@+id/action_splashFragment_to_guideFragment"
            app:destination="@id/guideFragment"
            app:enterAnim="@anim/anim_right_in"
            app:exitAnim="@anim/anim_left_out"
            app:popEnterAnim="@anim/anim_left_in"
            app:popExitAnim="@anim/anim_right_out"
            app:popUpTo="@id/nav_graph"
            app:popUpToInclusive="true"/>
    </fragment>
       
        <fragment
        android:id="@+id/guideFragment"
        android:name="com.ihuman.storydemo.fragment.guide.GuideFragment"
        android:label="GuideFragment"
        tools:layout="@layout/fragment_guide">
        <action
            android:id="@+id/action_guideFragment_to_mainFragment"
            app:destination="@id/mainFragment"
            app:enterAnim="@anim/anim_right_in"
            app:exitAnim="@anim/anim_left_out"
            app:popEnterAnim="@anim/anim_left_in"
            app:popExitAnim="@anim/anim_right_out"
            app:popUpTo="@id/nav_graph"
            app:popUpToInclusive="true"/>
    </fragment>

    <fragment
        android:id="@+id/mainFragment"
        android:name="com.ihuman.storydemo.fragment.main.MainFragment"
        android:label="fragment_main"
        tools:layout="@layout/fragment_main">
       
    </fragment>
       
</navigation>

Wish for help!