IssueTracker

since these are in public API (:/) we need to do this in 1.2

Opening diff shortly

Project: platform/frameworks/support
Branch: androidx-master-dev

commit d576cbdc911cba16638a44fd8223391a90a07ef7
Author: Mike Nakhimovich <digitalbuddha@users.noreply.github.com>
Date:   Tue Aug 11 09:30:34 2020

    [GH] Hide deprecated internal API.
   
    ## Proposed Changes
   
    * `RoomDatabase.java` has protected `mCallbacks` field which is leaking in the API docs, we should @Hide it.
   
    ## Testing
   
    Test: Ran unit tests locally
   
    ## Issues Fixed
   
    Fixes: 76109329
   
    This is an imported pull request from https://github.com/androidx/androidx/pull/61.
   
    Resolves #61
    Github-Pr-Head-Sha: 6440daa3a63752c7f9d5ba2a390248cd85bc634f
    GitOrigin-RevId: fe92d8466a59b44b218b6ca3cbd57dcda17992f7
   
    Change-Id: Id599cdf5b02b32bdae0166266fb7da967598fe92

A       room/runtime/api/current.ignore
M       room/runtime/api/current.txt
M       room/runtime/api/public_plus_experimental_current.txt
M       room/runtime/api/restricted_current.txt
M       room/runtime/src/main/java/androidx/room/RoomDatabase.java

https://android-review.googlesource.com/1396827

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!