How to change start destination of a navigation graph programmatically?

UPDATE:

When you have nav graph like this:

<fragment
    android:id="@+id/firstFragment"
    android:name="com.appname.package.FirstFragment" >
    <action
        android:id="@+id/action_firstFragment_to_secondFragment"
        app:destination="@id/secondFragment" /> 
</fragment>

<fragment
    android:id="@+id/secondFragment"
    android:name="com.appname.package.SecondFragment"/>

And you want to navigate to the second fragment and make it root of your graph, specify the next NavOptions:

NavOptions navOptions = new NavOptions.Builder()
        .setPopUpTo(R.id.firstFragment, true)
        .build();

And use them for the navigation:

Navigation.findNavController(view).navigate(R.id.action_firstFragment_to_secondFragment, bundle, navOptions);

setPopUpTo(int destinationId, boolean inclusive) – Pop up to a given destination before navigating. This pops all non-matching destinations from the back stack until this destination is found.

destinationId – The destination to pop up to, clearing all intervening destinations.

inclusive – true to also pop the given destination from the back stack.

ALTERNATIVE:

<fragment
    android:id="@+id/firstFragment"
    android:name="com.appname.package.FirstFragment" >
<action
    android:id="@+id/action_firstFragment_to_secondFragment"
    app:destination="@id/secondFragment"
    app:popUpTo="@+id/firstFragment"
    app:popUpToInclusive="true" /> 
</fragment>

<fragment
    android:id="@+id/secondFragment"
    android:name="com.appname.package.SecondFragment"/>

And then on your code:

findNavController(fragment).navigate(
    FirstFragmentDirections.actionFirstFragmentToSecondFragment())

Old answer

Deprecated: The clearTask attribute for actions and the associated API in NavOptions has been deprecated.

Source: https://developer.android.com/jetpack/docs/release-notes

If you want to change your root fragment to fragment 2 (e.g. after pressing back button on fragment 2 you will exit the app), you should put the next attribute to your action or destination:

app:clearTask="true"

Practically it looks in a next way:

<fragment
    android:id="@+id/firstFragment"
    android:name="com.appname.package.FirstFragment"
    android:label="fragment_first" >
    <action
        android:id="@+id/action_firstFragment_to_secondFragment"
        app:destination="@id/secondFragment"
        app:clearTask="true" /> 
</fragment>

<fragment
    android:id="@+id/secondFragment"
    android:name="com.appname.package.SecondFragment"
    android:label="fragment_second"/>

I’ve added app:clearTask="true" to action.

Now when you perform navigation from fragment 1 to fragment 2 use the next code:

Navigation.findNavController(view)
        .navigate(R.id.action_firstFragment_to_secondFragment);