How to handle back button when at the starting destination of the navigation component

Question

I've started working with the new navigation component and I'm really digging it! I do have one issue though - How am I supposed to handle the back button when I'm at the starting destination of the graph?

This is the code I'm using now:

findNavController(this, R.id.my_nav_host_fragment)
                .navigateUp()

When I'm anywhere on my graph, it's working great, it send me back, but when I'm at the start of it - the app crashes since the backstack is empty.

This all makes sense to me, I'm just not sure how to handle it.

While I can check if the current fragment's ID is the same as the one that I know to be the root of the graph, I'm looking for a more elegant solution like some bool flag of wether or not the current location in the graph is the starting location or not.

Ideas?

Answer 1

I had a similar scenario where I wanted to finish the activity when I was at the start destination and do a regular 'navigateUp' when I was further down the navigation graph. I solved this through a simple extension function:

fun NavController.navigateUpOrFinish(activity: AppCompatActivity): Boolean {
return if (navigateUp()) {
    true
} else {
    activity.finish()
    true
}

}

And then call it like:

override fun onSupportNavigateUp() = 
        findNavController(R.id.nav_fragment).navigateUpOrFinish(this)

However I was unable to use NavigationUI as this would hide the back arrow whenever I was at the start destination. So instead of:

NavigationUI.setupActionBarWithNavController(this, controller)

I manually controlled the home icon:

setSupportActionBar(toolbar)
supportActionBar?.setDisplayHomeAsUpEnabled(true)
supportActionBar?.setHomeAsUpIndicator(R.drawable.ic_navigation_back)

Answer 2

Override onBackPressed in your activity and check if the current destination is the start destination or not.

Practically it looks like this:

@Override
public void onBackPressed() {
    if (Navigation.findNavController(this,R.id.nav_host_fragment)
            .getCurrentDestination().getId() == R.id.your_start_destination) {
        // handle back button the way you want here
        return;
    }
    super.onBackPressed();
}