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

Answer 1

You shouldn't override "onBackPressed", you should override "onSupportNavigateUp" and put there

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

From the official documentation: You will also overwrite AppCompatActivity.onSupportNavigateUp() and call NavController.navigateUp

https://developer.android.com/topic/libraries/architecture/navigation/navigation-implementing

Answer 2

In Jetpack Navigation Componenet, if you want to perform some operation when fragment is poped then you need to override following functions.

1. Add OnBackPressedCallback in fragment to run your special operation when back is pressed present in system navigation bar at bottom.

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
   
        onBackPressedCallback = object : OnBackPressedCallback(true) {
            override fun handleOnBackPressed() {
                //perform your operation and call navigateUp
               findNavController().navigateUp()
            }
        }
    requireActivity().onBackPressedDispatcher.addCallback(onBackPressedCallback)
    }
   

2. Add onOptionsItemMenu in fragment to handle back arrow press present at top left corner within the app.

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
   
        setHasOptionsMenu(true)
    }
   
   override fun onOptionsItemSelected(item: MenuItem): Boolean {
      if (item.itemId == android.R.id.home) {
          //perform your operation and call navigateUp
          findNavController().navigateUp()
          return true
      }
      return super.onOptionsItemSelected(item)
   }
   

3. If there is no special code to be run when back is pressed on host fragment then use onSupportNavigateUp in Activity.

    override fun onSupportNavigateUp(): Boolean {
      if (navController.navigateUp() == false){
        //navigateUp() returns false if there are no more fragments to pop
        onBackPressed()
      }
      return navController.navigateUp()
    }
   

Note that onSupportNavigateUp() is not called if the fragment contains onOptionsItemSelected()

Answer 3

As my back button works correctly, and using NavController.navigateUp() crashed on start destination back button. I have changed this code to something like this. Other possibility will be to just check if currentDestination == startDestination.id but I want to close Activity and go back to other Activity.

override fun onSupportNavigateUp() : Boolean {
        //return findNavController(R.id.wizard_nav_host_fragment).navigateUp()
        onBackPressed()
        return true
    }

Answer 4

/** in your activity **/
private boolean doubleBackToExitPressedOnce = false;
        @RequiresApi(api = Build.VERSION_CODES.M)
        @Override
        public void onBackPressed() {
            int start = Navigation.findNavController(this, R.id.nav_host_fragment).getCurrentDestination().getId();
            if (start == R.id.nav_home) {
                if (doubleBackToExitPressedOnce) {
                    super.onBackPressed();
                    return;
                }
                this.doubleBackToExitPressedOnce = true;
                Toast.makeText(MainActivity.this, "Press back again to exits", Toast.LENGTH_SHORT).show();

                new Handler().postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        doubleBackToExitPressedOnce = false;
                    }
                }, 2000);
            } else {
                super.onBackPressed();
            }
        }

Answer 5

If you mean the start of your "root" navigation graph (just incase you have nested navigation graphs) then you shouldn't be showing an up button at all, at least according to the navigation principles.

Answer 6

Just call this in your back button Onclick

 requireActivity().finish()