Using androix backButtonPressedCallback

Question

I need to implement a custom onBackButtonPress method for Fragments. example in LogoutFragment, after logout is handled. user cannot go backStack but a message is shown like press again to exist and is exited. I used this solution. But is not working. Then I saw this Android Doc with onBackPressedDispatcher callback method CODE BELOW. I guess this will work. I added dependencies, but how to implement this in a different fragments, with only 1 activity and fragment container. Kotlin version.

    class LogoutFragment : DaggerFragment() {

       override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)           

                val callback = requireActivity().onBackPressedDispatcher.addCallback(this) {

                    ...
                }

               ...
        }
        ...
    }

score 1 · Accepted Answer · answered May 17 '20 at 07:58

onBackPressedDispatcher gives an opportunity to handle back press differently in every fragment. Hence you may have to give this piece of code in every fragment.

In the fragment:

var doubleBackToExitPressedOnce = false

 override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    requireActivity().onBackPressedDispatcher.addCallback(this, callback)
 }

 val callback = object : OnBackPressedCallback(true ) {
    override fun handleOnBackPressed() {
       if (doubleBackToExitPressedOnce)
          requireActivity().finish()
       Toast.makeText(requireContext(), "Press again to go back", 
          Toast.LENGTH_LONG).show()
       doubleBackToExitPressedOnce = true
    }
 }

If you want to handle back presses of all fragments in one place, you can do so by manipulating the doBack() method in your reference: How to implement onBackPressed() in Fragments?

//pseudocode

   fun doBack()
   {

     //find the fragment
     val fragment = supportFragmentManager.findFragmentByTag(
     //your fragment tag 
     )

     if(fragment is FragmentA)
     {

       //do something
     }
     else if(fragment is FragmentB)
     {

       //do something else
     }

     else
     {
        activity.getSupportFragmentManager().popBackStack(null, 
        FragmentManager.POP_BACK_STACK_INCLUSIVE);
     }

   }

Thanks a lot. this is working. I marked this as working answer @ACR — binrebin, May 17 '20 at 13:19
In the first case, there are no curly braces needed. In the second case , there can be chunks of code inside the if condition. Hence they stay. — ACR, May 17 '20 at 13:45
I was referering this `if (doubleBackToExitPressedOnce)` one. — binrebin, May 17 '20 at 13:52
No. It does not need it. If doubleBackToExitPressedOnce is false , the toast will be shown, which will be the first backpress. And next time, doubleBackToExitPressedOnce will be true, hence the activity will finish. — ACR, May 17 '20 at 13:56

Using androix backButtonPressedCallback

1 Answers1