按钮返回带有导航组件的 startDestination

Question

I need a second activity with a nav graph and have a return button in toolbar to the first activity that also contains a nav graph

In my second activity I have onSupportNavigateUp and setupActionBarWithNavController when entering the fragments if the arrow back button appears but in the activity no.

Try adding setHomeButtonEnabled and setDisplayHomeAsUpEnabled in both the activity and the fragment and if the button appears back, but when I enter some fragment in front and return to the fragment startDestination disappears the button back

I just need to keep the button back in the activity and solve my problem

Answer 1

You can do it by specifying a setFallbackOnNavigateUpListener:

private fun setupToolbar() {
    val navController = findNavController(R.id.nav_host_fragment)

    val appBarConfiguration =
        AppBarConfiguration.Builder()
            .setFallbackOnNavigateUpListener { onNavigateUp() }
            .build()

    dataBinding.toolbar.apply {
        setupWithNavController(navController, appBarConfiguration)
    }
}

And then do whatever you want in the Activity's:

override fun onNavigateUp(): Boolean {
    finish()
    return true
}

Answer 2

You can't, activity has their own toolbars and in your case they have two different NavControllers. So your second activity manage NavUp Button for his fragment and when start Destination fragment comes NavUpButton(Backbutton) disappear because it has no destination left behind. And if you programmatically show NavUp Button on start destination of that (2nd activity) and manage onClick and start first activity that always goes to Start destination of first activity's fragment because it has it's own Nav Controller.

Problem is that Navigation UI not works like that. The better approach is use only one activity with multiple fragments. And use any other approach to solve your problem within the same nav controller.

Answer 3

Add setHomeButtonEnabled func. to your returning action. If you are returning with button add it to onClick or with backPress, override backPress.

With this solve : You will set your button enable, when you try to return your startDestination.

Answer 4

I created an interface to show/hide up button from the nav host activity. Here is how the activity implements the interface methods to show/hide up button:

    override fun showUpButton() {
    val navController = this.findNavController(R.id.nav_host)
    val listener = AppBarConfiguration.OnNavigateUpListener { navController.navigateUp() }
    val abc = AppBarConfiguration.Builder().setFallbackOnNavigateUpListener(listener).build()
    NavigationUI.setupActionBarWithNavController(this, navController, abc)
}


override fun hideUpButton() {
    val navController = this.findNavController(R.id.nav_host)
    NavigationUI.setupActionBarWithNavController(this, navController)
}

Here the method in the activity when up button pressed:

    override fun onSupportNavigateUp(): Boolean {
    val navController = this.findNavController(R.id.nav_host)
    if(!navController.navigateUp()){ // When in start destination
        onBackPressed()
    }
    return navController.navigateUp()
}

In a fragment can listen whenever back button (NOT up button) pressed:

    private fun setupBackPress() {
    requireActivity()
        .onBackPressedDispatcher
        .addCallback(this, object : OnBackPressedCallback(true) {
            override fun handleOnBackPressed() { 
            }
        })
}