How to setOnNavigationItemListener on BottomNavigationView in android using Kotlin?

Question

I use kotlin-android-extension and I can call bottomNavigationView id from layout file to kotlin file. I can use bottomNavigationView.setOnNavigationItemSelectedListener(BottomNavigationView.OnNavigationItemSelectedListener {}), but whats next?

As far as I know in Java, there is another function called onNavigationItemSelected, but I can't find it in kotlin.

this is the example code I want to use in Java but cannot write it in kotlin.

bottomNavigationView.setOnNavigationItemSelectedListener(
    new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()) {
                case R.id.action_favorites:

                case R.id.action_schedules:

                case R.id.action_music:

            }
            return true;
        }
    });

this app already use kotlin from the start, so I can only write kotlin right? not write Java in kotline file. — j.elmer, Jun 18 '17 at 02:59

score 23 · Accepted Answer · edited May 05 '20 at 11:14

23

You can use this format of code:

bottomNavigation.setOnNavigationItemSelectedListener { item ->
    when (item.itemId) {
        R.id.action_favorites -> {
        }
        R.id.action_schedules -> {
        }
        R.id.action_music -> {
        }
    }
    true
}

edited May 05 '20 at 11:14

Miha_x64

5,973
1
41
63

answered Jun 18 '17 at 03:01

Glory

1,007
9
14

3

What does that "true" stand for? – Chris Nov 24 '17 at 20:40
Sorry, the "return" expression is mistakenly removed from code during the last edit. I've fixed it now. Thanks. @Chris – Glory Nov 26 '17 at 14:34
when i set return true this show errors message, only false keep error clean.? what is that.. – Ye Htun Z Jan 04 '18 at 15:37
2

It's not working with the `return true`, you have to add `else -> true` to the when block. Otherwise you will get the error "When expression must be exhaustive, add necessary else branch" – mbo Jan 12 '18 at 08:01
@Glory how to set one default menuitem among the items? – Eswar Jul 25 '18 at 14:25
@Chris return true to display the item as the selected item. https://developer.android.com/reference/android/support/design/widget/NavigationView.OnNavigationItemSelectedListener – Johnny Five Apr 17 '19 at 08:43

score 9 · Answer 2 · answered Aug 23 '18 at 17:15

9

You can use below code

bottom_navigation.setOnNavigationItemSelectedListener {
            var selectedFragment: Fragment = A()
            when (it.itemId) {
                R.id.action_item1 -> selectedFragment = A()
                R.id.action_item2 -> selectedFragment = B()
                R.id.action_item3 -> selectedFragment = C()
            }
            val transaction = fragmentManager.beginTransaction()
            transaction.replace(R.id.frame_layout, selectedFragment)
            transaction.commit()
            return@setOnNavigationItemSelectedListener true
        }

answered Aug 23 '18 at 17:15

Thanh vũ

169
3
1

1

Why we have to add @setOnNavigationItemSelectedListener after return word ? – twenk11k Oct 31 '19 at 13:15
wrong! the when is executed only once when the listener is set, so it's not equivalent to the java code in the question – AndrewBloom Nov 05 '19 at 15:21

Ali Hasan · Answer 3 · 2019-12-13T13:37:20.577

7

use must add annotation for return the lambda only

 bottomNavigation.setOnNavigationItemSelectedListener { item ->
        when(item.itemId){
            R.id.home -> {}

            R.id.group -> {}

            R.id.profile -> {}
        }
        return true
    }

edited Dec 13 '19 at 13:37

answered Feb 03 '18 at 16:49

Ali Hasan

653
9
8

wrong! the when is executed only once when the listener is set, so it's not equivalent to the java code in the question – AndrewBloom Nov 05 '19 at 15:21

score 4 · Answer 4 · answered Nov 05 '19 at 15:19

bottomNavigationView.setOnNavigationItemSelectedListener {
            when (it.itemId) {
                R.id.action_favorites -> { }
                R.id.action_schedules -> { }
                R.id.action_music -> { }
            }
            true
        }

note that last line seems to miss the return keyword, but

The last expression in a lambda is considered the return value:

from https://kotlinlang.org/docs/reference/lambdas.html

Moreover, @setOnNavigationItemSelectedListener creates a

local final fun <anonymous> (it: Menuitem) : Boolean

wrapping what follows, so in some answers this will have the effect of executing the when block only when the listener is set (only one time), and the callback will be just a return true statement.

score 4 · Answer 5 · answered Jan 19 '20 at 02:20

This is my code using the new Navigation component. Let me know if you need help with nav ui.

bottom_nav.setOnNavigationItemSelectedListener {
            when (it.itemId) {
                R.id.home -> {
                    findNavController(R.id.nav_host_fragment)
                        .navigate(R.id.mainFragment)
                }
                R.id.search -> {
                    findNavController(R.id.nav_host_fragment)
                        .navigate(R.id.searchFragment)
                }
                R.id.inapppurchases -> {
                    findNavController(R.id.nav_host_fragment)
                        .navigate(R.id.inappPurchasesFragment)
                }
                R.id.settings -> {
                    findNavController(R.id.nav_host_fragment)
                        .navigate(R.id.settingsFragment)
                }
            }
            true
        }

score 2 · Answer 6 · answered Feb 25 '18 at 14:35

return boolean in each block because setOnNavigationItemSelectedListener expects return type

fun initViews() {
    bottomNavigationView.setOnNavigationItemSelectedListener { item ->
        when (item.itemId) {
            R.id.action_menu_media -> {
                true
            }
            R.id.action_menu_tag -> {
                true
            }
            R.id.action_menu_home -> {
                true
            }
            else -> {
                true
            }
        }

    }
}

score 0 · Answer 7 · answered Jun 19 '22 at 13:43

kotlin: use setOnItemSelectedListener

bottomNavigationView.setOnItemSelectedListener { item: MenuItem ->
        when (item.itemId) {
             R.id. ... -> {
                Add your code
             true
        }
        
        else ->
            true
    }

How to setOnNavigationItemListener on BottomNavigationView in android using Kotlin?

7 Answers7