Попробуйте следующим образом
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*
import android.widget.Toast
import com.google.android.material.shape.CornerFamily
import com.google.android.material.shape.MaterialShapeDrawable
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val radius = resources.getDimension(R.dimen.roundcorner)
val navViewBackground = bottomNavigationView.getBackground() as MaterialShapeDrawable
navViewBackground.shapeAppearanceModel = navViewBackground.shapeAppearanceModel
.toBuilder()
.setTopRightCorner(CornerFamily.ROUNDED, radius)
.setTopLeftCorner(CornerFamily.ROUNDED, radius)
.build()
bottomNavigationView.setOnNavigationItemSelectedListener { item ->
when (item.itemId) {
R.id.action_home -> Toast.makeText(
this@MainActivity,
"HOME",
Toast.LENGTH_SHORT
).show()
R.id.action_favorite -> Toast.makeText(
this@MainActivity,
"Favorites",
Toast.LENGTH_SHORT
).show()
R.id.action_cart -> Toast.makeText(
this@MainActivity,
"CART",
Toast.LENGTH_SHORT
).show()
R.id.action_profile -> Toast.makeText(
this@MainActivity,
"PROFILE",
Toast.LENGTH_SHORT
).show()
}
true
}
}
}
ВЫХОД
Полный код можно найти здесь округлено-низкоуровневое навигационное сообщение