BottomNavFragment.kt

class RootFragment: KeyedFragment(R.layout.root_fragment) {
    private val binding by viewBinding(RootFragmentBinding::bind)

    private lateinit var firstFragment: FirstFragment
    private lateinit var secondFragment: SecondFragment
    private lateinit var thirdFragment: ThirdFragment

    private val fragments: Array<Fragment> get() = arrayOf(firstFragment, secondFragment, thirdFragment)
    private var selectedIndex = 0

    private val tabs: Array<TextView> get() = binding.run {
        arrayOf(buttonFirstTab, buttonSecondTab, buttonThirdTab)
    }

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

        if (savedInstanceState == null) {
            val firstFragment = FirstFragment().also { this.firstFragment = it }
            val secondFragment = SecondFragment().also { this.secondFragment = it }
            val thirdFragment = ThirdFragment().also { this.thirdFragment = it }

            childFragmentManager.beginTransaction()
                .add(R.id.containerBottomNavContent, firstFragment, "firstFragment")
                .add(R.id.containerBottomNavContent, secondFragment, "secondFragment")
                .add(R.id.containerBottomNavContent, thirdFragment, "thirdFragment")
                .selectFragment(selectedIndex)
                .commit()
        } else {
            selectedIndex = savedInstanceState.getInt("selectedIndex", 0)

            firstFragment = childFragmentManager.findFragmentByTag("firstFragment") as FirstFragment
            secondFragment = childFragmentManager.findFragmentByTag("secondFragment") as SecondFragment
            thirdFragment = childFragmentManager.findFragmentByTag("thirdFragment") as ThirdFragment
        }
    }


    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        tabs.forEachIndexed { index, textView ->
            textView.setOnClickListener {
                selectFragment(index)
            }
        }

        setupTabSelectedState(selectedIndex)
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putInt("selectedIndex", selectedIndex)
    }

    private fun FragmentTransaction.selectFragment(selectedIndex: Int): FragmentTransaction {
        fragments.forEachIndexed { index, fragment ->
            if (index == selectedIndex) {
                attach(fragment)
            } else {
                detach(fragment)
            }
        }

        return this
    }

    private fun setupTabSelectedState(selectedIndex: Int) {
        tabs.forEachIndexed { index, textView ->
            textView.setTextColor(when {
                index == selectedIndex -> ContextCompat.getColor(requireContext(), R.color.tab_selected)
                else -> ContextCompat.getColor(requireContext(), R.color.tab_unselected)
            })
        }
    }

    private fun selectFragment(indexToSelect: Int) {
        this.selectedIndex = indexToSelect

        setupTabSelectedState(indexToSelect)

        childFragmentManager.beginTransaction()
            .selectFragment(indexToSelect)
            .commit()
    }
}