FragmentAdapter.kt

class FragmentAdapter(
        fragmentManager: FragmentManager
) : FragmentPagerAdapter(fragmentManager) {

    val mFragmentList: ArrayList<Fragment> = ArrayList()
    private val mFragmentTitleList: MutableList<String> = mutableListOf()


    override fun getItem(position: Int): Fragment {
        return mFragmentList[position]
    }

    override fun getCount(): Int {
        return mFragmentList.count()
    }

    fun addFragment(fragment: Fragment, title: String = "") {
        mFragmentList.add(fragment)
        mFragmentTitleList.add(title)
    }

    override fun getPageTitle(position: Int): CharSequence {
        return mFragmentTitleList[position]
    }

MainFragment.kt

class MainFragment : Fragment() {
    private val tabFrag by lazy { tab_frag }
    private val pagerFrag by lazy { pager_frag }

    private lateinit var pagerAdapter: FragmentAdapter

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_main, container, false)
    }

    override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        pagerAdapter = FragmentAdapter(childFragmentManager)
        pagerAdapter.addFragment(FirstFragment(), "First")
        pagerAdapter.addFragment(ScndFragment(), "Second")
        pagerAdapter.addFragment(ThirdFragment(), "Third")
        pagerFrag.adapter = pagerAdapter
        tabFrag.setupWithViewPager(pagerFrag)
        tabFrag.setOnTabSelectedListener(OnTabCustomSelectedListener(context, tabFrag))
        val tabCount = tabFrag.tabCount
        for (i in 0 until tabCount) {
            val tab = tabFrag.getTabAt(i)
            if (tab != null) {
                val tabTextView = LayoutInflater.from(context).inflate(R.layout.item_tab_text, tabFrag, false) as TextView
                tabTextView.text = tab!!.text
                tabTextView.setTextAppearance(context,
                        if (i == 0) {
                            R.style.TextAppearance_Tabs_Selected
                        } else {
                            R.style.TextAppearance_Tabs
                        })
                tab.customView = tabTextView
            }
        }
    }