Navigation Architecture Componentでカスタムのhome iconを表示する方法

Problem

• Navigation Architecture Componentを使ってActionBarをセットアップした場合は、FragmentのonCreateView イベントより後でsupportActionBar?setHomeAsUpIndicatorを呼び出さないと、アイコンを変更できないようです。
  • 参考: https://stackoverflow.com/questions/53061213/navigation-architecture-component-how-to-set-change-custom-back-or-hamburger-i

Solution

MainActivity.kt

class MainActivity : AppCompatActivity() {
    private val navController get() = NavHostFragment.findNavController(nav_host_fragment)
    private val topLevelDestinationIds = setOf(R.id.homeFragment)

    // ...

    fun onFragmentViewCreated() {
        setHomeIcon(R.drawable.ic_custom_back)
    }

    private fun setHomeIcon(resourceID: Int) {
        val destinationID = navController.currentDestination?.id ?: return
        val isTopLevelDestination = (destinationID in topLevelDestinationIds)
        supportActionBar?.setDisplayHomeAsUpEnabled(isTopLevelDestination.not())
        supportActionBar?.setHomeAsUpIndicator(if (isTopLevelDestination) 0 else resourceID)
    }
}

BaseFragment.kt

abstract class BaseFragment : Fragment() {
    private val mainActivity: MainActivity? get() = (activity as? MainActivity)

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