作業ログ 2021/04/01

        // NavHostFragmentを取得（MainActivityのFragmentContainerから）
        val navHostFragment = supportFragmentManager.findFragmentById(R.id.main_fragment_container) as NavHostFragment
        setupActionBarWithNavController(
            // 取得したNavHostFragmentのNavControllerを指定
            navHostFragment.findNavController(),
            // 指定したFragmentでのみバックボタンを表示しないようにする
            AppBarConfiguration(setOf(R.id.categoryListFragment))
        )

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val navDestination =  when(condition){
            A -> {
                StartFragmentDirections.actionStartFragmentToAFragment()
            }
            B -> {
                StartFragmentDirections.actionStartFragmentToBFragment()
            }
            C -> {
                StartFragmentDirections.actionStartFragmentToCFragment()
            }
        }
        findNavController().navigate(navDestination)
    }

    dependencies {
        // ~~~
        def nav_version = "2.3.4"
        classpath "androidx.navigation:navigation-safe-args-gradle-plugin:$nav_version"
        // ~~~
    }

plugins {
    // ~~~
    id 'androidx.navigation.safeargs.kotlin'
}