画面上の戻るボタン

    override fun onCreate(savedInstanceState: Bundle?) {
..
        //戻る
        supportActionBar?.setDisplayHomeAsUpEnabled(true)
    }

    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        when (item.itemId) {
            android.R.id.home ->{
                finish()
                return true
            }
            else -> return super.onOptionsItemSelected(item)
        }
    }