How to use spinner

MainActivity.kt

class MainActivity : AppCompatActivity(), OnItemSelectedListener {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        findViewById<Spinner>(R.id.spinner).onItemSelectedListener = this
    }

    // 項目が選択された場合
    override fun onItemSelected(parent: AdapterView<*>?, view: View?, position: Int, id: Long) {
        Toast.makeText(this, "${parent?.selectedItem}が選択されました", Toast.LENGTH_SHORT).show()
    }

    // 項目が選択されなかった場合
    override fun onNothingSelected(parent: AdapterView<*>?) {
    }
}