ChooseDayDialog with optional request code

ChooseDayDialog.kt

import android.app.DatePickerDialog
import android.app.Dialog
import android.os.Bundle
import android.support.v4.app.DialogFragment
import java.util.*


class ChooseDayDialog : DialogFragment() {

    companion object {

        private val KEY_INITIAL_TIMESTAMP = "KEY_INITIAL_TIMESTAMP"
        private val KEY_REQUEST_CODE = "KEY_REQUEST_CODE"

        private val DEFAULT_REQUEST_CODE = 20

        fun createInstance(initialTimestamp: Long,
                           requestCode: Int = DEFAULT_REQUEST_CODE) =
                ChooseDayDialog().apply {
                    arguments = Bundle().apply {
                        putLong(KEY_INITIAL_TIMESTAMP, initialTimestamp)
                        putInt(KEY_REQUEST_CODE, requestCode)
                    }
                }

    }

    interface OnDayChosenListener {

        fun onDayChosen(requestCode: Int, year: Int, month: Int, dayOfMonth: Int)

    }

    private lateinit var listener: OnDayChosenListener

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val activity = activity
        if (activity is OnDayChosenListener) {
            listener = activity
        } else {
            throw IllegalStateException("Activity must implement OnDayChosenListener")
        }
    }

    override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
        val timestamp = arguments?.getLong(KEY_INITIAL_TIMESTAMP, -1L) ?: -1L
        if (timestamp == -1L) {
            throw IllegalStateException("no initial time given")
        }
        val requestCode = arguments?.getInt(KEY_REQUEST_CODE, DEFAULT_REQUEST_CODE)
                ?: DEFAULT_REQUEST_CODE

        val calendar = Calendar.getInstance().apply {
            timeInMillis = timestamp
        }
        return DatePickerDialog(activity,
                DatePickerDialog.OnDateSetListener { _, year, month, dayOfMonth ->
                    listener.onDayChosen(requestCode, year, month, dayOfMonth)
                },
                calendar.get(Calendar.YEAR),
                calendar.get(Calendar.MONTH),
                calendar.get(Calendar.DAY_OF_MONTH))
    }

}