class MainActivity : AppCompatActivity() {
    // ...

    /** Requests an overlay permission to the user if needed. */
    private fun requestOverlayPermission() {
        if (isOverlayGranted()) return
        val intent = Intent(
            Settings.ACTION_MANAGE_OVERLAY_PERMISSION,
            Uri.parse("package:$packageName")
        )
        startActivityForResult(intent, OVERLAY_PERMISSION_REQUEST_CODE)
    }

    /** Checks if the overlay is permitted. */
    private fun isOverlayGranted() =
        Build.VERSION.SDK_INT < Build.VERSION_CODES.M ||
        Settings.canDrawOverlays(this)
}

/** Terminates the app if the user does not accept an overlay. */
override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    super.onActivityResult(requestCode, resultCode, data)
    if (requestCode == OVERLAY_PERMISSION_REQUEST_CODE) {
        if (!isOverlayGranted()) {
            finish()  // Cannot continue if not granted
        }
    }
}

/** Settings for overlay view */
private val layoutParams = WindowManager.LayoutParams(
    WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY,  // Overlay レイヤに表示
    WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE  // フォーカスを奪わない
        or WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE
        or WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL
        or WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS,  // 画面外への拡張を許可
    PixelFormat.TRANSLUCENT
)

fun showAsOverlay() {
    windowManager.addView(myView, layoutParams)
}

class OverlayService : Service() {
    override fun onCreate() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            // Start as a foreground service
            val notification = MyNotification.build(this)
            startForeground(1, notification)
        }
        // ...

object MyNotification {
    private const val CHANNEL_ID = "channel_id_overlay_sample"
    private const val CHANNEL_NAME = "オーバーレイ表示の切り替え"
    private const val CHANNEL_IMPORTANCE = NotificationManager.IMPORTANCE_DEFAULT
    private const val FIRST_LINE = "オーバーレイ表示中"
    private const val SECOND_LINE = "ここから表示・非表示を切り替えられます。"
    private val ACTIVITY = MainActivity::class.java

    /**
     * Set the info for the views that show in the notification panel.
     */
    fun build(context: Context): Notification {
        // Create a notification channel
        val nm = context.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
        nm.createNotificationChannel(
            NotificationChannel(CHANNEL_ID, CHANNEL_NAME, CHANNEL_IMPORTANCE)
        )

        // The PendingIntent to launch our activity if the user selects this notification
        val pendingIntent = PendingIntent.getActivity(
            context, 0, Intent(context, ACTIVITY), 0
        )

        return Notification.Builder(context, CHANNEL_ID)
            .setAutoCancel(false)  // don't dismiss when touched
            .setContentIntent(pendingIntent)  // The intent to send when the entry is clicked
            .setContentTitle(FIRST_LINE)  // the label of the entry
            .setContentText(SECOND_LINE)  // the contents of the entry
            .setSmallIcon(R.drawable.cat)  // the status icon
            .setTicker(context.getText(R.string.app_name))  // the status text
            .setWhen(System.currentTimeMillis())  // the time stamp
            .build()
    }
}