====== Bevezető duma ======
Egy hibát nyomozok a Siskin IM nevű, nyílt forrású, iOS appban.

A hibaüzenet: 

{{:it:screenshot_20201014-143153.jpg?400|Siskin IM push reg error}}

Végignéztem a kódot, csak nem értem. :)

====== Kódrészletek ======
[[https://github.com/tigase/siskin-im/blob/ab733cbc3bc5bf6eafaf0d860546729e8da84b70/SiskinIM/AppDelegate.swift|AppDelegate.swift (585):]]

<code swift>
@objc func pushNotificationRegistrationFailed(_ notification: NSNotification) {
        let account = notification.userInfo?["account"] as? BareJID;
        let errorCondition = (notification.userInfo?["errorCondition"] as? ErrorCondition) ?? ErrorCondition.internal_server_error;
        let content = UNMutableNotificationContent();
        switch errorCondition {
        case .remote_server_timeout:
            content.body = "It was not possible to contact push notification component.\nTry again later."
        case .remote_server_not_found:
            content.body = "It was not possible to contact push notification component."
        case .service_unavailable:
            content.body = "Push notifications not available";
        default:
            content.body = "It was not possible to contact push notification component: \(errorCondition.rawValue)"; // <--- the error we get, with errorCondition.rawValue = undefined-condition
}
</code>


[[https://github.com/tigase/siskin-im/blob/a5a8643c295ad9acad3993ccac15cfae349c3cf1/SiskinIM/settings/AccountSettingsViewController.swift|AccountSettingsViewController.swift (238):]]


<code swift>
fileprivate func enablePushNotifications(action: UIAlertAction) {
        let accountJid = self.account!;
        let onError = { (_ errorCondition: ErrorCondition?) in  // <--- definition of onError
            DispatchQueue.main.async {
                var userInfo: [AnyHashable:Any] = ["account": accountJid];
                if errorCondition != nil {
                    userInfo["errorCondition"] = errorCondition;
                }
                NotificationCenter.default.post(name: Notification.Name("pushNotificationsRegistrationFailed"), object: self, userInfo: userInfo);
            }
        }
        // let's check if push notifications component is accessible
        if let pushModule: SiskinPushNotificationsModule = XmppService.instance.getClient(forJid: accountJid)?.modulesManager.getModule(SiskinPushNotificationsModule.ID), let deviceId = PushEventHandler.instance.deviceId {
            pushModule.registerDeviceAndEnable(deviceId: deviceId, pushkitDeviceId: PushEventHandler.instance.pushkitDeviceId, completionHandler: { result in // <--- actual registration attempt
                switch result {
                case .success(_):
                    break;
                case .failure(let errorCondition): // <--- errorCondition is set here 
                    DispatchQueue.main.async {
                        self.pushNotificationSwitch.isOn = false;
                        self.pushNotificationsForAwaySwitch.isOn = false;
                    }
                    onError(errorCondition);  // <--- onError is called here
                }
});
</code>


[[https://github.com/tigase/siskin-im/blob/275a939f1cb4a070473ae81c49e0fedffc43f6b4/SiskinIM/xmpp/SiskinPushNotificationsModule.swift|SiskinPushNotificationsModule.swift (99):]]

<code swift>
open func registerDeviceAndEnable(deviceId: String, pushkitDeviceId: String?, completionHandler: @escaping (Result<PushSettings,ErrorCondition>)->Void) {
        self.findPushComponent { result in
            switch result {
            case .success(let jid):
                self.registerDeviceAndEnable(deviceId: deviceId, pushkitDeviceId: pushkitDeviceId, pushServiceJid: jid, completionHandler: completionHandler);
            case .failure(_):
                self.registerDeviceAndEnable(deviceId: deviceId, pushkitDeviceId: pushkitDeviceId, pushServiceJid: self.defaultPushServiceJid, completionHandler: completionHandler);
            }
        }
}
</code>


[[https://github.com/tigase/siskin-im/blob/275a939f1cb4a070473ae81c49e0fedffc43f6b4/SiskinIM/xmpp/SiskinPushNotificationsModule.swift
|SiskinPushNotificationsModule.swift (265):]]

<code swift>
func findPushComponent(completionHandler: @escaping (Result<JID,ErrorCondition>)->Void) {
      self.findPushComponent(requiredFeatures: ["urn:xmpp:push:0", self.providerId], completionHandler: completionHandler);
}
</code>


====== Kérdések ======
** 1. Mi annak a szemantikája, hogy egy függvény meghívja önmagát? Vagy mi történik itt pontosan? **
 

Két példa is van rá:

<code swift>
func registerDeviceAndEnable (...) {
   ...
   self.registerDeviceAndEnable (...)
   ...
}   
</code>

   
<code swift>
func findPushComponent(completionHandler: @escaping (Result<JID,ErrorCondition>)->Void) {
        self.findPushComponent(
                requiredFeatures: ["urn:xmpp:push:0", self.providerId], 
                completionHandler: completionHandler
        );
}
</code>

A második eset még érdekesebb.

**2. Van plusz egy argumentum a hívásnál. Ez hogy lehetséges?**

**3. A függvénynek egy sora van, ami – ha jól értem – az implicit visszatérési értéke. Önmagával tér vissza a függvény? Ez mire jó?**

====== Felelősségkizárás és köszönet ======
Az ügy nem sürgős, csak akkor válaszolj, ha nincs fontosabb. 

És előre is nagyon köszi a segítséget. :)