How to call completionHandler from Singleton class? - ios

Question

I'm writing an App in Swift where I have implemented a singleton class handling the data retrieval via Moscapsule.
When I inform my app that there is new data with content-available: 1 the right function gets called where I'm currently only doing
let mqtt = MQTTManager.Instance
completionHandler(UIBackgroundFetchResult.NewData)
The manager creates a connection on init and the onMessageCallback then works with the new data.
If the app has run recently, is in background and the singleton class keeps up the network connection, this works. But this is not the right way.
I'm very new to iOS and Swift development, how can I wait the maximum amount of time? When the data comes in I could then send UIBackgroundFetchResult.NewData, if nothing comes, I could then send UIBackgroundFetchResult.Failed.
My first idea was to write a loop checking a variable from that class if a new message has come in every 500ms, and if so, call the completionHandler. But this doesn't feel right to me, so, what would you do?
EDIT 2015/02/18:
Heres the code from my AppDelegate.swift that gets executed once the app receives the silent push notification:
func application(application: UIApplication, didReceiveRemoteNotification userInfo: [NSObject : AnyObject], fetchCompletionHandler completionHandler: (UIBackgroundFetchResult) -> Void) {
log.debug("Got remote notification: \(userInfo)")
log.debug("Setting up MQTT client...")
let mqtt = MQTTManager.sharedInstance
completionHandler(UIBackgroundFetchResult.NewData)
}

Answer 1

You could define a property within that class to hold the completion handler (making it an optional):
var completionHandler: ((UIBackgroundFetchResult) -> Void)?
Then your didReceiveRemoteNotification would do something like:
func application(application: UIApplication, didReceiveRemoteNotification userInfo: [NSObject : AnyObject], fetchCompletionHandler completionHandler: (UIBackgroundFetchResult) -> Void) {
log.debug("Got remote notification: \(userInfo)")
log.debug("Setting up MQTT client...")
let mqtt = MQTTManager.sharedInstance
mqtt.completionHandler = completionHandler
}
Then, the onMessageCallback could then do something like:
self.completionHandler?(.NewData) // or .NoData or whatever
self.completionHandler = nil
I must confess that this doesn't feel right. It feels like the init function is not only instantiating the singleton, but also starting a connection, making a request, what have you. You haven't shared it with us, so it's hard to say, but it feels incorrect. The init should really only be creating the singleton. Then, let us configure it (e.g. specify the completionHandler) and only then would it call the separate connect function.