Perform background tasks when app is terminated

Question

I'm creating an application for my school which should check every n minutes if there is a new mark on the website. To do this when the user login for the first time, the number of the actual mark is saved in the "UserDefaults". When app is terminated, after n minutes, the number of mark is recounted and compared with the previous one and send a notification in case the number is changed.

What I'd like to know if there is a way to perform this task. I've tried to create a timer in -applicationWillTerminate- but it's fired only once. This is what I tried:

func applicationWillTerminate(_ application: UIApplication) {
    DispatchQueue.main.async {
        self.timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(AppDelegate.findMark), userInfo: nil, repeats: true)
        self.timer.fire()
    }
}

Selector findMark is the task.

Thanks in advance!

Answer 1

You have two options

1. Background App Refresh
2. Silent push notifications

Easiest one is Background App Refresh. Because later one needs a server to send the notification. You can check following API for the usage. Basically you set Background Fetch capability on Capabilities/Background Modes of your app. Then from time to time, iOS will wake up your app and call application(_:performFetchWithCompletionHandler:) delegate. You will have around 30-45 seconds to call your function and call completion handler. If you don't finish it on time, iOS will kill your app. If you don't obey the rules, iOS will give you less chances to wake up. For more detailed usage of Background Modes, you may check following tutorial

Answer 2

It's not possible to perform tasks like described in your question after the app is terminated. As described in the documentation:

App Termination

Apps must be prepared for termination to happen at any time and should not wait to save user data or perform other critical tasks. System-initiated termination is a normal part of an app’s life cycle. The system usually terminates apps so that it can reclaim memory and make room for other apps being launched by the user, but the system may also terminate apps that are misbehaving or not responding to events in a timely manner.

Suspended apps receive no notification when they are terminated; the system kills the process and reclaims the corresponding memory. If an app is currently running in the background and not suspended, the system calls the applicationWillTerminate: of its app delegate prior to termination. The system does not call this method when the device reboots.

In addition to the system terminating your app, the user can terminate your app explicitly using the multitasking UI. User-initiated termination has the same effect as terminating a suspended app. The app’s process is killed and no notification is sent to the app.

Link: https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/TheAppLifeCycle/TheAppLifeCycle.html

Edit:

You cannot perform any task after the application is terminated. What you can do is get that calculation done from server side and send a Push Notification to the device.

Answer 3

Accutally , the answers is yes. But you should not. We can use Location to archive your goal. Accroding to offical document in here: https://developer.apple.com/library/archive/documentation/UserExperience/Conceptual/LocationAwarenessPG/CoreLocation/CoreLocation.html#//apple_ref/doc/uid/TP40009497-CH2-SW10:

Note: If your app is terminated either by a user or by the system, the system doesn’t automatically restart your app when new location updates arrive. A user must explicitly relaunch your app before the delivery of location updates resumes. The only way to have your app relaunched automatically is to use region monitoring or the significant-change location service.

So we can archive it by using Starting the Significant-Change Location Service:

If you leave the significant-change location service running and your iOS app is subsequently suspended or terminated, the service automatically wakes up your app when new location data arrives. At wake-up time, the app is put into the background and you are given a small amount of time (around 10 seconds) to manually restart location services and process the location data. (You must manually restart location services in the background before any pending location updates can be delivered, as described in Knowing When to Start Location Services.) Because your app is in the background, it must do minimal work and avoid any tasks (such as querying the network) that might prevent it from returning before the allocated time expires. If it does not, your app will be terminated. If an iOS app needs more time to process the location data, it can request more background execution time using the beginBackgroundTaskWithName:expirationHandler: method of the UIApplication class.

Call location update in willFinishLaunchingWithOptions and applicationDidBecomeActive Then you can excute your own code right after

[_locationManager startUpdatingLocation];

But is is extremly drain your battery, you may be rejected by app store.