This code makes your iOS app run indefinitely in the background. Copy and paste the below methods into a singleton / manager which handles the tasks you need to perform in the background.

iOS Backgrounding

// @interface

// Declare Private property
@property (nonatomic) UIBackgroundTaskIdentifier backgroundTask;

//@end
// ...

// Copy into
//@implementation 

- (void)setupBackgrounding {
    self.backgroundTask = UIBackgroundTaskInvalid;
    
    [[NSNotificationCenter defaultCenter] addObserver: self selector: @selector(appBackgrounding:)
                                                 name: UIApplicationDidEnterBackgroundNotification
                                               object: nil];
    [[NSNotificationCenter defaultCenter] addObserver: self selector: @selector(appForegrounding:)
                                                 name: UIApplicationWillEnterForegroundNotification
                                               object: nil];
}

- (void)appBackgrounding: (NSNotification *)notification {
    [self keepAlive];
}

- (void) keepAlive {
    self.backgroundTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTask];
        self.backgroundTask = UIBackgroundTaskInvalid;
        [self keepAlive];
    }];
}

- (void)appForegrounding: (NSNotification *)notification {
    if (self.backgroundTask != UIBackgroundTaskInvalid) {
        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTask];
        self.backgroundTask = UIBackgroundTaskInvalid;
    }
}