Is NSTimer expected to fire when app is backgrounded?

Question

I don\'t understand it at all but NSTimer in my app definitely is running in background. I have a NSLog in method run by the timer and it is loggin

Answer 1

You can have a timer fire while in background execution mode. There are a couple of tricks:

• You need to opt into background execution with beginBackgroundTaskWithExpirationHandler.
• If you create the NSTimer on a background thread, you need to add it to the mainRunLoop manually.

---

- (void)viewDidLoad
{
    // Avoid a retain cycle
    __weak ViewController * weakSelf = self;

    // Declare the start of a background task
    // If you do not do this then the mainRunLoop will stop
    // firing when the application enters the background
    self.backgroundTaskIdentifier =
    [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{

        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundIdentifier];
    }];

    // Make sure you end the background task when you no longer need background execution:
    // [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskIdentifier];


    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        // Since we are not on the main run loop this will NOT work:
        [NSTimer scheduledTimerWithTimeInterval:0.5
                                         target:self
                                       selector:@selector(timerDidFire:)
                                       userInfo:nil
                                        repeats:YES];

        // This is because the |scheduledTimerWithTimeInterval| uses
        // [NSRunLoop currentRunLoop] which will return a new background run loop
        // which will not be currently running.
        // Instead do this:
        NSTimer * timer =
        [NSTimer timerWithTimeInterval:0.5
                                target:weakSelf
                              selector:@selector(timerDidFire:)
                              userInfo:nil
                               repeats:YES];

        [[NSRunLoop mainRunLoop] addTimer:timer
                                  forMode:NSDefaultRunLoopMode];
        // or use |NSRunLoopCommonModes| if you want the timer to fire while scrolling
    });
}

- (void) timerDidFire:(NSTimer *)timer
{
    // This method might be called when the application is in the background.
    // Ensure you do not do anything that will trigger the GPU (e.g. animations)
    // See: http://developer.apple.com/library/ios/DOCUMENTATION/iPhone/Conceptual/iPhoneOSProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html#//apple_ref/doc/uid/TP40007072-CH4-SW47
    NSLog(@"Timer did fire");
}

Notes

• Apps only get ~ 10 mins of background execution - after this the timer will stop firing.
• As of iOS 7 when the device is locked it will suspend the foreground app almost instantly. The timer will not fire after an iOS 7 app is locked.