UIBackgroundTaskIdentifier running NSTimer but not updating UILabel - iOS

Question

In my app, I have NSTimer which updates UILabel every second. I've added UIBackgroundTaskIdentifier before NSTimer to let it run in background.

__block UIBackgroundTaskIdentifier bgTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
                    [[UIApplication sharedApplication] endBackgroundTask:bgTask];
                    bgTask = UIBackgroundTaskInvalid;
                }];


timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(updateTime) userInfo:nil repeats:YES];

[[NSRunLoop mainRunLoop] addTimer:updateTimer forMode:NSRunLoopCommonModes];

. . .

-(void)updateTime
{
    secondsLeft -= 1;
    //NSLog(@"Update time : %d ......",secondsLeft);
    dispatch_async(dispatch_get_main_queue(), ^{
         _timeLbl.text = [NSString stringWithFormat:@"%d",secondsLeft];
    }); 
}

Problem is when we press home button app gets into background, timer is running but uilabel is not updating. E.g. Say, _timeLbl displaying '45'. Now if I press home button, app goes into background. After 10 sec. I click app icon to get app in foreground, and there I see '45' for a while (fraction of second) and then it display 35, instead of directly display 35. That means label is not getting updated with NSTimer. Is there any workaround to solve this ?

Thanks !

if you mean dispatch_async(dispatch_get_main_queue(), ^{ ...... }); .......tried this but no luck — iAkshay, Nov 28 '16 at 10:16
Try [this](http://stackoverflow.com/questions/11831412/ui-updates-with-background-tasks-on-ios) — Nikhil Manapure, Nov 28 '16 at 10:40
@Nikhil I've checked that but didn't get all of it. Could you please provide solution as reference to this link and my question ? — iAkshay, Nov 28 '16 at 10:52

score 1 · Answer 1 · answered Nov 28 '16 at 11:46

App Delegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions 
{

[[UIApplication sharedApplication]setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];

}



- (void)applicationWillResignActive:(UIApplication *)application {

[[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{

}];
 }

YOUR CONTROLLER.M

- (void)viewDidLoad {

[super viewDidLoad];
timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerTick:) userInfo:nil repeats:YES];

}

-(void) timerTick:(NSTimer *)timer1 {

int seconds = pauseTaskTime % 60;
int minutes = (pauseTaskTime - seconds) / 60;

lblTimer.text = [NSString stringWithFormat:@"%d:%.2d", minutes, seconds]
 }

When you complete your code then follows these steps :

1. Go to your project name.

2. Click Capabilities

3. On your Background Modes.

Thank You

score 0 · Answer 2 · edited Nov 28 '16 at 10:20

0

Try this

-(void)updateTime {
    secondsLeft -= 1;
    //NSLog(@"Update time : %d ......",secondsLeft);
    dispatch_async(dispatch_get_main_queue(), ^{
        timeLbl.text = [NSString stringWithFormat:@"%d",secondsLeft];
    });
}

If you change it on main queue, it will get updated. I faced it long back and sorted in this way.

edited Nov 28 '16 at 10:20

Rajat

10,977
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answered Nov 28 '16 at 10:18

Pushkraj Lanjekar

2,254
1
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34

Try this one http://stackoverflow.com/questions/16261620/timer-that-updates-label-running-in-background I am not able to see any linking between background task and update method in your code, so its not updating. You have 2 options: 1. Update your code to above one. or 2. Use didEnterForeground method to update label again. – Pushkraj Lanjekar Nov 28 '16 at 10:25
Thanks @Pushkraj, but I had visit this link before posting my question and did the same. – iAkshay Nov 28 '16 at 10:29

score 0 · Answer 3 · answered Nov 28 '16 at 10:22

0

Try updating the label using a reference date instead of instance variable secondsLeft.

secondsLeft = [[NSDate date] timeIntervalSinceDate:referenceDate]

answered Nov 28 '16 at 10:22

Windindi

412
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1. I can't do this since its for objective test and retrieved secondsLeft from database. .......... 2. How this make a trick for the problem ? – iAkshay Nov 28 '16 at 10:26
1

Updating UI in background is not possible as far my knowledge goes, I would suggest you display a minor loader when app comes in foreground till the UI gets updated. – Windindi Nov 28 '16 at 10:30

Subhajit Halder · Answer 4 · 2016-11-28T10:45:31.757

0

In your method -(void)updateTime after setting the text in your label write [_timeLbl layoutIfNeeded];.

E.g:

-(void)updateTime
{
    secondsLeft -= 1;
    //NSLog(@"Update time : %d ......",secondsLeft);
    _timeLbl.text = [NSString stringWithFormat:@"%d",secondsLeft]; 
   [_timeLbl layoutIfNeeded];
}

layoutIfNeeded checks that the view is needed to be updated or not and updates if needed. I one word it refreshes the view.

edited Nov 28 '16 at 10:45

answered Nov 28 '16 at 10:31

Subhajit Halder

1,427
13
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ok then i have to check it out when i reach home, and will make you know if i got a solution – Subhajit Halder Nov 28 '16 at 11:03

score 0 · Answer 5 · answered Nov 28 '16 at 10:34

0

this is working fine for me.

 [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:nil];
    countDown = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timeLeftSinceDate) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] addTimer:countDown forMode:NSRunLoopCommonModes];

answered Nov 28 '16 at 10:34

Logic

705
1
9
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the question's code contains bgTask and that is causing this issue. – Nikhil Manapure Nov 28 '16 at 10:38

score 0 · Answer 6 · answered Nov 28 '16 at 11:20

Use this Code

[[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:nil];
loop = [NSTimer scheduledTimerWithTimeInterval:0.25 target:self selector:@selector(Update) userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:loop forMode:NSRunLoopCommonModes];

score 0 · Answer 7 · edited May 23 '17 at 10:32

0

You need to use CLLocation Manager's api for background timer update otherwise NSTimer allow 10 min execution in background .

try this : https://stackoverflow.com/a/38472381/3901620

and also add this in info.plist

<key>UIBackgroundModes</key>
<array>
    <string>location</string>
</array>

edited May 23 '17 at 10:32

Community

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answered Nov 28 '16 at 12:04

KKRocks

8,222
1
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UIBackgroundTaskIdentifier running NSTimer but not updating UILabel - iOS

7 Answers7