iOS app Stopwatch and Timer stop counting when in background - ios

Question

I have an iPhone app which is basically a clock, stopwatch, and timer. Everything works fine, until you leave the app, and the stopwatch and timer stop counting. I think the way to solve this would be to start a counter when the app is put into the background, and add/subtract that from the stopwatch/timer when the app starts up again. Is this the best thing to do, and if so, how would I do this?
UPDATE: The timer, stopwatch, and clock are all in different view controllers. Here is the code for my stopwatch in StopwatchViewController.m:
- (void)stopwatch // Start counting the stopwatch
{
hourInt = [hourLabel.text intValue]; // Store integer values of time
minuteInt = [minuteLabel.text intValue];
secondInt = [secondLabel.text intValue];
if (secondInt == 59) { // Add on one second
secondInt = 0;
if (minuteInt == 59) {
minuteInt = 0;
if (hourInt == 23) {
hourInt = 0;
} else {
hourInt += 1;
}
} else {
minuteInt += 1;
}
} else {
secondInt += 1;
}
NSString *hourString = [NSString stringWithFormat:#"%02d", hourInt]; // Update text to show time
NSString *minuteString = [NSString stringWithFormat:#"%02d", minuteInt];
NSString *secondString = [NSString stringWithFormat:#"%02d", secondInt];
hourLabel.text = hourString;
minuteLabel.text = minuteString;
secondLabel.text = secondString;
[NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:#selector(stopwatch) userInfo:nil repeats:NO]; // Repeat every second
}
How do I get it to add on the elapsed time the app was in the background?

Answer 1

Add two observers for when app enter in background and for app enterForeGround
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(applicationWillEnterInBackGround) name:UIApplicationWillResignActiveNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(applicationWillEnterInForeground) name:UIApplicationWillEnterForegroundNotification object:nil];
- (void)applicationWillEnterInBackGround{
// stop your timer here
}
- (void)applicationWillEnterInForeground
{
// start your timer here
}
Example ------
in .h
NSTimer *timer;
float time;
in .m
- (void)viewDidLoad
{
[super viewDidLoad];
timer=[NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:#selector(stopwatch) userInfo:nil repeats:YES];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(applicationWillEnterInBackGround) name:UIApplicationWillResignActiveNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(applicationWillEnterInForeground) name:UIApplicationWillEnterForegroundNotification object:nil];
}
- (void)applicationWillEnterInBackGround
{
[timer invalidate];
}
- (void)applicationWillEnterInForeground
{
timer = [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:#selector(stopwatch) userInfo:nil repeats:YES];
}
- (void)stopwatch // Start counting the stopwatch
{
hourInt = [hourLabel.text intValue]; // Store integer values of time
minuteInt = [minuteLabel.text intValue];
secondInt = [secondLabel.text intValue];
if (secondInt == 59) { // Add on one second
secondInt = 0;
if (minuteInt == 59) {
minuteInt = 0;
if (hourInt == 23) {
hourInt = 0;
} else {
hourInt += 1;
}
} else {
minuteInt += 1;
}
} else {
secondInt += 1;
}
NSString *hourString = [NSString stringWithFormat:#"%02d", hourInt]; // Update text to show time
NSString *minuteString = [NSString stringWithFormat:#"%02d", minuteInt];
NSString *secondString = [NSString stringWithFormat:#"%02d", secondInt];
hourLabel.text = hourString;
minuteLabel.text = minuteString;
secondLabel.text = secondString;
}

Answer 2

The easiest way to proceed and save the current time when the application goes to background and then when come back to foreground compute the time interval and add it to your counters, something like:
.. going background ..
NSDate *backgroundTime = [[NSDate date] retain];
.. returning foreground ..
NSTimeInterval elapsed = [NSDate timeIntervalSinceDate:date];
[date release];
yourTimer -= elapsed;
To understand when exactly you should hook these events you should take a look here.