Counting time spent in background for an app using NSNotificationCenter - ios

Question

I am making a stopwatch, but it stops counting when the app is put into the background. I have tried to count the time that the app spends in the background, and then use NSNotificationCenter to send that time in seconds to my StopwatchViewController where I can add on the elapsed time. However, it does not seem to work:
In my AppDelegate.m file:
- (void)applicationDidEnterBackground:(UIApplication *)application
{
NSDate *currentDate= [NSDate date];
[[NSUserDefaults standardUserDefaults] setObject:currentDate forKey:#"backgroundDate"];
}
- (void)applicationDidBecomeActive:(UIApplication *)application
{
NSDate *dateWhenAppGoesBg= (NSDate *)[[NSUserDefaults standardUserDefaults] objectForKey:#"backgroundDate"];
NSTimeInterval timeSpentInBackground = [[NSDate date] timeIntervalSinceDate:dateWhenAppGoesBg];
NSNumber *n = [NSNumber numberWithDouble:timeSpentInBackground];
[[NSNotificationCenter defaultCenter] postNotificationName:#"NEWMESSAGE" object:n];
NSLog(#"%d", [n integerValue]);
}
In my StopwatchViewController.m file:
- (id)initWithNibName:(NSString *)nibName bundle:(NSBundle *)nibBundle { // Initialise view controller
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(newMessageReceived:) name:#"NEWMESSAGE" object:nil];
return self;
}
-(void)newMessageReceived:(NSNotification *) notification{
elapsedTime = [[notification object] intValue];
elapsedHours = elapsedTime / 3600;
elapsedTime = elapsedTime - (elapsedTime % 3600);
elapsedMinutes = elapsedTime / 60;
elapsedTime = elapsedTime - (elapsedTime % 60);
elapsedSeconds = elapsedTime;
secondInt = secondInt + elapsedSeconds;
if (secondInt > 59) {
++minuteInt;
secondInt -= 60;
}
minuteInt = minuteInt + elapsedMinutes;
if (minuteInt > 59) {
++hourInt;
minuteInt -= 60;
}
hourInt = hourInt + elapsedHours;
if (hourInt > 23) {
hourInt = 0;
}
}

Answer 1

If I am not completely missing the point, I think you are attacking the problem in the wrong way.
If you are creating a stopwatch, the only two interesting points in time are the point when you started the stopwatch and the current time. There is no reason to calculate the time that passed when your app was in the background.
Instead, just store the point in time where your stopwatch was started, then add e.g. a NSTimer that updates the timer display by comparing this time with the current time (i.e. [NSDate date). Then you won't have to worry about what happens when your app enter background mode.
EDIT Some ideas (disclaimer: did not have access to Xcode, so I just typed this up from my head):
When the user starts the timer, save the current time and start a NSTimer
- (void) didTapStart:(id)sender {
self->startTime = [NSDate date];
[NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:#selector(timerElapsed:) userInfo:nil repeats:YES];
}
Then update the display on the timer events
- (void) timerElapsed:(id)sender {
NSDateInterval elapsed = [[NSDate date] timeIntervalSinceDate:self->startTime];
int hours = (int)elapsed / 3600;
int minutes = ((int)elapsed / 60) % 60;
int seconds = (int)elapsed % 60;
NSString* elapsedString = [NSString stringWithFormat:#"Elapsed: %d:%02d:%02d",hours,minutes,seconds];
}