Error when convert float to datatime - objective-c

Question

I have this command that is being called each second in background mode, Every time he is called I add +1 to my variable of type float, and convert to date format:
xis = (xis + 1) / 000001;
//The Format which you want as output
NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
dateFormat.dateFormat = #"ss:mm:hh";
[dateFormat setTimeZone:[NSTimeZone systemTimeZone]];
//The Format in which your dateTime currently is
NSDateFormatter *dateFormat1 = [[NSDateFormatter alloc] init];
dateFormat1.dateFormat = #"hh.mm";
[dateFormat1 setTimeZone:[NSTimeZone systemTimeZone]];
NSString *timeStr = [NSString stringWithFormat:#"%.2f",xis];
NSDate *dates = [dateFormat1 dateFromString:timeStr];
NSLog(#"Time: %#", [dateFormat stringFromDate:dates]);
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
recordTime.text = [NSString stringWithFormat:#"%#",[dateFormat stringFromDate:dates]];
}];
But this code has a problem when the time comes 00:00:12, something goes wrong and the time is the value (null), which may be causing this and how can I solve?

Answer 1

There are two issues:
I would advise against using the NSTimer routine to keep track of elapsed time yourself. Timers may not be called with the frequency you expect. Also, if the user suspended the app and came back to it, you don't really want to try to keep this timer going while the app is no longer in the foreground. In short, you want to decouple the updating of the UI from the calculation of the elapsed time string representation.
So, instead, one should capture the "start time" and then have the routine get the current time, compare that to the "start time" and calculates the string representation of this elapsed time. But notably, there is no incrementing of variables for seconds elapsed, but rather one should rely on the system time of the device.
Your formatting problem stems from the awkward conversion of your numeric counter to a date string. The problem was complicated by the fact that the formatting string that was backwards, ss:mm:hh.
First (and to my above point), I'd probably calculate the time elapsed in seconds, by capturing the start time up front and store it in a CFAbsoluteTime property:
self.start = CFAbsoluteTimeGetCurrent();
And, if you really wanted to use NSDateFormatter, instead of using it to interpret a numeric value, I would only use it for converting to and from NSDate objects. You could, for example, get the NSDate for 00:00:00, use dateByAddingTimeInterval to add the elapsed time to the date, and then use the formatter to get the output string:
// calculate the time elapsed in seconds
CFTimeInterval elapsed = CFAbsoluteTimeGetCurrent() - self.start;
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
formatter.dateFormat = #"HH:mm:ss";
NSDate *start = [formatter dateFromString:#"00:00:00"];
NSDate *end = [start dateByAddingTimeInterval:elapsed];
NSString *elapsedString = [formatter stringFromDate:end];
Having said that, I'd probably favor a couple of different approaches. One is to just calculate hours, minutes, and seconds manually:
// calculate the time elapsed in seconds
CFTimeInterval elapsed = CFAbsoluteTimeGetCurrent() - self.start;
// convert this to hours, minutes, and seconds
double seconds;
double minutes;
double hours;
seconds = modf(elapsed / 60.0, &minutes) * 60.0;
minutes = modf(minutes / 60.0, &hours) * 60.0;
// create format string
NSString *elapsedString = [NSString stringWithFormat:#"%02.0f:%02.0f:%04.1f", hours, minutes, seconds];
Or a completely different approach would be to use NSCalendar. In this scenario, one might have a NSDate property that you initialize with the start time:
self.startDate = [NSDate date];
And then you could do something like:
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDate *now = [NSDate date];
NSDateComponents *components = [calendar components:NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond fromDate:self.startDate toDate:now options:0];
NSString *elapsedString = [NSString stringWithFormat:#"%02ld:%02ld:%02ld", (long)components.hour, (long)components.minute, (long)components.second];
Just a few approaches. But I'd advise against manually incrementing your counter manually, and I might suggest one of these latter techniques.