Very Interesting Variances with Time on Different Devices - iphone

Question

This timmer works great on all my devices (5, Ipad and two 4S) but it doesn't seem to work on the two 3GS I have. For some reason the time runs really slow on the 3s.
Heres a video explaining the problem:
http://youtu.be/4vdusgnIXcs
And heres the code that deals with the time:
- (void)showTime
{
int hours = 0;
int minutes = 0;
int seconds = 0;
int hundredths = 0;
NSArray *timeArray = [NSArray arrayWithObjects:self.hun.text, self.sec.text, self.min.text, self.hr.text, nil];
for (int i = [timeArray count] - 1; i >= 0; i--) {
int timeComponent = [[timeArray objectAtIndex:i] intValue];
switch (i) {
case 3:
hours = timeComponent;
break;
case 2:
minutes = timeComponent;
break;
case 1:
seconds = timeComponent;
break;
case 0:
hundredths = timeComponent;
hundredths++;
score++;
break;
default:
break;
}
}
if (hundredths == 100) {
seconds++;
hundredths = 0;
}
else if (seconds == 60) {
minutes++;
seconds = 0;
}
else if (minutes == 60) {
hours++;
minutes = 0;
}
self.hr.text = [NSString stringWithFormat:#"%.0d", hours];
self.min.text = [NSString stringWithFormat:#"%.2d", minutes];
self.sec.text = [NSString stringWithFormat:#"%.2d", seconds];
self.hun.text = [NSString stringWithFormat:#"%.2d", hundredths];
scoreLabel.text= [NSString stringWithFormat:#"%i",score];
Please help me try to figure out whats going on here. It works so well on the newer devices I'm just lost at what I need to do.
Thank you in advance!

Answer 1

If I understand your code correctly you run a NSTimer 100 times a second.
If that is correct you may primarily have a design problem and not a performance or NSTimer problem.
A NSTimer is not guaranteed to run on time. The only thing that is guaranteed is that it will not run earlier that it is supposed to be.
Since you don't know when a timer method runs you can't rely that it will run exactly 100 times a second. This means a timer is a bad way to "count" time. A better way would be to save the system time when you start the timer, and when you want to know how much time has elapsed you use the current system time and substract the start time. The NSTimer should be used for display purposes only.
Something like this:
// instance variables:
NSDate *startDate;
NSTimer *timer;
- (void)startTimer {
[timer invalidate];
startDate = [NSDate date]; // save current time
timer = [NSTimer timerWithTimeInterval:0.075 target:self selector:#selector(displayTime:) userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
}
- (void)displayTime:(NSTimer *)timer {
// the timer method is for display only. it doesn't "count" time
// calculate elapsed time from start time
NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSinceDate:startDate];
NSInteger ti = (NSInteger)elapsedTime;
// convert elapsed time (in seconds) into hours, minutes, seconds ...
double fractionalSeconds = fmod(elapsedTime, 1);
NSInteger hundreds = fractionalSeconds * 100;
NSInteger seconds = ti % 60;
NSInteger minutes = (ti / 60) % 60;
NSInteger hours = (ti / 3600);
NSLog(#"%02d:%02d:%02d.%02d", hours, minutes, seconds, hundreds);
}

Answer 2

Profile your programs with Instruments.app using both devices and compare. Since you have isolated the problem in the demo, the reports it generates should quickly expose the execution time differences. Once you understand the biggest problem area(s), you will know which parts of the program you can change in order to make it run faster. Then compare your updates with the initial runs to see how the speed has improved. Repeat as needed.