App crashes on drawing of UILabel - NSString is a zombie - objective-c

Question

I'm working on an iPad app that has a slider that is used to scroll through data. When scrolling, a map is displayed and the data is updated. The problem is, if you scroll fast enough (or somehow trigger the race condition), the app crashes on accessing a zombie NSString. I've been able to track it down in the Profiler and found this:
Event Type RefCt Timestamp Size Responsible Library Responsible Caller
Malloc 1 01:55.166.466 16 Foundation -[NSPlaceholderString initWithFormat:locale:arguments:]
Autorelease <null> 01:55.166.472 0 Foundation +[NSString stringWithFormat:]
CFRetain 2 01:55.166.473 0 My Program -[StateView updateVotes:]
CFRetain 3 01:55.166.476 0 UIKit -[UILabel setText:]
CFRelease 2 01:55.166.504 0 My Program -[StateView updateVotes:]
CFRelease 1 01:55.177.661 0 Foundation -[NSAutoreleasePool release]
CFRelease 0 01:55.439.090 0 UIKit -[UILabel setText:]
Zombie -1 01:55.439.109 0 UIKit -[NSString(UIStringDrawing) drawAtPoint:forWidth:withFont:lineBreakMode:letterSpacing:includeEmoji:]
I'm using ARC on iOS5, so I'm not in control of the retain/release at all. Even if I was, looking at the above, it is correct. The problem seems to be a race condition between the drawing function and the UILabel string actually changing. The UILabel releases the first string, as a new one has been set, but the drawing function is holding a reference to it somehow, but did not retain it.
As a note, I have not modified the UILabel in any way.
Any ideas?
--- Code added as update:
Slider update:
-(void)sliderValueChanged:(UISlider *)slider {
float position = slider.value - 1790.0f;
int year;
if(position <= 0.0f) {
year = 1789;
} else {
year = 1792 + (floor(position / 4.0f)*4);
}
[self setYear:year];
}
setYear:
-(void)setYear:(int)year {
if (year == currentYear) {
// year didn't change, so don't do anything
return;
}
[yearLabel setText:[[NSString alloc] initWithFormat:#"%i", year]];
currentYear = year;
[self getMapForYear:year];
}
getMapForYear:
-(void) getMapForYear:(int)year {
[self setToMap:[historicalData objectForKey:[NSNumber numberWithInt:year]];
}
setToMap:
-(void) setToMap:(HistoricalMap *)map {
// Label the map
for (State *state in [map states]) {
[mapController setVotes:[state votes] forState:[state abbreviation]];
}
}
setVotes:forState:
-(void)setVotes:(NSNumber *)votes forState:(NSString *)stateAbbreviation {
StateView *state = [states objectForKey:stateAbbreviation];
if (state == nil) {
NSLog(#"Invalid State Votes -- %#", stateAbbreviation);
return;
}
[state updateVotes:votes];
[state setNeedsDisplay];
}
updateVotes:
-(void)updateVotes:(NSNumber *)newVotes {
[self setVotes:newVotes];
NSString *voteString = [[NSString alloc] initWithFormat:#"%#", newVotes];
[voteLabel setText:voteString];
if ([newVotes isEqual:[NSNumber numberWithInt:0]]) {
[[self voteLabel] setHidden:YES];
[[self stateAbbreviationLabel] setHidden:YES];
} else {
[[self stateAbbreviationLabel] setHidden:NO];
[[self voteLabel] setHidden:NO];
}
}

Answer 1

I think you are attempting to do far too much during the slider's movement. Creating and executing core data fetch requests alone would seem to be overkill, let alone updating the entire GUI and a screenful of labels. Have you tested the performance of this on a device?
It could be worth profiling these sections of code and seeing where the time is spent. You could look at caching the fetch requests or the results, for example, or you may have to resort to updating only when the slider has stopped, or only for every n increments along the path.

Answer 2

You havw several memory-leaks with NSString:
[yearLabel setText:[[NSString alloc] initWithFormat:#"%i", year]]; // leak
Create string with stringWithFormat method instead
[yearLabel setText:[NSString stringWithFormat:#"%i", year]];

Answer 3

[NSString stringWithFormat: **is the best way formatting the string than any other..**