Xcode Objective C - Help with NSAutoreleaseNoPool error using NSThread - objective-c

Question

Hey experts, I'm having a little trouble with NSThread. Xcode keeps on giving me "* __NSAutoreleaseNoPool(): Object 0x5694dc0 of class NSCFString autoreleased with no pool in place - just leaking" errors.
I'm correctly declaring the pool with the line
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
then at the end of my loop I use:
[pool release];
Is it because I'm using a delegate method as the performSelectorInBackground?
Thanks stackoverflow.
- (void)preFetch { //process filenames to be downloaded and assign types to each one
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSArray *regions = [NSArray arrayWithObjects: #"dr_national", #"ds_ir", #"conus_FL360", #"FL360_conus", #"dr_nw", #"dr_nc", #"dr_ne", #"dr_sw", #"dr_sc", #"dr_se", #"ds_ir_nw", #"ds_ir_nc", #"ds_ir_ne", #"ds_ir_sw", #"ds_ir_sc", #"ds_ir_se", nil];
NSError* error;
for (NSString *regionDir in regions) {
NSLog(#"region now: %#", regionDir); foo = 0;
NSString *regUrl = [NSString stringWithFormat:#"http://someUrl/%#/index.lst", regionDir ];
NSString* text1 = [NSString stringWithContentsOfURL:[NSURL URLWithString:regUrl ] encoding:NSASCIIStringEncoding error:&error];
NSArray *listItems = [text1 componentsSeparatedByString:#"\n"];
for (int k=0; k<[listItems count]; k++) {
if ([[listItems objectAtIndex:k] length] != 0){
NSString *newpath = [NSString stringWithFormat:#"http://someUrl/%#", [listItems objectAtIndex:k]];
NSLog(#"newpath: %#",newpath);
[self performSelectorInBackground:#selector(moveProgressBar) withObject:nil];
[self fetchImages:newpath:type]; //pass multiple arguments to fetchImages, newpath and type
}
}
}
[pool release];
}
- (void)moveProgressBar{
[delegate increaseAmount];
}

Answer 1

You should just set up an autorelease pool in your method, since that's being called on a different thread.
- (void)moveProgressBar
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[delegate increaseAmount];
[pool drain];
}
Edit
Having said that, looking at the code itself, it seems that you might be trying to update the UI from a background thread? Any code that does that should be executed on the main thread.
If you have a long running process that you want to run which doesn't lock the UI, and keeps the user updated on progress, the typical pattern would be to do the processing itself on a background thread, and periodically update the UI using performSelectorOnMainThread:.