Generating images in the background - objective-c

Question

I am currently building an application that generates images and stores them in an NSMutableArray, which then used in a UINavigation (Cell.imageView.image). I need to be able to handle up to 2000 images without causing lag in my application.
Currently how I set this generating is by calling the generating method when cellForRowAtIndexPath is accessed. Which seems to cause a 4-5 second lag before the next navigation is called.
fortunately after those 4-5 seconds the generating is done and there is no issues.
In the world of iProducts waiting 4-5 seconds isn't really an option. I am wondering what my options are for generating these images in the background. I tried using threads [self performSelectorInBackground:#selector(presetSnapshots) withObject:nil];
but that only gave me issues about vectors for some reason.
Heres the generating code:
-(void)presetSnapshots{
//NSAutoreleasePool* autoReleasePool = [[NSAutoreleasePool alloc] init];
for (int i = 0; i < [selectedPresets count]; ++i){
GraphInfo* graphInfo = [selectedPresets objectAtIndex:i];
graphInfo.snapshot = [avc takePictureOfGraphInfo:graphInfo PreserveCurrentGraph:false];
[graphInfo.snapshot retain];
}
//[autoReleasePool drain];
presetSnapshotFinished = YES;
}
inside - (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath { is
if (presetSnapshotFinished == NO){
[self presetSnapshots];
//[self performSelectorInBackground:#selector(presetSnapshots) withObject:nil];
}
cell.imageView.image = [[selectedPresets objectAtIndex:indexPath.row] snapshot];
Edit:
I also rather not use coreData for this. The images are 23x23 and coming out to about 7kb. So its about 6MB being use at any giving time in memory.

Answer 1

You can use Grand Central Dispatch (GCD) to fire up [self presetSnapshots]
dispatch_queue_t working_queue = dispatch_queue_create("com.yourcompany.image_processing", NULL);
dispatch_queue_t high = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,NULL);
dispatch_set_target_queue(working_queue,high);
dispatch_queue_t main_queue = dispatch_get_main_queue();
dispatch_async(working_queue,^{
if (presetSnapshotFinished == NO){
[self presetSnapshots];
}
dispatch_async(main_queue,^{
cell.imageView.image = [[selectedPresets objectAtIndex:indexPath.row] snapshot];
});
});