for- loop in background thread accesses each index more than once --- iPhone dev

Question

I have a method getImageData which I call as [self performSelectorInBackground:@selector(getImagesData ) withObject:nil]; in my viewDidLoad. getImageData has a for-loop i realised that each index in the loop is called more than once. I also access a static NSMutableArray in the loop. When i don't retain the array it gives me exc bad access. If I retain it the loop runs endlessly

Answer 1

Sounds like a concurrency issue (and, furthermore, that you aren't really addressing the problem by understanding it -- I would suggest reading the Cocoa Memory Management Guide [same as iPhone] and one of the Concurrency Guides provided in the documentation).

Specifically, might you be calling -performSelectorInBackground: twice?

A for loop won't magically re-visit indices without their being something in your code that is:

1. changing the loop invariant
2. corrupting memory in exactly the right way
3. you are inadvertently executing the loop multiple times -

If I retain it the loop runs endlessly indicates that you are doing 3. That it crashes otherwise is just because you aren't managing memory correctly; it is a symptom of a different problem.