Sometimes it's like this:
Tried lots of different things, like restructuring the classes, moving methods up and down the hierarchy, add lots of NSLog and checking object allocation/deallocation, check for memory leak in Instruments...etc.
Search on the net (see this one for example) and learned that the error is more about trying to call method of a object that's already deleted, not the memory leak. Have to use "Enable NSZombie detection" as shown below to check.
And I got it straight away!
After a while finally worked out that I was incorrectly doing something like this;
recordOfComputersMove = [recordOfComputersMove stringByAppendingFormat:@"%@%i",kStringSeparater,curEmptySpot];
which should actually be like this, adding the "retain" at the back fixed the problem.
recordOfComputersMove = [[recordOfComputersMove stringByAppendingFormat:@"%@%i",kStringSeparater,curEmptySpot] retain];
while working on the issue, I also read a few articles talking about this thing called "Enable Guard Malloc" (see here and here), and it took me a while to find out how to do it in XCode 4 (see here) as shown below inside "Edit Scheme".
However I think I don't understand how to use it "properly" yet as after it's been enabled, all I can see is this message as shown below, saying "stack log" been created in this file. But when I tried to open that file, it's in some sort of special format which I can't find a proper reader/editor to interpret it into a readable way.
Well, I guess most important thing for me is to identify the problem and work out a fix. That "Guard Malloc" way is definitely a bit too advanced for my level I guess...
Yeah, feels good to learn new things every day!