Hello,
For several months now I've been trying to track down a particular bug that can
cause a crash in my app, and finally I figured I would cry for help in case
there is anything obvious that I can try that I'm not already trying. Here's
the situation:
Occasionally, if you close a document (NSDocument subclass) in my app and then
open another document (or reopen the same document), the app crashes. I've
debugged this to the extent that I now know the cause, at least. What is
happening is that sometimes when a document gets closed, a particular
NSArrayController subclass instance isn't getting deallocated - but only
sometimes. Then, when another document gets loaded, one of the notifications it
sends out is picked up by the array controller still in memory which was
registered as an observer for that particular notification; the array
controller then tries to call out to other parts of its associated document -
but of course they no longer exist, and neither should it, so it crashes.
The array controller in question is stored (and instantiated) in the main
document .nib (the one named MyDocument.nib by default in new Xcode projects).
File's owner is a subclass of NSWindowController. I am aware of certain
retain-loop issues with NSArrayController that have been improved since Tiger
but are still not 100% fixed to the best of my knowledge, so I'm careful to
work around them by not hooking up the content in IB - instead I set and remove
content in code.
So, I know the cause of the crash and I know exactly which class isn't getting
deallocated; I even have some ideas about how I might approach fixing it. The
problem is that it is so random, and doesn't happen very often (then again,
sometimes it happens more often than others), so implementing any potential
fixes will really be like stabbing in the dark as testing will be impossible.
My app is fairly complex so I figured I may need to go through and try doing
lots of different things - that is, trying one feature, then closing and
reopening a project, then trying another feature and closing and reopening
again, and on and on, so that I could narrow down what affected it; the
thinking being that there must be one particular thing causing the retain loop.
But I just made it crash, launched my app again and then followed the exact
same steps - and the array controller deallocated fine the second time around.
So my question is, what is the best approach to track down tricky, elusive
memory bugs like this? I already have NSDebugEnabled, NSZombieEnabled,
MallocStackLogging and MallocStackLoggingNoCompact set to YES in the arguments
pane of my executable's properties. They helped me find out where the crash was
happening but not how or where my array controller is being retained.
Has anybody got any good advice (or links to good advice) on how to track down
something like this? At the moment I'm stumped because I can't seem to make it
reproduceable every time.
Many thanks and all the best,
Keith
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