iPhone Memory Debugging with NSZombie and Instruments

When your iPhone app crashes with 'BAD ACCESS' you're in trouble - a memory bug where you tried to call a method on a object that was already deleted. Instruments has support for NSZombie - a feature that makes it easy to find the source of the bug by showing you a full history of every alloc, retain, release, and autorelease of the object that caused the crash! Wow. Here's how: Basic steps are:

  1. Run in Performance tool 'Object Allocations'
  2. Stop running and set options on ObjAllocations instrument: 'Enable NSZombie' Detection and 'Record Reference Counts'
  3. Re-run from instruments, when it crashes, click the arrow in the popup zombie dialog
  4. Open up the stack trace view and see the full memory history of the problem object
  5. Wow, how awesome is that?
  6. If you love this post, do me a favor and check out our Free app Focus for Facebook
  7. You might also like my memory management tutorial and other posts under 'App Development'.

Enable Zombies in Instruments

Here's a link to the demo program: ZombieDebug Demo Project. The code we're looking at in the video is:

@implementation ZombieDebugViewController

@synthesize objArray;

-(void)rewriteText {
NSMutableString* s = [NSMutableString stringWithCapacity:100];
for (id obj in objArray) {
[s appendFormat:@"%@,\n",obj];
[obj release];
}
label.text = s;
}

- (void)viewDidLoad {
[super viewDidLoad];
self.objArray = [NSMutableArray arrayWithCapacity:10];
[objArray addObject:@"I'm a string object"];
[self rewriteText];
}

-(IBAction) tapButton:(id)button {
NSNumber* n = [NSNumber numberWithLong:random()];
[objArray addObject:n];
[self rewriteText];
}

-(void)dealloc {
[super dealloc];
self.objArray=nil;
}

@end