The document discusses various debugging techniques for iOS applications, including: 1. Enabling zombies to detect messages to deallocated objects. 2. Using NSAssert statements to validate assumptions and catch bugs. 3. Setting breakpoint actions and tracing method calls for debugging. 4. Catching exceptions by setting a breakpoint on objc_exception_throw. 5. Writing custom exception handlers to catch crashes and save user data.

1. DEBUGGING
OR HOW I LEARNED TO STOP WORRYING AND LOVE
EXC_BAD_ACCESS

2. 1. ZOMBIES!
RETAAAAAIN!
RETAAAAAIN
RETAAAAIN!

3. I will never, ever use Comic Sans in a
presentation again.
I will never, ever use Comic Sans in a
presentation again.
I will never, ever use Comic Sans in a
presentation again.
I will never, ever use Comic Sans in a
presentation again.
I will never, ever use Comic Sans in a
presentation again.
I will never, ever use Comic Sans in a
presentation again.
I will never, ever use Comic Sans in a
presentation again.

4. 1. ZOMBIES!
Once retainCount hits zero, an object is typically
deallocated. Call a method on a pointer to that object
and BOOM.
When zombies are enabled, deallocated objects’ class
is changed to _NSZombie and memory isn’t freed.
Subsequent messages to the zombie object cause a
nice, predictable, break-pointable exception.

5. In Xcode 3, add the environment variable in the Executable
Info window

6. In Xcode 4, add the environment variable in the Scheme
Editor (⌥ + “Play” Button)

7. HE MADE A COMMON
MISTAKE ...

8. DON’T DO THIS
NSZombieEnabled=1

9. 2. NSAssert()
Call this everywhere you make an assumption about
the state of the world.
Expect a parameter to be non-nil or non-negative?
NSAssert that shit.
Fall through a switch statement on an HTTP result
code? NSAssert that shit.
Get a return code from a system function you don’t
explicitly handle? NSAssert that shit.

10. NS_BLOCK_ASSERTIONS

11. BUT WAIT...

12. So yeah, you don’t want to ship an app with
NSAssert enabled, but it’d be cool if at least some of
your test builds had them.
Because, in the words of Von Moltke the Elder:
“No code survives first contact with users.”

13. Of course, if your app asserts out when it’s not
connected to the debugger, BOOM.
Users hate that shit.

14. REAL USER
HATING THAT SHIT

15. 3. CUSTOM ASSERT HANDLERS
So Simple. Just subclass NSAssertionHandler...
- (void)handleFailureInFunction:(NSString *)functionName
file:(NSString *)fileName
lineNumber:(NSInteger)line
description:(NSString *)format, ... {
va_list args;
va_start(args, format);
[self showAssertUIAlert:functionName file:fileName lineNumber:line description:format args:args];
}
- (void)handleFailureInMethod:(SEL)selector
object:(id)object
file:(NSString *)fileName
lineNumber:(NSInteger)line
description:(NSString *)format, ... {
va_list args;
va_start(args, format);
NSString *methodName = NSStringFromSelector(selector);
[self showAssertUIAlert:methodName file:fileName lineNumber:line description:format args:args];
}

17. 4. BREAKPOINT
ACTIONS
I don’t want to start a religious war here, but I never, ever
use NSLog() for debugging.
But I do trace like a mofo. All the time. Using breakpoint
actions!

19. 5. objc_exception_throw()
Set a breakpoint on this function in the DEBUG
configuration/scheme.
In every project.
Always.

20. XCODE 3

21. XCODE 4

22. RUNS WITH
objc_exception_throw
BREAKPOINT

23. DOESN’T

24. 6. CUSTOM EXCEPTION
HANDLERS
So what about when we ship our app?
Crash reports are a great resource, especially on iOS,
since Apple collects them when devices are synced.
But wouldn’t it be great if you could catch crashes before
they happen, save the user’s data, and perhaps even post
the crash to your handy crash reporting web service?

25. IT’S PRETTY FLIPPIN’ EASY!
void InstallUncaughtExceptionHandler()
{
NSSetUncaughtExceptionHandler(&HandleException);
signal(SIGABRT, SignalHandler);
signal(SIGILL, SignalHandler);
signal(SIGSEGV, SignalHandler);
signal(SIGFPE, SignalHandler);
signal(SIGBUS, SignalHandler);
signal(SIGPIPE, SignalHandler);
}
void SignalHandler(int signal)
{
// do whatever you want here, save the user’s work,
// post a crash report to your web service, show
// a dialog apologizing for your shoddy workmanship... whatever!
}
void HandleException(NSException *exception)
{
// Unhandled exception no more!
}
Shamelessly lifted from http://cocoawithlove.com/2010/05/handling-unhandled-exceptions-and.html

26. ESSENTIAL RESOURCES
iOS Debugging Magic TN2239
Understanding and Analyzing iOS
Application Crash Reports TN2151
Debugging Autorelease
Custom Uncaught Exception Handler

27. art@tapsquare.com @artgillespie