After almost a year, my third update to the ObjC beginner caveats blurb. 8 new caveats added today (marked with [NEW])

Language/SDK

1. NSArray/NSMutableArray cannot hold ints or NSInteger
>> although NSInteger is notationally similar to NSNumber, NSNumber is an object while NSInteger is an alias for int or long (depending on whether you’re running a 32/64 bits architecture). NSArray can only hold objects. If you want to add a number to an NSArray, use NSNumber and initialise your number with something like [NSNumber numberWithInt:26].
(Yes, you can use standard C arrays, there are quite a few posts about this around)

2. NSNumber doesn’t auto-unbox
>> If you try to assign NSNumber to an int (or, better, an NSInteger), you’re assigning a pointer, not the actual number. To get the actual number, use something like: [MyNSNumber integerValue]

3. Strings don’t concatenate with [+] or [.]
>> If something like “Score: “+score is what you’re used to, try making friends with:
[NSString stringWithFormat:@"Score: %i", score];

4. In, (void)applicationDidFinishLaunching:(UIApplication *)application {…}not all IBOutlets depending on your nib file will initialize until your view is added to the main window
after [window makeKeyAndVisible]; it should be safe to access all IBOutlets attached to your viewController.

5. I updated the frame property of a subview, but the coordinates look incorrect.
>> after updating the UIView.frame property on a subview, invoke setNeedsLayout on the parent view.

6. NSTimer fires at the wrong time / fires too many times
>> When you allocate/init NSTimer with the usual code, you don’t need to invoke fire(). NSTimer is setup as soon as it’s initialized and fire() shouldn’t be called directly as it, *doh!* fires the timer.
>> Whatever you name your callback method, it should take a unique NSTimer argument ( the selector argument would typically look like @selector(myCallback:) with the semi-colon at the end).
What happened to me is that my timer kept firing over and over even though repeat:NO was set. Adding the timer argument to the callback fixed the problem.

7. You cannot declare new variables inside a switch statement

switch (foo) {

case BAR:

char foobar=’*'; // compile error

break;

}

Use this instead:

char foobar;

switch (foo) {

case BAR:

foobar=’*'; // OK

break;

}

Memory Management

8. Use NSZombieEnabled to crash and get a call stack when your program attempts accessing a deallocated object [NEW]

Google it…

9. You need a symbolic breakpoint to hit malloc_error_break and get a call stack [NEW]

Google it (I feel lazy).

10. Unless otherwise stated, objects allocated as a side effect of calling methods other than [alloc] are autoreleased.

This is especially applicable to objects created using factory methods. Take the following example:

[NSMutableArray arrayWithCapacity:5]; // will be released automatically

[[NSMutableArray alloc]initWithCapacity:5]; // we just created a zombie!

At first, it would seem that [NSMutableArray arrayWith...] forms are just shorthands for [[NSMutableArray alloc] initWith…]. Hell, not quite.

[NSMutableArray arrayWith...] and other, similar factory methods, ‘emulate stack allocation’ for reference types. This means that we don’t need to worry about releasing them, as the so-called autorelease pool will take care of them for us.

When we check our code for memory issues, knowing which objects are kept in memory (on the heap) is essential. One way to learn about these is to scan for alloc, retain and release statements. The so called ‘shorthand forms’ allow using objects transiently, without worrying about memory issues, so this convention can help reduce the time spent on memory management.

There are downsides to this for unfortunate beginners:

(1) When we start with objective c, the [alloc[init form appears verbose and cumbersome. So we're likely to create our own shortcuts -- generating 'silent allocations' that make memory management harder.
(2) If we use the 'shorthand' forms, we quickly end up with weird, hard to fix bugs, because we might end up assigning auto-releaseable objects to variables, then these objects get deleted implicitly, and finally we end up accessing... ...garbage(!!!).

11. Beware of  abusing autorelease [NEW]

Autorelease is useful and in some cases you cannot avoid using it:

  • Objects returned by factory methods
    A(n essential) convention dictates that objects created using [alloc] have a reference count of 1. In contrast, callers of factory methods are not responsible for releasing returned objects unless they retain them (see #10)
    Since we can’t use [release] before returning an object we just created, we need to use [autorelease]
  • An object on the call stack may get deallocated if [release] is called.
    Now, it may be argued that only bad design can cause this to happen. Nevertheless one way to solve the problem (it can get nasty) is to use [autorelease]

Now that this is out of the way, beware of practices involving using [autorelease] as a ‘default safe way’. Here’s why:

If an object deallocates unexpectedly following a call to [autorelease] the call stack above [dealloc] doesn’t tell you what caused this object to deallocate.

I don’t see how a coding style involving loosing track of deallocation events can be safe.

12. You can crash the leaks tool in Instruments (seen in Instruments 4.0, 4.1) [NEW]

Recently I managed quirky memory management code that didn’t crash on device or in the simulator, yet crashed instruments. Checking the details, I stumbled on something like ‘invalid leaks data’.

I was about to use up one of my support tickets but reflected that sending my code over and waiting for the ticket to process would take a lot longer than reverting my changes and use the weak muscle to figure it out.

On the downside I got lucky before I managed to narrow it down.

Even if you’re not part of a team, consider using versioning (SVN or whatever new-fangled stuff you’d like).

Kind reminder:

No versioning
=> no diff tool
=> no way to accurately revert changes
=> panic
=> loss of money
=> loss of sleep
=> loss of hair(*)

(*)If you are bald I trust this won’t be an issue.

13. WTF my code crashes when ‘unplugged’ [NEW]

Consider the following case:

  • Your code doesn’t crash when debugging on-device.
  • Your code doesn’t crash when debugging in the simulator.
  • Your code crashes when running in the simulator after pressing the app icon.
  • Your code crashes on-device when running after pressing the app icon.

Then try this:

  • Disable NSZombieEnabled. Quite possibly you’ll see a fairly non-descript crash in the debugger when running the same code.
  • Try to isolate the stack that causes the crash (it’s not meant to be fun but using a dichotomy you might get it done in less than an hour)
  • Re-enable NSZombieEnabled. At this point you may see inconsistent variable assignments in the debug window.
  • Set breakpoints on [dealloc]. Quite possibly you’re deallocating an object that’s already ‘somewhere up’ on the call stack. On sunny days this crashes the device/simulator (hey, it’s actually a GOOD thing). On rainy days it drives the debugger crazy.

Build errors & warnings

14. Warning: Multiple Build Commands for output file …

In XCode, the name of a resource (e.g. a picture to include in the build as resource) is different from the path the resource is retrieved from. Several resources can have the same name, linked with different paths. So for example we can have:

  • foo.jpg (/images/foo.jpg)
  • foo.jpg (/mypics/draft/foo.jpg)

In the final bundle, however these resources are in conflict, they both target /foo.jpg. XCode will issue a warning if resources conflict in this way.

Unsurprisingly, this typically happens when reorganizing project resources.

15. Linking C++ libraries [NEW]

If your project depends on C++/ObjC++ libraries, you may need to add -lstdc++ to other linker flags to your build (see here).

Nibbling UIs (Interface Builder)

16. Action mappings are one-to-many

Yesterday I copied a button from my UI and bound an action to it. Then I was rather puzzled to find that, when I clicked on this button, my game started off with an ominous GL error code. I thought binding an action to a button overrides the previously bound action. It doesn’t. The same user action on the same widget can bind several IBAction targets. In my case this caused the game’s start method to be called twice.

Asset management

17. Use the blue folders [NEW]

If you have hundreds of assets, adding them manually will be a pain. However you can link a whole folder; in XCode’s ‘add files wizard’ select your folder and tick create folder references automatically…

XCode is getting better at detecting changed/added assets between builds (but see #18, below)

18. Sometimes you need to remove your app from the simulator/test device [NEW]

If your code relies on a mechanism allowing to retrieve a resource from one of several locations, you need to delete your app from the simulator / device whenever you remove assets from a blue folder.

Let’s say your sound folder is organized like this:

default_sounds/
---- KO_sound.caf
---- goblin_sounds/
-------- KO_sound.caf

Now suppose you deleted KO_sound.caf under goblin_sounds. Well the goblin specific sound recorded from your little sister’s performance will still load and play because it doesn’t get removed automatically.

Between builds, no files are deleted from package contents. I don’t know whether it’s a bug or not. It’s often annoying.