Dan Wood: The Eponymous Weblog (Archives)

Third past today! Since I just posted a "gripe" entry, I thought I would also post something constructive. I wanted to talk about how cool class posing is for debugging.

We have used class posing quite a few times just to insert some extra warnings for our debug builds. A lot of times, you can catch a nasty condition before it happens and provide some information about what's happening. For instance, if you are only supposed to call a particular method from the main thread, a posed subclass could define your entry points to log an error message if it's being called from a background thread.

For example, I wanted to log some information whenever an NSThread got detached, so I could see where all my application's threads were coming from. So I created a subclass of NSThread called, unimaginatively, MyThread:

@interface MyThread : NSThread
@end

This class has two methods (in an @implementation block, of course). The first is used to force the subclass to be used in place of its parent class:

+ (void) load
{
    [self poseAsClass:[NSThread class]];
}

The second is a redefinition of +[NSThread detachNewThreadSelector: toTarget: withObject:]. It logs some useful information and then calls the "super" implementation to actually do the work. I've simplified my code a bit here but you should get the idea:

+ (void)detachNewThreadSelector:(SEL)selector
                       toTarget:(id)target
                     withObject:(id)argument;
{
    LOG((@"Detach -[%@ %@%@]", target, NSStringFromSelector(selector),
        argument));
    [super detachNewThreadSelector:selector
                          toTarget:target
                        withObject:argument];
}

Generally I don't need this logging to happen, so the code is normally commented out with #ifdef 0. But when I need to know what's going on with threads, this is very helpful.

I've also use this technique to debug my bindings and log each instance of -[NSObject bind:toObject: withKeyPath: options:] and -[NSObject unbind:], all the initialization methods on QTMovie to make sure that the movies are created on the main thread, and so forth.