Why does this swizzle of UIAlertView's alloc method cause a recursion?

Question

I'm playing around with swizzling, and can't quite figure this out. My alloc swizzle looks like this:

@interface UIAlertView (Custom)
+ (id)allocCustom;
@end

@implementation UIAlertView (Custom)    
+ (void)load
{
    Method original;
    Method mock;
    original = class_getClassMethod(self, @selector(alloc));
    mock = class_getClassMethod(self, @selector(allocCustom));
    method_exchangeImplementations(mock, original);
}

+ (id)allocCustom
{
    NSLog(@"Custom!");
    return [self allocCustom];
}    
@end

If I pause at the NSLog statement, which gets called repeatedly, in the call stack I see:

* thread #1: tid = 0x2c388a, 0x00003a2f My App`+[UIAlertView(self=0x03825c74, _cmd=0x0327e663) allocCustom] + 31 at MYClass.m:62, queue = 'com.apple.main-thread, stop reason = breakpoint 4.1
    frame #0: 0x00003a2f My App`+[UIAlertView(self=0x03825c74, _cmd=0x0327e663) allocCustom] + 31 at iBFGClientAppDelegate.m:62
    frame #1: 0x036c7b42 CoreFoundation`+[NSTimeZone timeZoneWithName:] + 34
    frame #2: 0x036c7a52 CoreFoundation`+[NSTimeZone systemTimeZone] + 626
    frame #3: 0x036c7767 CoreFoundation`+[NSTimeZone defaultTimeZone] + 71
    frame #4: 0x036e82cd CoreFoundation`CFTimeZoneCopyDefault + 45
    frame #5: 0x036f8b90 CoreFoundation`CFCalendarCreateWithIdentifier + 544
    frame #6: 0x03705f2c CoreFoundation`__CFLogCString + 124
    frame #7: 0x03705e6e CoreFoundation`_CFLogvEx + 270
    frame #8: 0x01581fbc Foundation`NSLogv + 137
    frame #9: 0x01581f28 Foundation`NSLog + 27
    frame #10: 0x00003a3a My App`+[UIAlertView(self=0x03825c74, _cmd=0x0327e663) allocCustom] + 42 at MYClass.m:62
    frame #11: 0x036c7b42 CoreFoundation`+[NSTimeZone timeZoneWithName:] + 34
    frame #12: 0x036c7a52 CoreFoundation`+[NSTimeZone systemTimeZone] + 626
    frame #13: 0x036c7767 CoreFoundation`+[NSTimeZone defaultTimeZone] + 71
    frame #14: 0x036e82cd CoreFoundation`CFTimeZoneCopyDefault + 45
    frame #15: 0x036f8b90 CoreFoundation`CFCalendarCreateWithIdentifier + 544
    frame #16: 0x03705f2c CoreFoundation`__CFLogCString + 124
    frame #17: 0x03705e6e CoreFoundation`_CFLogvEx + 270
    frame #18: 0x01581fbc Foundation`NSLogv + 137
    frame #19: 0x01581f28 Foundation`NSLog + 27
    frame #20: 0x00003a3a My App`+[UIAlertView(self=0x01800f3c, _cmd=0x0327e663) allocCustom] + 42 at MYClass.m:62
    frame #21: 0x023fb8ae UIKit`UIApplicationMain + 60
    frame #22: 0x0000293b My App`main(argc=1, argv=0xbfffedac) + 75 at main.m:18

It seems as though timeZoneWithName: is calling back into my custom alloc method. Clearly there is no recursion when alloc is not swizzled, but I'm confused what's happening here.

Answer 1

Your problem is most likely occurring because you're swizzling a method allocCustom on UIAlertView with the method alloc, which is a method on NSObject. From what I have found, you can successfully swizzle NSObject's alloc method, but it doesn't appear you can do this from a subclass. You can see evidence that this is the case by adding the following to your UIAlertView category:

+ (id)alloc
{
    return [super alloc];
}

By doing that, it will now work correctly. But since alloc is a method on NSObject and clearly not being overridden by UIAlertView anyway, you could just remove your method swizzling, and add your code to your overridden alloc method:

+ (id)alloc
{
    NSLog(@"Custom!");
    return [super alloc];
}

Now this might be a little dangerous to do in case Apple at a future time changed their implementation and for some reason were overriding alloc in UIAlertView. In that case, I think your implementation would override theirs, although I think the exact behavior is technically undefined.

Answer 2

I'm no expert in method swizzling, but I'm guessing that because your allocCustom is a class method, self is actually referring to the class itself. What you really want is something like this

+(id) allocCustom {

 id obj = calloc(class_getInstanceSize(self), 1);
 obj->isa = self;
 obj->retainCount = 1;
 return obj;
}

this link may be of use to you: https://www.mikeash.com/pyblog/friday-qa-2013-01-25-lets-build-nsobject.html