Classes

struct objc_class {
    Class isa  OBJC_ISA_AVAILABILITY;

#if !__OBJC2__
    Class super_class                      OBJC2_UNAVAILABLE;
    const char *name                       OBJC2_UNAVAILABLE;
    long version                           OBJC2_UNAVAILABLE;
    long info                              OBJC2_UNAVAILABLE;
    long instance_size                     OBJC2_UNAVAILABLE;
    struct objc_ivar_list *ivars           OBJC2_UNAVAILABLE;
    struct objc_method_list **methodLists  OBJC2_UNAVAILABLE;
    struct objc_cache *cache               OBJC2_UNAVAILABLE;
    struct objc_protocol_list *protocols   OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;

Classes are opaque in the new runtime, but we still can learn from the old stuff. Cache: hash table from selectors to code. Talk about meta-classes.

Message forwarding

+(BOOL)resolveInstanceMethod:(SEL)aSEL;

-(void)forwardInvocation:(NSInvocation*)anInvocation;

-(NSMethodSignature*)methodSignatureForSelector:(SEL)sel;

-(BOOL)respondsToSelector:(SEL)aSelector;

Example: Introspection

unsigned int methodCount;
Method* methods = class_copyMethodList([NSObject class], 
    &methodCount);

for (int i = 0; i < methodCount; i++) {
    SEL selector = method_getName(methods[i]);
    char returnType[255];
    method_getReturnType(methods[i], returnType, 255);
    NSLog(@"%s %@", returnType, 
        NSStringFromSelector(selector));
}

Mention type encodings / @encode

Method swizzling

// Implementation by Mike Ash

void Swizzle(Class c, SEL orig, SEL new) {
   Method origMethod = class_getInstanceMethod(c, orig);
   Method newMethod = class_getInstanceMethod(c, new);

   if (class_addMethod(c, orig, 
       method_getImplementation(newMethod), 
       method_getTypeEncoding(newMethod))) {
     class_replaceMethod(c, new, 
        method_getImplementation(origMethod), 
        method_getTypeEncoding(origMethod));
   } else {
     method_exchangeImplementations(origMethod, newMethod);
   } 
}

From UIViewController.h

- (void)attentionClassDumpUser:(id)arg1 
  yesItsUsAgain:(id)arg2 
  althoughSwizzlingAndOverridingPrivateMethodsIsFun:(id)arg3 
  itWasntMuchFunWhenYourAppStoppedWorking:(id)arg4 
  pleaseRefrainFromDoingSoInTheFutureOkayThanksBye:(id)arg5;

(thanks to @steipete for discovering this)

Example: Customize edit-mode

Class class = NSClassFromString(
  @"UITableViewCellEditControl");
SEL originalSelector = NSSelectorFromString
  (@"_toggleRotateAnimationDidStop:finished:");
SEL newSelector = 
  @selector(toggleMinusButtonAnimationDidStop:finished:);

IMP imp = imp_implementationWithBlock(^(id target) {
    [target performSelector:newSelector];

    [self toggleMinusButtonImageForControlView:target];
});

Method m = class_getInstanceMethod(class, originalSelector);
if (!class_addMethod(class, newSelector, 
      imp, method_getTypeEncoding(m))) {
    return;
}

Swizzle(class, originalSelector, newSelector);

Implementation, part 1

@implementation HOM

-(void)forwardInvocation:(NSInvocation*)anInvocation {
   id result = [xxTarget performSelector:xxSelector 
       withObject:anInvocation];
   [anInvocation setReturnValue:&result];
}

-methodSignatureForSelector:(SEL)aSelector {
   return [[xxTarget objectAtIndex:0] 
       methodSignatureForSelector:aSelector];
}

-xxinitWithTarget:aTarget selector:(SEL)newSelector {
   xxTarget = aTarget;
   xxSelector = newSelector;
   return self;
}

Implementation, part 2

@implementation NSArray(hom)

-(NSArray*)collect:(NSInvocation*)anInvocation {
    NSMutableArray *resultArray = [NSMutableArray array];

    for (id obj in self ) {
        id resultObject;
        [anInvocation invokeWithTarget:obj];
        [anInvocation getReturnValue:&resultObject];
        [resultArray addObject:resultObject];
    }

    return resultArray;
}

-collect {
    return [HOM homWithTarget:self 
        selector:@selector(collect:)];
}