Override Methods with Categories

A+EatApple.m

#import "A.h"
#import <objc/runtime.h>

@implementation A (EatApple)

- (void)printAppleOverride
{
    [self printAppleOverride];
    NSLog(@"Eat that Apple");
}

+ (void)load
{
    method_exchangeImplementations(class_getInstanceMethod(self, @selector(printApple)), class_getInstanceMethod(self, @selector(printAppleOverride)));
}

@end

A.m

@implementation A

- (void)printApple
{
    NSLog(@"The original Apple");
}

@end