Hook.h

#import <objc/runtime.h>

#define HookMethod(cls_, sel_, ...) \
  (^{ \
    SEL _cmd = sel_; \
    __block IMP __original = method_setImplementation(class_getInstanceMethod(cls_, _cmd), imp_implementationWithBlock(__VA_ARGS__)); \
  })()

#define CallOriginal(...) \
  __original(self, _cmd, __VA_ARGS__)

test.m

#import <Foundation/Foundation.h>

#import "Hook.h"

@interface Example : NSObject
@end

@implementation Example

- (NSString *)example {
    return @"one";
}

@end

void main() {
    @autoreleasepool {
        HookMethod([Example class], @selector(example), ^(Example *self) {
            return [CallOriginal() stringByAppendingString:@" two"];
        });

        Example *e = [[Example alloc] init];
        NSLog(@"example: %@", [e example]);
    }
}