Custom_KVO.m

#import <Foundation/Foundation.h>
#import <objc/message.h>
#import <objc/runtime.h>

OBJC_EXPORT id objc_msgSendSuper2(void); /* objc_super superInfo = { self, [self class] }; */

@interface MyObject : NSObject {
    NSUInteger _integer;
}
@property (assign, nonatomic, setter=custom_setInteger:) NSUInteger integer;
@end
@implementation MyObject
+ (id)_createValueSetterWithContainerClassID:(id)classID key:(NSString *)key {
    if ([key isEqualToString:@"integer"]) {
        Method method = class_getInstanceMethod(self, @selector(custom_setInteger:));
        id setter = ((id (*)(id, SEL, id ,id, Method))objc_msgSend)([objc_lookUpClass("NSKeyValueMethodSetter") alloc], sel_registerName("initWithContainerClassID:key:method:"), classID, key, method);
        return setter;
    }
    
    struct objc_super superInfo = { self, [self class] };
    return ((id (*)(struct objc_super *, SEL, id, id))objc_msgSendSuper2)(&superInfo, _cmd, classID, key);
}

- (NSUInteger)integer {
    return _integer;
}
- (void)custom_setInteger:(NSUInteger)integer {
    _integer = integer;
}
@end

@interface Observer : NSObject
@end
@implementation Observer
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    NSLog(@"%@ %@ %@", keyPath, object, change);
}
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MyObject *object = [MyObject new];
        Observer *observer = [Observer new];
        
        [object addObserver:observer forKeyPath:@"integer" options:NSKeyValueObservingOptionNew context:NULL];
        object.integer = 3;
    }
    return 0;
}