NSObject+ResponderBlock.h

#import <Foundation/Foundation.h>

@interface NSObject (ResponderBlock)

- (id)responderBlockForSelector:(SEL)selector;
- (void)setResponderBlock:(id)block forSelector:(SEL)selector;

@end

NSObject+ResponderBlock.m

#import "NSObject+ResponderBlock.h"
#import <objc/runtime.h>

@implementation NSObject (ResponderBlock)

- (id)responderBlockForSelector:(SEL)selector
{
    Method method = class_getInstanceMethod([self class], selector);
    IMP implementation = method_getImplementation(method);
    
    return imp_getBlock(implementation);
}

- (void)setResponderBlock:(id)block forSelector:(SEL)selector
{
    Class class = [self class];
    
    IMP newImplementation = imp_implementationWithBlock(block);
    IMP oldImplementation = class_getMethodImplementation(class, selector);
    
    if (oldImplementation) {
        class_replaceMethod(class, selector, newImplementation, nil);
    } else {
        class_addMethod(class, selector, newImplementation, nil);
    }
}

@end