@interface UINavigationController (Extensions)

- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated willPopHandler:(void (^)(void))inHandler;

@end

#### SNIP HERE ############

#import "UINavigationController+Extensions.h"

#import <objc/runtime.h>

@interface CNavigationControllerExtensionsHelper : NSObject <UINavigationControllerDelegate>
@property (readwrite, nonatomic, retain) NSMutableDictionary *handlersForControllers;
- (void)addHandler:(void (^)(void))inHandler forController:(UIViewController *)inViewController;
@end

#pragma mark -

static void *kNavigationControllerExtensionsHelperKey;

@implementation UINavigationController (Extensions)

- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated willPopHandler:(void (^)(void))inHandler
    {
    CNavigationControllerExtensionsHelper *theHelper = objc_getAssociatedObject(self, &kNavigationControllerExtensionsHelperKey);
    NSAssert(self.delegate == NULL || self.delegate == theHelper, @"Cannot use on navigation controller that already has a delegate.");
    if (theHelper == NULL)
        {
        theHelper = [[CNavigationControllerExtensionsHelper alloc] init];
        objc_setAssociatedObject(self, &kNavigationControllerExtensionsHelperKey, theHelper, OBJC_ASSOCIATION_RETAIN);
        self.delegate = theHelper;
        }
    [theHelper addHandler:inHandler forController:self.topViewController];

    [self pushViewController:viewController animated:animated];
    }

@end

#pragma mark -

@implementation CNavigationControllerExtensionsHelper

@synthesize handlersForControllers;

- (id)init
    {
    if ((self = [super init]) != NULL)
        {
        handlersForControllers = [[NSMutableDictionary alloc] init];
        }
    return self;
    }

- (void)addHandler:(void (^)(void))inHandler forController:(UIViewController *)inViewController
    {
    NSValue *theKey = [NSValue valueWithPointer:(__bridge void *)inViewController];
    [self.handlersForControllers setObject:[inHandler copy] forKey:theKey];
    }

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated;
    {
    NSValue *theKey = [NSValue valueWithPointer:(__bridge void *)viewController];
    void (^theHandler)(void) = [self.handlersForControllers objectForKey:theKey];
    if (theHandler != NULL)
        {
//        NSUInteger theIndex = [navigationController.viewControllers indexOfObject:viewController];
//        NSLog(@"WILL SHOW: %d / %d", theIndex, [navigationController.viewControllers count]);

        theHandler();

        [self.handlersForControllers removeObjectForKey:theKey];

        if (self.handlersForControllers.count == 0)
            {
            objc_setAssociatedObject(self, &kNavigationControllerExtensionsHelperKey, NULL, OBJC_ASSOCIATION_RETAIN);
            navigationController.delegate = NULL;
            }
        }
    }

@end