UINavigationController+Completion.h

@interface UINavigationController(Completion)

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

@end

UINavigationController+Completion.m

#import "UINavigationController+Completion.h"
#import <objc/runtime.h>

@interface UINavigationController(Completion_Private)<UINavigationControllerDelegate>

@property (nonatomic, copy) void (^per_completionBlock)();
@property (nonatomic) id<UINavigationControllerDelegate> per_oldDelegate;

@end

@implementation UINavigationController(Completion)

- (void) pushViewController:(UIViewController *)viewController animated:(BOOL)animated completion:(void (^)(void))completion
{
    self.per_oldDelegate = self.delegate;
    self.delegate = self;
    
    self.per_completionBlock = completion;
    
    [self pushViewController:viewController animated:animated];
}

@end

@implementation UINavigationController(Completion_Private)

- (void) setPer_oldDelegate:(id<UINavigationControllerDelegate>)per_oldDelegate
{
    objc_setAssociatedObject(self, @selector(per_oldDelegate), per_oldDelegate, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (id<UINavigationControllerDelegate>) per_oldDelegate
{
    return objc_getAssociatedObject(self, @selector(per_oldDelegate));
}

- (void) setPer_completionBlock:(void (^)())per_completionBlock
{
    objc_setAssociatedObject(self, @selector(per_completionBlock), per_completionBlock, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

- (void (^)())per_completionBlock
{
    return objc_getAssociatedObject(self, @selector(per_completionBlock));
}

- (void) navigationController:(UINavigationController *)navigationController didShowViewController:(UIViewController *)viewController animated:(BOOL)animated
{
    if (self.per_completionBlock)
    {
        self.per_completionBlock();
    }
    
    if ([self.per_oldDelegate respondsToSelector:@selector(navigationController:didShowViewController:animated:)])
    {
        [self.per_oldDelegate navigationController:navigationController didShowViewController:viewController animated:animated];
    }
    
    self.delegate = self.per_oldDelegate;
}

- (void) navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated
{
    if ([self.per_oldDelegate respondsToSelector:@selector(navigationController:willShowViewController:animated:)])
    {
        [self.per_oldDelegate navigationController:navigationController willShowViewController:viewController animated:animated];
    }
}

- (id<UIViewControllerInteractiveTransitioning>) navigationController:(UINavigationController *)navigationController interactionControllerForAnimationController:(id<UIViewControllerAnimatedTransitioning>)animationController
{
    if ([self.per_oldDelegate respondsToSelector:@selector(navigationController:interactionControllerForAnimationController:)])
    {
        return [self.per_oldDelegate navigationController:navigationController interactionControllerForAnimationController:animationController];
    }
    
    return nil;
}

- (UIInterfaceOrientation) navigationControllerPreferredInterfaceOrientationForPresentation:(UINavigationController *)navigationController
{
    if ([self.per_oldDelegate respondsToSelector:@selector(navigationControllerPreferredInterfaceOrientationForPresentation:)])
    {
        return [self.per_oldDelegate navigationControllerPreferredInterfaceOrientationForPresentation:navigationController];
    }
    
    switch ([[UIDevice currentDevice] orientation])
    {
        case UIDeviceOrientationUnknown:
        case UIDeviceOrientationFaceUp:
        case UIDeviceOrientationFaceDown:
        case UIDeviceOrientationPortrait:
        {
    		return UIInterfaceOrientationPortrait;
        }
        case UIDeviceOrientationLandscapeLeft:
        {
            return UIInterfaceOrientationLandscapeLeft;
        }
        case UIDeviceOrientationLandscapeRight:
        {
            return UIInterfaceOrientationLandscapeRight;
        }
        case UIDeviceOrientationPortraitUpsideDown:
        {
            return UIInterfaceOrientationPortraitUpsideDown;
        }
    }
}

- (NSUInteger) navigationControllerSupportedInterfaceOrientations:(UINavigationController *)navigationController
{
    if ([self.per_oldDelegate respondsToSelector:@selector(navigationControllerSupportedInterfaceOrientations:)])
    {
        return [self.per_oldDelegate navigationControllerSupportedInterfaceOrientations:navigationController];
    }
    
    return UIInterfaceOrientationMaskAll;
}

@end