思路及效果：

利用runtime实现方法交换(method_exchangeImplementations)和利用runtime 给分类动态绑定属性kOriginDelegate。kOriginDelegate绑定原self.interactivePopGestureRecognizer.delegate代理来响应代理方法，添加一个接口 -(BOOL)navigationShouldPopOnBackButton;供外部访问， 这个接口返回YES正常跳转，返回NO即可拦截返回按钮的响应事件。

具体实现如下：

UIViewController+BackButtonHandler.h文件：

#import <UIKit/UIKit.h>

@protocol BackButtonHandlerProtocol <NSObject>
@optional
// Override this method in UIViewController derived class to handle 'Back' button click
- (BOOL)navigationShouldPopOnBackButton;
@end

@interface UIViewController (BackButtonHandler) <BackButtonHandlerProtocol>

@end

UIViewController+BackButtonHandler.m文件：

#import "UIViewController+BackButtonHandler.h"
#import <objc/runtime.h>

@implementation UIViewController (BackButtonHandler)

@end

static NSString *const kOriginDelegate = @"kOriginDelegate";

@implementation UINavigationController (ShouldPopOnBackButton)

+ (void)load
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        
        SEL originSelector = @selector(viewDidLoad);
        SEL swizzledSelector = @selector(new_viewDidLoad);
        
        Method originMethod = class_getInstanceMethod(class, originSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL didAddMethod = class_addMethod(class,
                                            originSelector,
                                            method_getImplementation(swizzledMethod),
                                            method_getTypeEncoding(swizzledMethod));
        if (didAddMethod) {
            class_replaceMethod(class,
                                swizzledSelector,
                                method_getImplementation(originMethod),
                                method_getTypeEncoding(originMethod));
        } else {
            method_exchangeImplementations(originMethod, swizzledMethod);
        }
    });
}

- (void)new_viewDidLoad
{
    [self new_viewDidLoad];
    
    objc_setAssociatedObject(self, [kOriginDelegate UTF8String], self.interactivePopGestureRecognizer.delegate, OBJC_ASSOCIATION_ASSIGN);
    self.interactivePopGestureRecognizer.delegate = (id<UIGestureRecognizerDelegate>)self;
}

#pragma mark - 按钮
- (BOOL)navigationBar:(UINavigationBar *)navigationBar shouldPopItem:(UINavigationItem *)item {
    
    if([self.viewControllers count] < [navigationBar.items count]) {
        return YES;
    }

    BOOL shouldPop = YES;
    UIViewController* vc = [self topViewController];
    if([vc respondsToSelector:@selector(navigationShouldPopOnBackButton)]) {
        shouldPop = [vc navigationShouldPopOnBackButton];
    }

    if(shouldPop) {
        dispatch_async(dispatch_get_main_queue(), ^{
            [self popViewControllerAnimated:YES];
        });
    } else {
        for(UIView *subview in [navigationBar subviews]) {
            if(0. < subview.alpha && subview.alpha < 1.) {
                [UIView animateWithDuration:.25 animations:^{
                    subview.alpha = 1.;
                }];
            }
        }
    }
    return NO;
}

#pragma mark - 手势
- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer
{
    if (gestureRecognizer == self.interactivePopGestureRecognizer) {
        UIViewController *vc = [self topViewController];
        if([vc respondsToSelector:@selector(navigationShouldPopOnBackButton)]) {
            return [vc navigationShouldPopOnBackButton];
        }
        id<UIGestureRecognizerDelegate> originDelegate = objc_getAssociatedObject(self, [kOriginDelegate UTF8String]);
        return [originDelegate gestureRecognizerShouldBegin:gestureRecognizer];
    }
    return YES;
}
@end

使用实例：

/**
 * 协议中的方法，获取返回按钮的点击事件
 */
- (BOOL)navigationShouldPopOnBackButton
{
    if ([self.mark isEqualToString:@"绑定手机号"]) {
        UIAlertView *view = [[UIAlertView alloc] initWithTitle:@"" message:@"绑定尚未完成，确认退出？" delegate:self cancelButtonTitle:@"取消" otherButtonTitles:@"确定", nil];
        [view show];
        return NO;
    }else{
        return YES;
    }
}