//主要实现原理@implementation UINavigationController (ShouldPopOnBackBtn)- (BOOL)navigationBar:(UINavigationBar *)navigationBar shouldPopItem:(UINavigationItem *)item{    BOOL shouldPop = YES;    // 看一下当前控制器有没有实现代理方法 currentViewControllerShouldPop    // 如果实现了，根据当前控制器的代理方法的返回值决定    // 没过没有实现 shouldPop = YES    UIViewController* currentVC = [self topViewController];    if([currentVC respondsToSelector:@selector(currentViewControllerShouldPop)]) {        shouldPop = [currentVC currentViewControllerShouldPop];    }    if(shouldPop)    {        dispatch_async(dispatch_get_main_queue(), ^{            [self popViewControllerAnimated:YES];        });        // 这里要return, 否则这个方法将会被再次调用        return YES;    }    else    {        // 让系统backIndicator 按钮透明度恢复为1        for(UIView *subview in [navigationBar subviews]) {            if(0. < subview.alpha && subview.alpha < 1.) {                [UIView animateWithDuration:.25 animations:^{                    subview.alpha = 1.;                }];            }        }        return NO;    }}@end//如何使用// 第一步：导入分类头文件#import "UIViewController+BackBtnEventIntercept.h"// 第二步：实现代理方法，return NO 则拦截了系统的返回按钮事件- (BOOL)currentViewControllerShouldPop{    return NO;}如何禁用系统?右滑返回手势- (void)viewDidAppear:(BOOL)animated{    [super viewDidAppear:animated];    // 为当前控制器禁用?右滑返回手势    if ([self.navigationController respondsToSelector:@selector(interactivePopGestureRecognizer)]) {        self.navigationController.interactivePopGestureRecognizer.enabled = NO;    }}- (void)viewWillDisappear:(BOOL)animated{    [super viewWillDisappear:animated];    // 为其他控制器开启?右滑返回手势    if ([self.navigationController respondsToSelector:@selector(interactivePopGestureRecognizer)]) {        self.navigationController.interactivePopGestureRecognizer.enabled = YES;    }}