拦截UIViewController的popViewController事件

实现拦截`UIViewController`的`pop`操作有两种方式:


* 自定义实现返回按钮,即设置`UIBarButtonItem`来实现自定义的返回操作。
* 创建`UINavigatonController`的`Category`,来定制`navigationBar: shouldPopItem:`的逻辑。



**UIViewController+BackButtonHandler.h:**


```objc
#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:**


```objc
#import "UIViewController+BackButtonHandler.h"

@implementation UIViewController (BackButtonHandler)

@end

@implementation UINavigationController (ShouldPopOnBackButton)

- (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 {
		// Workaround for iOS7.1. Thanks to @boliva - http://stackoverflow.com/posts/comments/34452906
		for(UIView *subview in [navigationBar subviews]) {
			if(0. < subview.alpha && subview.alpha < 1.) {
				[UIView animateWithDuration:.25 animations:^{
					subview.alpha = 1.;
				}];
			}
		}
	}

	return NO;
}

```

**使用:**


* 在`UIViewController`当中引入头文件


```
#import "UIViewController+BackButtonHandler.h"
```
* 在`UIViewController`中实现`navigationShouldPopOnBackButton`方法。


```objc
- (BOOL)navigationShouldPopOnBackButton{
    [[[UIAlertView alloc] initWithTitle:@"提示" message:@"确定返回上一界面?"
                               delegate:self cancelButtonTitle:@"取消" otherButtonTitles:@"确定", nil] show];
    //renturn no 拦截pop事件
    return NO;
}
```




**参考:**


* [iOS拦截导航栏返回按钮事件的正确方式](http://www.jianshu.com/p/25fd027916fa)

* [UIViewController-BackButtonHandler](https://github.com/onegray/UIViewController-BackButtonHandler)
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