產(chǎn)生的原因：
當(dāng)一個(gè)對象添加了notification之后，如果dealloc的時(shí)候，仍然持有notification，就會(huì)出現(xiàn)NSNotification類型的crash。NSNotification類型的crash多產(chǎn)生于程序員寫代碼時(shí)候犯疏忽，在NSNotificationCenter添加一個(gè)對象為observer之后，忘記了在對象dealloc的時(shí)候移除它。

iOS9之前會(huì)crash，iOS9之后蘋果系統(tǒng)已優(yōu)化。在iOS9之后，即使開發(fā)者沒有移除observer，Notification crash也不會(huì)再產(chǎn)生了。

解決方案：
NSNotification Crash的防護(hù)原理很簡單， 利用method swizzling hook NSObject的dealloc函數(shù)，再對象真正dealloc之前先調(diào)用一下：[[NSNotificationCenter defaultCenter] removeObserver:self]，即可。

具體方式：

#import <Foundation/Foundation.h>

/**
 當(dāng)一個(gè)對象添加了notification之后，如果dealloc的時(shí)候，仍然持有notification，就會(huì)出現(xiàn)NSNotification類型的crash。
 
 iOS9之后專門針對于這種情況做了處理，所以在iOS9之后，即使開發(fā)者沒有移除observer，Notification crash也不會(huì)再產(chǎn)生了
 */

NS_ASSUME_NONNULL_BEGIN

@interface NSObject (NSNotificationCrash)

+ (void)xz_enableNotificationProtector;

@end

NS_ASSUME_NONNULL_END

#import "NSObject+NSNotificationCrash.h"
#import "NSObject+XZSwizzle.h"
#import <objc/runtime.h>


static const char *isNSNotification = "isNSNotification";

@implementation NSObject (NSNotificationCrash)


+ (void)xz_enableNotificationProtector {
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSObject *objc = [[NSObject alloc] init];
        
        [objc xz_instanceSwizzleMethod:@selector(addObserver:selector:name:object:) replaceMethod:@selector(xz_addObserver:selector:name:object:)];
        
        // 在ARC環(huán)境下不能顯示的@selector dealloc。
        [objc xz_instanceSwizzleMethod:NSSelectorFromString(@"dealloc") replaceMethod:NSSelectorFromString(@"xz_dealloc")];
    });
}

- (void)xz_addObserver:(id)observer selector:(SEL)aSelector name:(nullable NSNotificationName)aName object:(nullable id)anObject {
    
    // 添加標(biāo)志位，在delloc中只有isNSNotification是YES，才會(huì)移除通知
    [observer setIsNSNotification:YES];
    [self xz_addObserver:observer selector:aSelector name:aName object:anObject];
}


- (void)setIsNSNotification:(BOOL)yesOrNo {
    objc_setAssociatedObject(self, isNSNotification, @(yesOrNo), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (BOOL)isNSNotification {
    NSNumber *number = objc_getAssociatedObject(self, isNSNotification);;
    return  [number boolValue];
}

/**
 如果一個(gè)對象從來沒有添加過通知，那就不要remove操作
 */
- (void)xz_dealloc
{
    if ([self isNSNotification]) {
        NSLog(@"CrashProtector: %@ is dealloc，but NSNotificationCenter Also exsit",self);
        [[NSNotificationCenter defaultCenter] removeObserver:self];
    }
    
    [self xz_dealloc];
}

@end