KVO在項(xiàng)目中使用很多诈泼，主要是兩種原因會(huì)使KVO崩潰

• 1膝迎、KVO沒有被移除
• 2颈嚼、KVO移除的次數(shù)比添加的次數(shù)多

設(shè)計(jì)思路

• 1未玻、利用runtime交換了addObserver:forKeyPath:options:context:
• 2灾而、在替換的addObserver:forKeyPath:options:context:中
  • a、創(chuàng)建一個(gè)Map扳剿，keyPath作為key旁趟，Value是KVOItem對(duì)象，而KVOItem保存是的被監(jiān)聽的對(duì)象庇绽，監(jiān)聽的屬性锡搜，這個(gè)Map是保存利用關(guān)聯(lián)屬性保存的
  • b、調(diào)用原生的addObserver:forKeyPath:options:context:方法瞧掺，
  • c耕餐、最后利用method_setImplementation修改了監(jiān)聽者的dealloc方法的實(shí)現(xiàn)，里面先是判斷是否被當(dāng)作監(jiān)聽對(duì)象辟狈，如果有肠缔，遍歷并移除KVO，然后調(diào)用原有的dealloc方法 哼转。沒有使用交換方法的原因是明未，沒有被當(dāng)作監(jiān)聽的對(duì)象在dealloc方法中，也會(huì)判斷被當(dāng)作監(jiān)聽的對(duì)象壹蔓。
• 3趟妥、但是在項(xiàng)目中，可能會(huì)有開發(fā)人員佣蓉、系統(tǒng)自己在dealloc中移除KVO披摄，這樣就會(huì)一個(gè)問題，因?yàn)樵谡{(diào)用dealloc之前勇凭， KVO已經(jīng)被移除了疚膊，這時(shí)候再次移除會(huì)崩潰，所以利用runtime交換removeObserver:forKeyPath:方法虾标，由于removeObserver:forKeyPath:context底層也是調(diào)用removeObserver:forKeyPath:, 所以這個(gè)方法不用替換酿联。
• 4、在替換的removeObserver:forKeyPath:中
  • a、利用observer對(duì)象取出對(duì)應(yīng)的Map贞让，判斷是否存在對(duì)應(yīng)的keyPath, 如果存在周崭，就移除KVO，并且從Map移除對(duì)應(yīng)的KeyPath

//
//  NSObject+KVO.m
//  CrashSafe
//
//  Created by 無頭騎士 GJ on 2019/1/26.
//  Copyright ? 2019 無頭騎士 GJ. All rights reserved.
//

#import "NSObject+KVO.h"
#import <objc/message.h>

static const char KVOArrayKey;

@interface WTKVOItem: NSObject

@property (nonatomic, weak) id obj;

@property (nonatomic, strong) NSString *keyPath;


@end

@implementation WTKVOItem


@end

@implementation NSObject (KVO)

+ (void)load
{
    Method addObserver = class_getInstanceMethod(self, @selector(addObserver:forKeyPath:options:context:));
    Method wt_addObserver = class_getInstanceMethod(self, @selector(wt_addObserver:forKeyPath:options:context:));
    method_exchangeImplementations(addObserver, wt_addObserver);
    
    Method removeObserver = class_getInstanceMethod(self, @selector(removeObserver:forKeyPath:));
    Method wt_removeObserver = class_getInstanceMethod(self, @selector(wt_removeObserver:forKeyPath:));
    method_exchangeImplementations(removeObserver, wt_removeObserver);
}


- (void)wt_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context
{
    if (observer == nil || keyPath == nil || keyPath.length == 0) return;
    
    NSMutableDictionary *keyPathdict = objc_getAssociatedObject(observer, &KVOArrayKey);
    if (keyPathdict == nil)
    {
        keyPathdict = [NSMutableDictionary dictionary];
        objc_setAssociatedObject(observer, &KVOArrayKey, keyPathdict, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    
    WTKVOItem *item = [WTKVOItem new];
    item.keyPath = keyPath;
    item.obj = self;
    
    keyPathdict[keyPath] = item;

    [self wt_addObserver: observer forKeyPath: keyPath options: options context: context];
    
    
    [self replaceImpl: observer.class];
}

- (void)wt_removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath
{
    NSMutableDictionary *keyPaths = objc_getAssociatedObject(observer, &KVOArrayKey);
    if (keyPaths == nil) return;
    
    if ([keyPaths objectForKey: keyPath])
    {
        [self wt_removeObserver: observer forKeyPath: keyPath];
        
        [keyPaths removeObjectForKey: keyPath];
    }
}


- (void)wt_removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(void *)context
{
    NSMutableDictionary *keyPaths = objc_getAssociatedObject(observer, &KVOArrayKey);
    if (keyPaths == nil) return;
    
    if ([keyPaths objectForKey: keyPath])
    {
        [self wt_removeObserver: observer forKeyPath: keyPath context: context];
        
        [keyPaths removeObjectForKey: keyPath];
    }
}

- (void)replaceImpl:(Class)cls
{
    Method dealloc = class_getInstanceMethod([self class], NSSelectorFromString(@"dealloc"));
    
    __block IMP deallocIMP = method_setImplementation(dealloc, imp_implementationWithBlock(^(__unsafe_unretained id self){
        
        ((void(*)(id, SEL))objc_msgSend)(self, @selector(cleanupSEL));
        
        ((void(*)(id, SEL))deallocIMP)(self, NSSelectorFromString(@"dealloc"));
    
    }));
}

- (void)cleanupSEL
{
    NSMutableDictionary *keyPaths = objc_getAssociatedObject(self, &KVOArrayKey);
    if (keyPaths == nil) return;
    
    [keyPaths enumerateKeysAndObjectsUsingBlock:^(NSString * _Nonnull key, WTKVOItem * _Nonnull obj, BOOL * _Nonnull stop) {
        
        [obj.obj removeObserver: self forKeyPath: obj.keyPath];
        
        
    }];
    
    objc_setAssociatedObject(self, &KVOArrayKey, NULL, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}


@end