1、了解KVO

@interface ViewController ()
@property (nonatomic,strong)Person *p1;
@property (nonatomic,strong)Person *p2;
@end

@implementation ViewController

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"变了---key:%@ ---change:%@",keyPath,change);
}


- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    _p1 = [[Person alloc]init];
    _p2 = [[Person alloc]init];
    _p1.personName = @"lily";
    _p2.personName = @"lucy";
    NSLog(@"监听之前---p1：%p ---p2：%p",[_p1 methodForSelector:@selector(setPersonName:)],[_p2 methodForSelector:@selector(setPersonName:)]);
    
    [_p1 addObserver:self forKeyPath:@"personName" options:NSKeyValueObservingOptionNew context:nil];
    _p1.personName = @"macBookPro";
    
    /*
     (lldb) po _p1
     <Person: 0x6000033b43f0>

     (lldb) po object_getClassName(_p1)
     "NSKVONotifying_Person"
     */
    NSLog(@"监听之后---p1：%p ---p2：%p",[_p1 methodForSelector:@selector(setPersonName:)],[_p2 methodForSelector:@selector(setPersonName:)]);
}

打印结果：

2021-05-22 15:54:52.280855+0800 hahahah[20817:1289137] 监听之前---p1：0x104136c90 ---p2：0x104136c90
2021-05-22 15:54:52.281967+0800 hahahah[20817:1289137] 变了---key:personName ---change:{
    kind = 1;
    new = macBookPro;
}
2021-05-22 15:54:52.282090+0800 hahahah[20817:1289137] 监听之后---p1：0x7fff207bbb57 ---p2：0x104136c90

2、自定义实现KVO

.h

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface Person : NSObject
@property (nonatomic,copy)NSString *personName;
@end

NS_ASSUME_NONNULL_END

.m

#import "Person.h"
#import <objc/message.h>

@implementation Person

void setterMethed(id self,SEL _cmd,NSString *key){
    //1、重写父类方法
    struct objc_super superClass = {
        self,
        class_getSuperclass([self class])
    };
    objc_msgSendSuper(&superClass,_cmd,key);
    //2、通知观察者,执行回调方法
    id observer = objc_getAssociatedObject(self, (__bridge const void *)@"myObjc");
    //key--
    NSString *methedName = NSStringFromSelector(_cmd);
    NSString *myKey = getMethedName(methedName);
    objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject: change:context:),myKey,self,@{myKey:key},nil);
}

NSString * getMethedName(NSString *setter){
    NSRange range = NSMakeRange(3, setter.length-4);
    NSString *key = [setter substringWithRange:range];
    NSString *letter = [[key substringToIndex:1]lowercaseString];
    key = [key stringByReplacingCharactersInRange:(NSMakeRange(0, 1)) withString:letter];
    return key;
}

- (void)hh_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context{
    
    NSString *oldClassStr = NSStringFromClass([self class]);
    NSString *newClassStr = [NSString stringWithFormat:@"hhKVO_%@",oldClassStr];
    
    //1、初始化类对
    Class newClass = objc_allocateClassPair([self class], newClassStr.UTF8String, 0);
    //2、注册类对
    objc_registerClassPair(newClass);
    //3、修改isa
    object_setClass(self, newClass);
    //4、重写set方法
    NSString *newSetMethed = [NSString stringWithFormat:@"set%@",keyPath.capitalizedString];
    SEL sel = NSSelectorFromString(newSetMethed);
    class_addMethod(newClass, sel, (IMP)setterMethed, "v@:@");
    
    objc_setAssociatedObject(self, (__bridge const void *)@"myObjc", observer, OBJC_ASSOCIATION_ASSIGN);
}

@end