NSObject+kvo.h

#import <Foundation/Foundation.h>

@interface NSObject (kvo)

- (void)hk_addObserver:(NSObject *_Nonnull)observer forKeyPath:(NSString *_Nullable)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;


@end

NSObject+kvo.m

#import "NSObject+kvo.h"
#import <objc/message.h>//消息发送!!

/*
 1.创建HANKKVO_Perosn子类
 2.修改调用者的isa指针
 3.重写set --> 添加!
 */
@implementation NSObject (kvo)

- (void)hk_addObserver:(NSObject *_Nonnull)observer forKeyPath:(NSString *_Nullable)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context
{
    //1.动态生成一个类
    NSString * oldName = NSStringFromClass([self class]);
    NSString * newName = [@"HankKVO_" stringByAppendingString:oldName];
    //继承谁,叫什么
    Class MyClass = objc_allocateClassPair([self class], newName.UTF8String, 0);
    //注册
    objc_registerClassPair(MyClass);
    
    //2.动态修改一个对象的类型!!
    object_setClass(self, MyClass);
    
    //3.添加setName方法
    NSString *aSelectorName = [NSString stringWithFormat:@"set%@:",[keyPath capitalizedString]];
    SEL selector = NSSelectorFromString(aSelectorName);
    class_addMethod(MyClass, selector, (IMP)setProperty, "v@:@");
    
    //将观察者保存到当前对象中!!
    objc_setAssociatedObject(self, @"observer", observer, OBJC_ASSOCIATION_ASSIGN);
    //将keyPath保存到当前对象中!!
    objc_setAssociatedObject(self, @"keyPath", keyPath, OBJC_ASSOCIATION_ASSIGN);
}


//OC 的方法,每次调用,有两个隐式参数!!
void setProperty(id self,SEL _cmd,NSString * value){
    //保存class
    id class = [self class];
    //拿出保存到在当前对象中的属性
    NSString *keyPath = objc_getAssociatedObject(self, @"keyPath");
    //改变类型 self -> super
    object_setClass(self, class_getSuperclass([self class]));
    //获取Set方法
    NSString *aSelectorName = [NSString stringWithFormat:@"set%@:",[keyPath capitalizedString]];
    SEL selector = NSSelectorFromString(aSelectorName);
    //发送父类方法 [super setName:newName]
    objc_msgSend(self, selector,value);
    //改回子类  super -> self
    object_setClass(self, class);
    //拿出保存到在当前对象中的属性
    id observer = objc_getAssociatedObject(self, @"observer");
    //发送回调方法
    objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject:change:context:),keyPath,self,nil,nil);
}

@end

ViewController.m

#import "ViewController.h"
#import "Person.h"
#import "NSObject+kvo.h"

@interface ViewController ()

@property (nonatomic, strong) Person *p;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    Person * p = [[Person alloc] init];
    [p hk_addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew context:nil];
    _p = p;
}

//通知!!
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"发现属性变化：%@",keyPath);
}


- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    _p.age = @"18";
    NSLog(@"现在的Age:%@",_p.age);
}