原理：动态生成子类，重写set方法，分发observeValueForKeyPath消息

自定义KVO用到的一些runtime接口

//动态生成一个root类或者某类的派生类
objc_allocateClassPair(Class superclass, const char *name, 
                                         size_t extraBytes) 
//注册类
void objc_registerClassPair(Class cls) 
//给类添加一个方法
BOOL class_addMethod(Class cls, SEL name, IMP imp, 
                                 const char *types) 

//修改对象的指针
Class object_setClass(id obj, Class cls) 

//添加关联属性
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)

自定义KVO例子

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


@implementation NSObject (WHJKVO)
- (void)whj_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context {
    // 创建类
    NSString * superClass = NSStringFromClass([self  class]);
    NSString *subClassName = [NSString stringWithFormat:@"whj_%@",superClass];
    const char *name = [subClassName UTF8String];
    Class myClass =  objc_allocateClassPair([self class], name, 0);
    // 注册类
    objc_registerClassPair(myClass);
    // 添加方法
    NSString *methodName = [NSString stringWithFormat:@"set%@:",[keyPath capitalizedString]];
    objc_setAssociatedObject(self, @"methodName", methodName, OBJC_ASSOCIATION_RETAIN_NONATOMIC); //Person 添加关联属性
    class_addMethod(myClass, NSSelectorFromString(methodName), (IMP)myImp, "V@:@");

    //修改指针
    object_setClass(self, myClass);//执行这一行后self，就指向 whj_Person了
    // 添加关联对象,目的保存observer，以备给observer发送消息
    objc_setAssociatedObject(self, @"objc", observer, OBJC_ASSOCIATION_ASSIGN); //whj_Person 添加关联属性

}


void myImp(id self, SEL _cmd,id param) {

    //调用super 的实现
    //先保存子类
    id class = [self class];
    //指针改到父类
    object_setClass(self,class_getSuperclass(class));
    NSString *methodName = objc_getAssociatedObject(self, @"methodName"); // Person 获取关联属性
    //调用父类的方法
    objc_msgSend(self,NSSelectorFromString(methodName),param);

    // 发消息
    id observer =  objc_getAssociatedObject(self, @"objc"); // Person 获取关联属性
    objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject:change:context:),param,self,nil,nil);
    
    //指针改回来 ,这样下次属性变化还回来whj_Person类的myImp方法
    object_setClass(self, class);
    
}