Apple 使用了 isa 混写（isa-swizzling）来实现 KVO 。当观察对象A时，KVO机制动态创建一个新的名为：NSKVONotifying_A 的新类，该类继承自对象A的本类，且 KVO 为 NSKVONotifying_A 重写观察属性的 setter 方法，setter 方法会负责在调用原 setter 方法之前和之后，通知所有观察对象属性值的更改情况。

自己实现KVO

由于KVO要对所有的NSObject都可以使用，所以创建NSObject的Category

NSObject+NNKVO.h

#import <Foundation/Foundation.h>
@interface NSObject (NNKVO)
    - (void)NN_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;
@end

NSObject+NNKVO.m

#import "NSObject+NNKVO.h"
#import <objc/message.h>
@implementation NSObject (NNKVO)

    - (void)NN_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context {
        //1. 创建子类
        NSString *originClassName = NSStringFromClass(self.class);
        NSString *newClassName = [@"NSKVONotifying_" stringByAppendingString:originClassName];
        
        Class newClass = objc_allocateClassPair(self.class, newClassName.UTF8String, 0);
        objc_registerClassPair(newClass);//创建子类后，需要register
        
        //2. 创建方法，需要重写父类的setter方法
        class_addMethod(newClass, @selector(setName:), (IMP)(NN_SetMethod), "");
        
        //3. 更改类型
        object_setClass(self, newClass);
        
        [[self getObservers] setObject:observer forKey:keyPath];
    }
    
    // setter方法
    void NN_SetMethod(id self, SEL method, id setValue) {
        Class subClass = object_getClass(self); // 获取当前类
        Class superClass = class_getSuperclass(subClass); // 获取父类
        
        object_setClass(self, superClass); // 更改当前对象的类型
        ((void (*) (id, SEL, id)) objc_msgSend) (self, method, setValue); // 调用setter方法，进行数据更改
        object_setClass(self, subClass); // 更改回原来的子类
        
        // 触发回调observe更新了属性
        for (id objc in [self getObservers].allValues) {
            ((void (*) (id, SEL, id, id, id ,id)) objc_msgSend)(objc, @selector(observeValueForKeyPath:ofObject:change:context:), @"name", self, nil, nil);
    }
    
    - (NSMutableDictionary *)getObservers {
        NSMutableDictionary *observers = objc_getAssociatedObject(self, _cmd);
        if (!observers) {
            observers = [NSMutableDictionary dictionary];
            objc_setAssociatedObject(self, _cmd, observers, OBJC_ASSOCIATION_RETAIN);
        }
        return observers;
    }
}

@end