■ 核心：当一个类有观察者时，生成一个中间类，把它的isa指向中间类。

(生成中间类就方便去重写它的set方法。KVO是当你调用了set方法时它才会被触发。我重新生成set方法，里面也添加进去发送通知的方法。当我去调用set方法时，就会去触发它的KVO机制)

原理:

■ 当一个object有观察者时，动态创建这个object的类的子类

■ 对于每个被观察的property，去重写其set方法

■ 在重写的set方法中调用-willChangeValueForKey：和-didChangeValueForKey：

image.png

手动控制发送通知的时机，需要的核心方法

■ -(void)willChangeValueForKey:(NSString *)key

■ -(void)didChangeValueForKey:(NSString *)key

■ +(BOOL)automaticallyNotifiesObserverForKey:(NSString*)key //通过这个方法去判定key的情况，不让系统自动去发送。我自己手动控制发送通知，可以返回一个NO

CFPerson.h

#import <Foundation/Foundation.h>

@interface CFPerson : NSObject
@property(nonatomic,copy)NSString *name;
@property(nonatomic,copy)NSString *age;

- (void)setInfo:(NSDictionary *)info;
@end

CFPerson.m

#import "CFPerson.h"

@implementation CFPerson

- (void)setName:(NSString *)name
{
    if(_name!=name){
        [self willChangeValueForKey:@"name"];
        _name = name;
        [self didChangeValueForKey:@"name"];
    }
}

- (void)setInfo:(NSDictionary *)info
{
    [self willChangeValueForKey:@"age"];
    [self willChangeValueForKey:@"name"];
    _name = info[@"name"];
    _age = info[@"age"];
    [self didChangeValueForKey:@"name"];
    [self didChangeValueForKey:@"age"];
}

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
    if([key isEqualToString:@"name"]||[key isEqualToString:@"age"])
    {
        return NO;
    }else
    {
        return [super automaticallyNotifiesObserversForKey:key];
    }
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"属性%@改变之前的值为：%@",keyPath,change[NSKeyValueChangeOldKey]);
    NSLog(@"属性%@改变之后的值为：%@",keyPath,change[NSKeyValueChangeNewKey]);
}
@end

main.m

#import <Foundation/Foundation.h>
#import "CFPerson.h"
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        CFPerson *person = [[CFPerson alloc]init];
        person.name = @"lilei";
        
        //添加observer
        [person addObserver:person forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
        [person addObserver:person forKeyPath:@"age" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
//        person.name = @"zhangsan";
        
//        //手动发送通知
//        [person willChangeValueForKey:@"name"];
//        person.name = @"lilei";
//        [person didChangeValueForKey:@"name"];
        
          //调用重写的setter方法
//          person.name = @"zhangsan";
        
         //一个操作发送两个通知
        [person setInfo:@{@"name":@"wanger",@"age":@"20"}];
    }
    return 0;
}

2019-03-13 15:43:52.037056+0800 手动调用KVO[14200:1509438] 属性name改变之前的值为：lilei
2019-03-13 15:43:52.037277+0800 手动调用KVO[14200:1509438] 属性name改变之后的值为：wanger
2019-03-13 15:43:52.037328+0800 手动调用KVO[14200:1509438] 属性age改变之前的值为：<null>
2019-03-13 15:43:52.037347+0800 手动调用KVO[14200:1509438] 属性age改变之后的值为：20