1. KVO 简单使用

//添加一个监听

• (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;
  //响应kvo
• (void)observeValueForKeyPath:(nullable NSString *)keyPath ofObject:(nullable id)object change:(nullable NSDictionary<NSKeyValueChangeKey, id> *)change context:(nullable void *)context;

新建一个控制器 TagetViewController 控制器 和一个 Company 类
在我们的 Company 类创建一个对象 监听它的key值

Company 类
@interface Company : NSObject
@property(strong,nonatomic) NSString * companyName;
@end

#import "Company.h"
@implementation Company
@end

//在 TagetViewController 中 实例化Company并添加一个companyName属性值监听
  _pany = [Company  new];
  [_pany addObserver:self forKeyPath:NSStringFromSelector(@selector(companyName)) options:(NSKeyValueObservingOptionNew) context:nil];

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

//简单触发监听
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    _pany.companyName = @"5555";
}

2.如何开启手动添加触发

如果我们有需求需要去手动开起KVO 我们需要在观察的对象中实现
//开启手动触发

• (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key; YES 自动 NO 手动
  手动触发方法实现

• (void)willChangeValueForKey:(NSString *)key;
• (void)didChangeValueForKey:(NSString *)key;

开启手动模式
#import "Company.h"

@implementation Company

+(BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
   //return  YES; // 自动
 // return  NO ;  //手动
    
    if ([key isEqualToString:@"companyName"]) {
        return  NO;
    }
    return  YES;
}


@end


 TagetViewController 控制器里面
//触发
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    
     [_pany willChangeValueForKey:@"companyName"];
     [_pany didChangeValueForKey:@"companyName"];
    _pany.companyName = @"5555";

    
}

由此可以看出 kvo 的手动触发跟值的改变没有关系。 只是跟 willChangeValueForKey：didChangeValueForKey：的调用有关。

3.添加属性依赖

现在在我们的Company类里面新增了一个Member 类 我们的Member类中有很多属性。当每一个属性值发生改变我们都需要触发KVO 但是我们不能对每一个属性都去添加监听。所以这里我们就需要添加属性依赖。添加了依赖之后我们只需要去监听我们的me mber 对象 当他添加依赖的属性值发生改变我们就能监听。
Member类

#import <Foundation/Foundation.h>

@interface Member : NSObject

///姓名
@property (nonatomic, copy) NSString * name ;
///编号
@property (nonatomic, strong) NSString * number;
///职位
@property (nonatomic, strong) NSString * job;
@end

@implementation Member
@end

company 新增 
///员工
@property (nonatomic, strong) Member * member ;

//实例化
- (instancetype)init
{
    self = [super init];
    if (self) {
        _member = [Member new];
        
    }
    return self;
}

在 company中添加我们的依赖对象

/**添加属性依赖*/

+(NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{
    NSSet * keyPaths = [super keyPathsForValuesAffectingValueForKey:key];
    
    if ([key isEqualToString:@"member"])
    {
        // 添加依赖对象
        keyPaths = [[NSSet alloc]initWithObjects:@"_member.name",@"_member.number",@"_member.job", nil];
        
    }
    return  keyPaths;   
}

添加了属性依赖之后只需要监听我们的member对象 当其属性值发生改变我们都能收到监听

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    _pany = [Company  new];
//只需要监听member对象
    [_pany addObserver:self forKeyPath:NSStringFromSelector(@selector(member)) options:(NSKeyValueObservingOptionNew) context:nil];
     
    
}

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

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    

    //_pany.member.name = @"腾讯";
   // _pany.member.job = @"IOS 开发";
    _pany.member.number =@"3958";
    
    
}

4.KVO底层实现（自定义KVO）

• 创建一个子类！ NSKVONotyfind_XXXX
• 重写setName 方法！
• 外界改变isa指针！
  新建一个nsobject 扩展类

import "NSObject+PBKVO.h"

#import <Foundation/Foundation.h>

@interface NSObject (PBKVO)

///自定义KVO
-(void)pb_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;


@end

import "NSObject+PBKVO.m"

#import "NSObject+PBKVO.h"
#import <objc/message.h>
#import <UIKit/UIKit.h>
@implementation NSObject (PBKVO)
-(void)pb_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context
{
    NSString * oldName =NSStringFromClass(self.class);
    NSString * newName = [@"PBKVO" stringByAppendingString:oldName];
    //创建一个子类
    Class  myClass =  objc_allocateClassPair(self.class, newName.UTF8String, 0);
    objc_registerClassPair(myClass);
    //重写setName方法 其实就是添加一个方法
   
    class_addMethod(myClass, @selector(setName:), (IMP)setName, "v@:@");
    //改变isa  指针
    object_setClass(self, myClass);
    
    //保存当前观察者对象 - 用弱对象
    objc_setAssociatedObject(self, @"observer", observer, OBJC_ASSOCIATION_ASSIGN);
    
}

void  setName (id self,SEL _cmd, NSString * newName)
{
    //调用父类的 setName方法
    Class  class = [self class];
    //先把自己指向父类
    object_setClass(self,  class_getSuperclass([self class]));
    objc_msgSend(self,@selector(setName:),newName);
    id observer = objc_getAssociatedObject(self, @"observer");
    if (observer) {
        objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject:change:context:),@"name",self,@{@"new":newName},nil);
    }
    
    NSLog(@"哥们 我来了");
    //换回来
    object_setClass(self, class);
    
}

@end

修改监听方法
[_company pb_addObserver:self forKeyPath:@"name" options:(NSKeyValueObservingOptionNew) context:nil];

点击触发 也简单完成了监听回调。