• 让一个对象监听另一个对象属性的改变

// MJPerson.h
#import <Foundation/Foundation.h>
@interface MJPerson : NSObject

@property(nonatomic, assign)NSInteger age;

@end

// MJPerson.m
@implementation MJPerson
@end

// MJDog.h
#import <Foundation/Foundation.h>
@interface MJDog : NSObject
@end

// MJDog.m
#import “MJDog.h”
@implementation MJDog
@end

// MJViewController.h
#import <Foundation/Foundation.h>
@interface MJViewController : UIViewController
@end

// MJViewController.m
#import “MJViewController.h”
#import “MJPerson.h”
#import “MJDog”
@interface MJViewController () // 这是类扩展，类别的一种

@property (nonatomic, strong)MJPerson *person;
@property (nonatomic, strong)MJDog *dog;

@end

@implementation MJViewController

- (void)viewDidLoad {
  self.person = [[MJPerson alloc] init];
  self.dog = [[MJDog alloc] init];
  // 让self.dog监听self.person的age属性的改变
  [self.person addObsever:self.dog forKeyPath:@”age” options:0 context:nil];
}

- (void)touchesBegan:(NSSet *)touches withEvents:(UIEvent *)event {
  self.person.age = 30;
}

@end

KVO内部实现原理
运行过程中，系统会自动生成一个NSKVONortifying_MJPerson ：MJPerson类，调用person的setAge方法，会调用子类的setAge方法，runtime中会使用到KVO。

// NSKVONortifying_MJPerson.h
#import “MJPerson.h”
@interface NSKVONortifying_MJPerson : MJPerson
@end

// NSKVONortifying_MJPerson.m
#import “NSKVONortifying_MJPerson.h”
@implementation NSKVONortifying_MJPerson

- (void)setAge:(int)age {
  [super setAge:age];
  [self willChangeValueForKey:@”age”];
  [self didChangeValueForKey:@”age”];
}

@end