1.创建被观察的对象

PersonInfo.h

#import <Foundation/Foundation.h>

@interface PersonInfo : NSObject

@property (nonatomic, assign) NSInteger age;

+ (instancetype)create;

@end

PersonInfo.m

#import "PersonInfo.h"

@implementation PersonInfo

+ (instancetype)create {
  PersonInfo *personInfo = [PersonInfo new];
  return personInfo;
}

@end

2.观察者为ViewController的实例对象, 被观察的是PersonInfo的age属性

ViewController.h

#import <UIKit/UIKit.h>
#import "PersonInfo.h"

@interface ViewController : UIViewController

@property (nonatomic, strong) PersonInfo *personInfo;

@end

ViewController.m

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

#pragma mark - ViewController LifeCycle

- (void)viewDidLoad {
  [super viewDidLoad];
  [self initPersonInfo];  //创建PersonInfo
  [self addObserverForPersonAge]; //添加观察者
}

- (void)dealloc {
  [self removeObserverForPersonAge]; //移除观察者
}

#pragma mark - IBAction Methods

- (IBAction)changeAgeButtonClicked:(id)sender {
  self.personInfo.age = arc4random()%200; //点击按钮, 随机产生一个年龄
}

#pragma mark - Observe Methods

/*
 *KeyPath: 被观察属性的字段名称
 *context: 联系上下文, 可用来传值
 *options: 被观察属性的旧值或新值得设置项 (4个), 不同的组合, 可获取不同维度的值
 *NSKeyValueObservingOptionOld   只可获取旧值
 *NSKeyValueObservingOptionNew  只可获取新值
 *NSKeyValueObservingOptionInitial 注册即会调用一次, 只可获取新值 (需配合: NSKeyValueObservingOptionNew)
 *NSKeyValueObservingOptionPrior 调用两次, 在值改变之前和值改变之后
 */

- (void)addObserverForPersonAge {
  [self.personInfo addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew context:nil];
}

- (void)removeObserverForPersonAge {
  [self.personInfo removeObserver:self forKeyPath:@"age" context:nil];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
  NSInteger oldAge = [[change objectForKey:NSKeyValueChangeOldKey] integerValue];
  NSLog(@"Old age: %ld", (long)oldAge);

  NSInteger newAge = [[change objectForKey:NSKeyValueChangeNewKey] integerValue];
  NSLog(@"New age: %ld", (long)newAge);
}

#pragma mark - Private Methods

- (void)initPersonInfo {
  self.personInfo = [PersonInfo create];
  self.personInfo.age = 6;
}

@end