先来定义一个类,要被我们监听的类

YSPerson.h 文件


#import <Foundation/Foundation.h>

@interface YSPerson : NSObject
@property (assign, nonatomic) int age;
@end

YSPerson.m文件


@implementation YSPerson
{
    int _age;
}

- (void)setAge:(int)age
{
    _age = age;
}

- (int)age
{
    return _age;
}

@end

在控制器中监听YSPerson 实例对象的 age


- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[YSPerson alloc] init];
    self.person1.age = 1;
    
    NSLog(@"person1添加KVO监听之前 - %@",
          object_getClass(self.person1));
    NSLog(@"person1添加KVO监听之前 - %p",
          [self.person1 methodForSelector:@selector(setAge:)]);

    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];

    NSLog(@"person1添加KVO监听之后 - %@",
          object_getClass(self.person1));
    NSLog(@"person1添加KVO监听之后 - %p ",
          [self.person1 methodForSelector:@selector(setAge:)]);


  /*
  通过上面的打印输出,可以证明,监听前后的实例对象的类型和
  setAge的方法地址都是不一样的,验证了我们的猜想
*/

}


- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
//    self.person1.age = 21;

    // NSKVONotifying_YSPerson是使用Runtime动态创建的一个类，是YSPerson的子类
    // self.person1.isa == NSKVONotifying_YSPerson
    [self.person1 setAge:21];
    
}


- (void)dealloc {
    [self.person1 removeObserver:self forKeyPath:@"age"];
}

// 当监听对象的属性值发生改变时，就会调用
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context);
}

当我们点击屏幕的时候,给self.person1的age赋值 21时候,我们就能监听到值的改变
实现原理是利用Runtime来完成
当我们监听了YSPerson的属性的时候,系统会在Runtime阶段为我们创建一个NSKVONotifying_YSPerson ,这个类继承自YSPerson
在NSKVONotifying_YSPerson的实现中插入了以下代码

@implementation NSKVONotifying_YSPerson

- (void)setAge:(int)age
{
    _NSSetIntValueAndNotify();
}

// 伪代码
void _NSSetIntValueAndNotify()
{
    [self willChangeValueForKey:@"age"];
    [super setAge:age];
    [self didChangeValueForKey:@"age"];
}

- (void)didChangeValueForKey:(NSString *)key
{
    // 通知监听器，某某属性值发生了改变
    [oberser observeValueForKeyPath:key ofObject:self change:nil context:nil];
}

@end