什么是KVO

KVO(Key-Value Observing)，俗称键值监听。它提供一种机制，当指定的对象的属性被修改后，则对象就会接受到通知。简单的说就是每次指定的被观察的对象的属性被修改后，KVO就会自动通知相应的观察者了。

监听方法

- (void)addObserver:(NSObject*)observer forKeyPath:(NSString*)keyPath options:(NSKeyValueObservingOptions)options context:(void*)context;

• observer: 观察者,也就是KVO通知的订阅者
• keyPath: 描述将要观察的属性，相对于被观察者
• options: KVO的一些属性配置；有四个选项
  1. NSKeyValueObservingOptionNew:change字典包括改变后的值
  2. NSKeyValueObservingOptionOld:change字典包括改变前的值
  3. NSKeyValueObservingOptionInitial:注册后立刻触发KVO通知
  4. NSKeyValueObservingOptionPrior:值改变前是否也要通知
• context: 上下文，这个会传递到订阅着的函数中，用来区分消息

移除监听

- (void)removeObserver:(NSObject*)anObserver forKeyPath:(NSString*)keyPath;

- (void)removeObserver:(NSObject*)observer forKeyPath:(NSString*)keyPath context:(void*)context;

回调监听

- (void)observeValueForKeyPath:(NSString*)keyPath ofObject:(id)object change:(NSDictionary*)change context:(void*)context;

KVO使用步骤

1. 注册，指定被观察者的属性
2. 实现回调方法
3. 移除观察

代码展示

@interfacePerson:NSObject

@property(nonatomic,assign)NSIntegerage;

@end

@implementationPerson

@end

self.person1 = [[Person alloc]init];

self.person2 = [[Person alloc]init];

NSLog(@"监听前%@ --- %@",object_getClass(self.person1),object_getClass(self.person2));

NSKeyValueObservingOptionsoptions =NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld;

[self.person2 addObserver:selfforKeyPath:@"age"options:options context:nil];

self.person2.age =20;

NSLog(@"监听后%@ --- %@",object_getClass(self.person1),object_getClass(self.person2));


[self.person2 removeObserver:selfforKeyPath:@"age"];

打印结果：

KVODemo[2879:697131] 监听前Person --- Person

KVODemo[2879:697131] 监听后NSKVONotifying_Person--- Person

监听前

• self.person1的isa指向Person
• self.person2的isa指向Person

监听后

• self.person1的isa指向NSKVONotifying_Person
• self.person2的isa指向Person

RuntimeAPI动态生成一个NSKVONotifying_Person，并且让instance对象的isa指向这个全新的NSKVONotifying_Person

打印NSKVONotifying_Person内部方法

我们需要用到这个分类DLIntrospection已经有大神帮我们封装好了

+ (NSArray*)classes;

+ (NSArray*)properties;

+ (NSArray*)instanceVariables;

+ (NSArray*)classMethods;

+ (NSArray*)instanceMethods;

+ (NSArray*)protocols;

+ (NSDictionary*)descriptionForProtocol:(Protocol *)proto;

+ (NSString*)parentClassHierarchy;

断点调试结果

KVO[23897:840022] 监听前Person --- Person

(lldb) po [object_getClass(self.person1) instanceMethods]

<__NSArrayI0x604000429720>(

- (void)setAge:(q)arg0 ,

- (q)age

)

(lldb) po [object_getClass(self.person2) instanceMethods]

<__NSArrayI0x600000245ca0>(

- (void)setAge:(q)arg0 ,

- (class)class,

- (void)dealloc,

- (BOOL)_isKVOA

)

KVO[23897:840022] 监听后Person ---NSKVONotifying_Person

(lldb) po [object_getClass(self.person1) instanceMethods]

<__NSArrayI0x60000022a560>(

- (void)setAge:(q)arg0 ,

- (q)age

)

(lldb) po [object_getClass(self.person2) instanceMethods]

<__NSArrayI0x6040002217a0>(

- (void)setAge:(q)arg0 ,

- (q)age

)

结论

• RuntimeAPI动态生成一个NSKVONotifying_Person，并且让instance对象的isa指向这个全新的NSKVONotifying_Person
• -setAge: 修改age会调用Foundation的_NSSetXXXValueAndNotify函数(下面有打印)
• _NSSetXXXValueAndNotify函数会调用①willChangeValueForKey:，②然后调用原来的setter方法，③最后调用didChangeValueForKey:，在内部触发监听方法( observeValueForKeyPath:ofObject:change:context:）
• 为了隐藏NSKVONotifying_Person子类,重写了class方法，返回Person(苹果真会搞事情)
• -dealloc 清理现场
• -_isKVO内部表示，判断这个类有没有动态生成子类

我们来证明一下

@implementationPerson

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

NSLog(@"%@",NSStringFromSelector(_cmd));

[superwillChangeValueForKey:key];

}

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

NSLog(@"begin---%@",NSStringFromSelector(_cmd));

[superdidChangeValueForKey:key];

NSLog(@"end---%@",NSStringFromSelector(_cmd));

}

@end

KVO[24586:884853] willChangeValueForKey:

KVO[24586:884853] begin---didChangeValueForKey:

KVO[24586:884853] {

kind =1;

new =20;

old =0;

}

KVO[24586:884853] end---didChangeValueForKey: