KVO的全稱是Key-Value Observing，俗稱“鍵值監(jiān)聽”，可以用于監(jiān)聽某個(gè)對(duì)象屬性值的改變

1.KVO使用方法

1. 添加監(jiān)聽:

addObserver:forKeyPath:options:context:

self.person = [[Person alloc] init];
self.person.age = 1;

NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
[self.person addObserver:self forKeyPath:@"age" options:options context:@"123"];

2. 監(jiān)聽回調(diào)：

當(dāng)監(jiān)聽對(duì)象的屬性值發(fā)生改變時(shí)肺稀，就會(huì)調(diào)用observeValueForKeyPath:ofObject:change: context:

// 當(dāng)監(jiān)聽對(duì)象的屬性值發(fā)生改變時(shí)赘艳，就會(huì)調(diào)用
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"監(jiān)聽到%@的%@屬性值改變了 - %@ - %@", object, keyPath, change, context);
}

2.探索KVO的本質(zhì)

@interface ViewController ()
@property (strong, nonatomic) Person *person1;
@property (strong, nonatomic) Person *person2;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[Person alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[Person alloc] init];
    self.person2.age = 2;
    
    // 給person1對(duì)象添加KVO監(jiān)聽
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
//    self.person1.age = 21;
//    self.person2.age = 22;
    
    // NSKVONotifying_Person是使用Runtime動(dòng)態(tài)創(chuàng)建的一個(gè)類，是Person的子類
    
    // self.person1.isa == NSKVONotifying_Person
    [self.person1 setAge:21];
    
    // self.person2.isa = Person
    [self.person2 setAge:22];
}

- (void)dealloc {
    [self.person1 removeObserver:self forKeyPath:@"age"];
}
// 當(dāng)監(jiān)聽對(duì)象的屬性值發(fā)生改變時(shí)辆脸，就會(huì)調(diào)用
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"監(jiān)聽到%@的%@屬性值改變了 - %@ - %@", object, keyPath, change, context);
}

@end

在上面的代碼中但校，僅僅對(duì)person1添加了KVO監(jiān)聽，通過(guò)斷點(diǎn)分別獲取到person1的isa->NSKVONotifying_Person,
person2的isa->Person

image

未使用KVO監(jiān)聽

當(dāng)實(shí)例對(duì)象添加了KVO監(jiān)聽状囱，person1的isa指向的類對(duì)象就變成了另外一個(gè)NSKVONotifying_Person，而NSKVONotifying_Person類對(duì)象的superclass指針才指向了Person倘是。
NSKVONotifying_Person是使用Runtime動(dòng)態(tài)創(chuàng)建的一個(gè)類亭枷，是Person的子類，結(jié)構(gòu)如下：

使用KVO監(jiān)聽

當(dāng)調(diào)用setAge:方法的時(shí)候搀崭，通過(guò)查看IMP地址叨粘，實(shí)際上調(diào)用的是Foundation框架里的_NSSetIntValueAndNotify函數(shù)，

調(diào)用過(guò)程如下（偽代碼）：

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

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

- (void)didChangeValueForKey:(NSString *)key
{
    // 通知監(jiān)聽器门坷，某某屬性值發(fā)生了改變
    [oberser observeValueForKeyPath:key ofObject:self change:nil context:nil];
}

_NSSetIntValueAndNotify函數(shù)里面先是調(diào)用了[self willChangeValueForKey:@"age"] -> [super setAge:age]->[self didChangeValueForKey:@"age"];,然后在didChangeValueForKey:里面通過(guò)監(jiān)聽對(duì)象[oberser observeValueForKeyPath:key ofObject:self change:nil context:nil]

3.查看_NSSet*AndNotify的存在

根據(jù)對(duì)監(jiān)聽對(duì)象類型的不同宣鄙，生成的方法也不同

4._NSSet*ValueAndNotify的內(nèi)部實(shí)現(xiàn)

NSSet*ValueAndNotify的內(nèi)部實(shí)現(xiàn)

• 調(diào)用willChangeValueForKey:
• 調(diào)用原來(lái)的setter實(shí)現(xiàn)
• 調(diào)用didChangeValueForKey:
• didChangeValueForKey:內(nèi)部會(huì)調(diào)用observer的observeValueForKeyPath:ofObject:change:context:方法