KVO的全稱是Key-Value Observing，俗稱“鍵值監聽”，可以用于監聽某個對象屬性值的改變

1.KVO使用方法

1. 添加監聽:

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. 監聽回調：

當監聽對象的屬性值發生改變時，就會調用observeValueForKeyPath:ofObject:change: context:

// 當監聽對象的屬性值發生改變時，就會調用
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"監聽到%@的%@屬性值改變了 - %@ - %@", object, keyPath, change, context);
}

2.探索KVO的本質

@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對象添加KVO監聽
    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動態創建的一個類，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"];
}
// 當監聽對象的屬性值發生改變時，就會調用
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"監聽到%@的%@屬性值改變了 - %@ - %@", object, keyPath, change, context);
}

@end

在上面的代碼中，僅僅對person1添加了KVO監聽，通過斷點分別獲取到person1的isa->NSKVONotifying_Person,
person2的isa->Person

image

未使用KVO監聽

當實例對象添加了KVO監聽，person1的isa指向的類對象就變成了另外一個NSKVONotifying_Person，而NSKVONotifying_Person類對象的superclass指針才指向了Person。
NSKVONotifying_Person是使用Runtime動態創建的一個類，是Person的子類，結構如下：

使用KVO監聽

當調用setAge:方法的時候，通過查看IMP地址，實際上調用的是Foundation框架里的_NSSetIntValueAndNotify函數，

調用過程如下（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
{
    // 通知監聽器，某某屬性值發生了改變
    [oberser observeValueForKeyPath:key ofObject:self change:nil context:nil];
}

_NSSetIntValueAndNotify函數里面先是調用了[self willChangeValueForKey:@"age"] -> [super setAge:age]->[self didChangeValueForKey:@"age"];,然后在didChangeValueForKey:里面通過監聽對象[oberser observeValueForKeyPath:key ofObject:self change:nil context:nil]

3.查看_NSSet*AndNotify的存在

根據對監聽對象類型的不同，生成的方法也不同

4._NSSet*ValueAndNotify的內部實現

NSSet*ValueAndNotify的內部實現

• 調用willChangeValueForKey:
• 調用原來的setter實現
• 調用didChangeValueForKey:
• didChangeValueForKey:內部會調用observer的observeValueForKeyPath:ofObject:change:context:方法