2. OC語法-KVO和KVC

一、KVO

1.1 KVO

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

1.2 未使用KVO監聽的對象

1.3 使用了KVO監聽的對象

1.4 _NSSet*ValueAndNotify的內部實現

[selfwillChangeValueForKey:@"age"];

// 原來的setter實現

[self didChangeValueForKey:@"age"];

• 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

1.5 KVO基本使用

// 1. 給person1對象的age和height屬性添加KVO監聽
NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
[self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];

// 2. 移除KVO監聽
- (void)dealloc {
    [self.person1 removeObserver:self forKeyPath:@"age"];
    [self.person1 removeObserver:self forKeyPath:@"height"];
}

// 3. 監聽屬性的改變
- (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);
}

二、KVC

2.1 KVC

• KVC的全稱是Key-Value Coding，俗稱“鍵值編碼”，可以通過一個key來訪問某個屬性
• 常見的API有

- (void)setValue:(id)value forKeyPath:(NSString *)keyPath; 
- (void)setValue:(id)value forKey:(NSString *)key;
- (id)valueForKeyPath:(NSString *)keyPath;
- (id)valueForKey:(NSString *)key;

2.2 setValue:forKey:的原理

• accessInstanceVariablesDirectly方法的默認返回值是YES
  

2.3 valueForKey:的原理