KVO－－－Key Value Obersving

KVO键值观察机制，就是MVC中Model变化的时候通知View更新，使用该机制避免了胶水代码，即在任何数据更新的地方都得写更新UI的更新代码。具体系统系统：NSObject，NSArray，NSSet三种KVO机制，API如下@interface NSObject(NSKeyValueObserverRegistration)

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

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

@end

@interface NSArray(NSKeyValueObserverRegistration)

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

- (void)removeObserver:(NSObject *)observer fromObjectsAtIndexes:(NSIndexSet *)indexes forKeyPath:(NSString *)keyPath;

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

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

@end

@interface NSSet(NSKeyValueObserverRegistration)

- (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context NS_AVAILABLE(10_4, 2_0);

- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath NS_AVAILABLE(10_4, 2_0);

@end

只解释@interfaceNSObject(NSKeyValueObserverRegistration)

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

例如：

Class Model：NSObject

｛

NSString ＊text；

｝

Class Controller：NSObject

｛

Model ＊model；

｝

此时如果在Controller中想使用KVO机制得如下操作

［model addObserver：self forKeyPath：@“text” options：NSKeyValueObservingOptionNew context：nil］；

这条语句意思是：当model的text属性变化时通知self，其中NSKeyValueObservingOptionNew 可以变换，context参数是void＊类型，因此可以传递自己想传递的任何类型。

除此在Controller中还得实现

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

text属性变化时系统会回调这个函数，因此在这个函数中更新UI。

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

删除观察者记得在释放本对象之前一定要remove observer，比如在dealloc中remove observer。

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

向数组中某个对象集合添加观察者