Key-value observing provides a mechanism that allows objects to be notified of changes to specific properties of other objects.A controller object typically observes properties of model objects, and a view object observes properties of model objects through a controller.
class 必須首先遵從 NSKeyValueCoding 、然后是 NSKeyValueObserving
Registering for Key-Value Observing
Receiving Notification of a Change
Removing an Object as an Observer
You must perform the following steps to enable an object to receive key-value observing notifications for a KVO-compliant property:
//Register the observer with the observed object
//passing itself as the observer and the key path of the property to be observed.
//The observer additionally specifies an options parameter and a context pointer to manage aspects of the notifications.
//A safer and more extensible approach is to use the context to ensure notifications you receive are destined for your observer and not a superclass.
- addObserver:forKeyPath:options:context:
//accept change notification messages
- observeValueForKeyPath:ofObject:change:context:
//Unregister the observer,At a minimum, invoke this method before the observer is released from memory.
- removeObserver:forKeyPath:
注意：The key-value observing addObserver:forKeyPath:options:context:method does not maintain strong references to the observing object, the observed objects, or the context.
注冊監(jiān)聽器：
- addObserver:forKeyPath:options:context: 注冊一個監(jiān)聽器用于監(jiān)聽指定Key路徑
- removeObserver:forKeyPath: 為指定Key路徑刪除指定的監(jiān)聽器
- removeObserver:forKeyPath:context: 為指定Key路徑刪除指定的監(jiān)聽器，只是多了一個context參數(shù)。
監(jiān)聽器開始觀察：
- observeValueForKeyPath:ofObject:change:context:
- (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
返回yes（默認返回YES）,不用調用willChangeValueForKey
返回no斗埂，在改變被觀察的屬性值時盛杰，需要自己調用willChangeValueForKey 或 didChangeValueForKey 方法
如果不使用存取器裁厅，需要在每次修改屬性的實例變量時調用 willChangeValueForKey: 和 調用 didChangeValueForKey:
Key-Value Observing Implementation Details
Automatic key-value observing is implemented using a technique called isa-swizzling.
The isa pointer, as the name suggests, points to the object's class which maintains a dispatch table. This dispatch table essentially contains pointers to the methods the class implements, among other data.
When an observer is registered for an attribute of an object the isa pointer of the observed object is modified, pointing to an intermediate class rather than at the true class. As a result the value of the isa pointer does not necessarily reflect the actual class of the instance.
You should never rely on the isa pointer to determine class membership. Instead, you should use the class method to determine the class of an object instance.
