kvo,key value observing,一种数据变化监听机制 ，kvc,一种协议，实现后，访问对象时候 可以直接根据名字来了，
This document describes the NSKeyValueCoding informal protocol, which defines a mechanism allowing applications to access the properties of an object indirectly by name (or key), rather than directly through invocation of an accessor method or as instance variables.
You should read this document to gain an understanding of how to use key-value coding in your applications and how to make your classes key-value coding compliant for interacting with other technologies. Key-value coding is a fundamental technology when working with key-value observing, Cocoa bindings, Core Data, and making your application AppleScript-able. You are expected to be familiar with the basics of Cocoa development and the Objective-C language.
// KVO change notification
- (void)willChangeValueForKey:(NSString *)key;
- (void)didChangeValueForKey:(NSString *)key;
- (void)willChangeValueForKey:(NSString *)inKey withSetMutation:(NSKeyValueSetMutationKind)inMutationKind usingObjects:(NSSet *)inObjects;
- (void)didChangeValueForKey:(NSString *)inKey withSetMutation:(NSKeyValueSetMutationKind)inMutationKind usingObjects:(NSSet *)inObjects;