- 全稱：Key Value Coding（鍵值編碼）
- 賦值
// 能修改私有成員變量
- (void)setValue:(id)value forKey:(NSString *)key;
- (void)setValue:(id)value forKeyPath:(NSString *)keyPath;
- (void)setValuesForKeysWithDictionary:(NSDictionary *)keyedValues;
- 取值
// 能取得私有成員變量的值
- (id)valueForKey:(NSString *)key;
- (id)valueForKeyPath:(NSString *)keyPath;
- (NSDictionary *)dictionaryWithValuesForKeys:(NSArray *)keys;
