动态识别类型

-(BOOL) isKindOfClass: classObj	is object a descendent or member of classObj 此对象是否是 classObj 的子孙或一员
-(BOOL) isMemberOfClass: classObj	is object a member of classObj 此对象是否是 classObj 的一员
-(BOOL) respondsToSelector: selector	does the object have a method named specifiec by the selector 此对象是否有叫做selector 的 method
+(BOOL) instancesRespondToSelector: selector	does an object created by this class have the ability to respond to the specified selector 此对象是否是由有能力响应指定 selector 的对象所产生
-(id) performSelector: selector	invoke the specified selector on the object 唤起此对象的指定selector