能够通过NSObject的一些方法获取运行时信息或动态运行一些消息：
/*Returns a Boolean value that indicates whether the receiving class is a subclass of, or identical to, a given class.*/
+ (BOOL)isSubclassOfClass:(Class)aClass;
/*Returns a Boolean value that indicates whether instances of the receiver are capable of responding to a given selector.*/
+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
?比如：if?([[Retangle class]?respondsToSelector:?@selector(print)]==YES)
{……}?
/*Returns a Boolean value that indicates whether the receiver is an instance of a given class.*/
- (BOOL)isMemberOfClass:(Class)aClass;
?比如：[obj isMemberOfClass:[Retangle?class]];
除此之外还有：
conformsToProtocol 检查对象是否实现了指定协议类的方法
methodForSelector? 返回指定方法实现的地址
performSelector:withObject 运行SEL 所指代的方法
?
- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;
这三个方法，均为同步运行。与线程无关。主主线程和子一程中均可调用成功。等同于直接调用该方法。在须要动态的去调用方法的时候去使用。
比如：[self performSelector:@selector(test2)] 与[self test2];运行效果上全然同样。
- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray
*)modes;
- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay;
这两个方法为异步运行。即使delay传參为0，仍为异步运行。仅仅能在主线程中运行，在子线程中不会调到aSelector方法。可用于当点击UI中一个button会触发一个消耗系统性能的事件，在事件运行期间button会一直处于高亮状态，此时能够调用该方法去异步的处理该事件，就能避免上面的问题。
在方法未到运行时间之前。取消方法为：
+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget selector:(SEL)aSelector object:(id)anArgument;
+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget;
注意：调用该方法之前或在该方法所在的viewController生命周期结束的时候去调用取消函数，以确保不会引起内存泄露。
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait modes:(NSArray
*)array;
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait;
这两个方法，在主线程和子线程中均可运行，均会调用主线程的aSelector方法
假设wait传YES，则为异步运行，假设wait传NO，则为同步运行。
注意：apple不同意程序猿在主线程以外的线程中对ui进行操作，此时我们必须调用performSelectorOnMainThread函数在主线程中完毕UI的更新
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(id)arg waitUntilDone:(BOOL)wait modes:(NSArray
*)array;
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(id)arg waitUntilDone:(BOOL)wait;
调用指定线程中的某个方法。分析效果同3。
- (void)performSelectorInBackground:(SEL)aSelector withObject:(id)arg ?起子线程在后台运行