NSObject分类

- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray *)modes;

- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay;

+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget selector:(SEL)aSelector object:(id)anArgument;

+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget;//取消目标对象所有绑定的后台延时事件。