cancelPreviousPerformRequestsWit

• (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray<NSRunLoopMode> *)modes;
• (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay

performSelector N秒钟之后 执行方法

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

cancelPreviousPerformRequestsWithTarget：取消performSelector方法注册过的方法，就是说当上面这个方法正在运行，比如希望5秒钟之后执行某一个方法，但是没到5秒钟的情况下取消performSelector，就是用cancelPreviousPerformRequestsWithTarget方法来实现的