scheduledTimerWithTimeInterval

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)seconds target:(id)target selector:(SEL)aSelector userInfo:(id)userInfo repeats:(BOOL)repeats

Description:

Creates and returns a new NSTimer object and schedules it on the current run loop in the default mode.

After seconds seconds have elapsed, the timer fires, sending the message aSelector to target.

定时器，每隔一段时间，就会调用目标函数