nstimer,需要注册到 runloop里面去的，

创建Timer的三种方法

１.scheduling a timer with the current run loop　

２.creating a timer that you later register with a run loop

３.initializing a timer with a given fire date

 

Scheduled Timers

以下两个方法自动注册新创建的timer到当前NSRunLoop对象，NSRunLoop的模式为默认的NSDefaultRunLoopMode

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