为什么iphonesdk NSTimer不调用我告诉它的函数呢?

if(!myTimer)
{
    NSLog(@"Setting up the timer!");
    myTimer=[NSTimer timerWithTimeInterval:1 
                                    target:self
                                  selector:@selector(timerTicked)
                                  userInfo:nil 
                                   repeats:YES];
}

感谢NSLog函数,我知道设置计时器的代码正在关闭,但它没有调用函数:

-(void)timerTicked:(NSTimer*)theTimer
{
//NSLOG that tells me that this function isn't being fired
}