NSTimer 导致UIViewController无法正常释放

 self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(updateMonitorTime) userInfo:nil repeats:YES];
 [[NSRunLoop currentRunLoop]addTimer:self.timer forMode:NSRunLoopCommonModes];

- (void)updateMonitorTime
{
    NSLog("请求数据");
}

- (void)viewWillDisappear:(BOOL)animated
{
    if (self.timer != nil) {
         [self.timer invalidate];
         self.timer = nil;
    }
}