iPhone中心活动指示器,无论屏幕方向如何

activity.center = CGPointMake(self.view.frame.size.width/2,self.view.frame.size.height/2);

在viewDidLoad中,注册设备轮换的通知：

- (void)viewDidLoad {
   [[UIDevice currentDevice] beginGeneratingDeviceOrientationNotifications];
   [[NsnotificationCenter defaultCenter] addObserver:self
        SELEctor:@SELEctor(didRotate:)
        name:UIDeviceOrientationDidChangeNotification object:nil];
}

并实现didRotate：

-(void)didRotate:(Nsnotification *)notification {   
   if (activity) {
    activity.center = CGPointMake(self.view.frame.size.width/2,self.view.frame.size.height/2);
    }
}