Spin animation of indefinite duration (not INFINITE). It can be stopped at an even quarter turn if needed. Also, the acceleration ramps up during the first 90 degrees, and decelerates during the last 90 degrees (after a stop has been requested).

gistfile1.m

// an ivar for your class:
BOOL animating;

- (void)spinWithOptions:(UIViewAnimationOptions)options
{
   // this spin completes 360 degrees every 2 seconds
   [UIView animateWithDuration: 0.5f
                         delay: 0.0f
                       options: options
                    animations: ^{
                    
                       self.imageToMove.transform = CGAffineTransformRotate(imageToMove.transform, M_PI / 2);
                    }
                    completion: ^(BOOL finished)
                    {
                       if (finished)
                       {
                          if (animating)
                          {
                             // if flag still set, keep spinning with constant speed
                             [self spinWithOptions: UIViewAnimationOptionCurveLinear];
                          }
                          else
                          if (options != UIViewAnimationOptionCurveEaseOut)
                          {
                             // one last spin, with deceleration
                             [self spinWithOptions: UIViewAnimationOptionCurveEaseOut];
                          }
                       }
                    }];
}

- (void) startSpin
{
   if (!animating)
   {
      animating = YES;
      [self spinWithOptions: UIViewAnimationOptionCurveEaseIn];
   }   
}

- (void) stopSpin
{
    // set the flag to stop spinning after one last 90 degree increment
    animating = NO;
}