gistfile1.m

- (void)transitionToNewRootViewController:(UIViewController *)newRootVC animated:(BOOL)animated moveToLeft:(BOOL)moveToLeft {
  UIViewController *rootVC = self.window.rootViewController;
  if (rootVC == newRootVC) {
    return;
  }

  CGFloat direction = (moveToLeft) ? 1 : -1;

  CGRect initialFromFrame = rootVC.view.frame;
  CGRect initialToFrame = initialFromFrame;
  initialToFrame.origin.x += direction * initialFromFrame.size.width;

  CGRect finalFromFrame = initialFromFrame;
  finalFromFrame.origin.x -= direction * initialFromFrame.size.width;
  CGRect finalToFrame = initialFromFrame;

  newRootVC.view.frame = initialToFrame;
  [self.window addSubview:newRootVC.view];
 
  [UIView animateWithDuration:(animated) ? 0.3 : 0.0 animations:^{
    rootVC.view.frame = finalFromFrame;
    newRootVC.view.frame = finalToFrame;
  } completion:^(BOOL finished) {
    [rootVC.view removeFromSuperview];
    self.window.rootViewController = toVC;
  }];
}