childViewControllerをスライドアニメーションで切り替える（ContainerViewController）

- (void)switchToViewController:(UIViewController *)viewController transition:(SwithingTransition)transition
{
  UIViewController *toViewController = viewController;

  if (!toViewController) {
    return;
  }

  UIViewController *fromViewController = self.currentEmbededViewController;

  if (!fromViewController) {
    // 遷移前ViewControllerが存在しない場合、そのままViewに追加
    [self addChildViewController:toViewController];
    [self.view addSubview:toViewController.view];
    [toViewController didMoveToParentViewController:self];
  } else {
    CGRect toViewStartFrame = self.view.frame;
    CGRect fromViewEndFrame = self.view.frame;

    // 開始、終了フレームの設定
    switch (transition) {
      case SwitchingTransitionToRight:
        toViewStartFrame.origin.x -= self.view.frame.size.width;
        fromViewEndFrame.origin.x += self.view.frame.size.width;
        break;
      case SwitchingTransitionToLeft:
        toViewStartFrame.origin.x += self.view.frame.size.width;
        fromViewEndFrame.origin.x -= self.view.frame.size.width;
        break;
      default:
        break;
    }

    toViewController.view.alpha = 0.0;
    toViewController.view.frame = toViewStartFrame;

    [self addChildViewController:toViewController];
    // 削除開始
    [fromViewController willMoveToParentViewController:nil];

    [self transitionFromViewController:fromViewController
                      toViewController:toViewController
                              duration:1.0
                               options:UIViewAnimationOptionTransitionNone
                            animations:^{
                              fromViewController.view.frame = fromViewEndFrame;
                              toViewController.view.frame   = self.view.frame;

                              fromViewController.view.alpha = 0.0;
                              toViewController.view.alpha   = 1.0;
                            } completion:^(BOOL finished) {
                              // 追加完了
                              [toViewController didMoveToParentViewController:self];
                            }];
  }

  self.currentEmbededViewController = toViewController;
}