Custom view controller transition without alignment jump

Animator.m

// from animator.m

- (void)animatePush:(id<UIViewControllerContextTransitioning>)transitionContext {
  // seems that all the steps here have specific orders
  // add toVC to container view
  // auto layout step on toVC after toVC added to container
  // get image frames after AL step
  // have to set toVC frame offscreen after grab final image frames
  // backdrop view added to container, needs to be under toVC.view
  
  // can you add toVC to container, then set the frame later?
  // if so, don't have to split toVC section of set frame/add to container, then reset frame at end
  
  PhotoCollectionViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
  PhotoDetailViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
  UIView *container = [transitionContext containerView];
  
  toVC.view.alpha = 0.0;
  CGRect finalFrame = [transitionContext finalFrameForViewController:toVC];
  // seems to be ok if you don't set toVC's frame - because its already set?
  //toVC.view.frame = finalFrame;
  
  // adding toVC after AL pass seemed to cause placeholder image to end up at top of screen,
  // underneath the nav bar, causing a visual jump
  [container addSubview:toVC.view];
  
  // need an AL pass, or won't be setup correctly on phone sizes not designed in IB
  // I would expect that you get the view after its AL pass
  [toVC.view setNeedsLayout];
  [toVC.view layoutIfNeeded];
  
  UIImageView *fromImageView = [self getCellImageView:fromVC];
  UIImageView *toImageView = toVC.photoImageView;
  
  CGRect fromImageViewFrame = fromImageView.frame;
  CGRect fromImageViewFrameConverted = [container convertRect:fromImageViewFrame fromView:fromImageView];
  
  CGRect toImageFrame = toImageView.frame;
  // converting to toImageView frame to container coordinates ends up with an image that is
  // too low, by the height of the nav bar. It seems like its double counting the nav bar
  // CGRect toImageFrameConverted = [container convertRect:toImageFrame fromView:toImageView];
  
  UIView *placeholder = [fromImageView snapshotViewAfterScreenUpdates:NO];
  placeholder.frame = fromImageViewFrameConverted;
  fromImageView.hidden = YES;
  toImageView.hidden = YES;
  
  [container addSubview:placeholder];

  // now move inital frame position fro toVC
  // doing before messes up toImageFrame calc
  CGRect frame = finalFrame;
  frame.origin.y += frame.size.height;
  toVC.view.frame = frame;
  
  // backdrop view swap to get rid of "curtain" effect
  UIView *backdrop = [[UIView alloc] initWithFrame:finalFrame];
  backdrop.backgroundColor = toVC.view.backgroundColor;
  backdrop.alpha = 0;
  toVC.view.backgroundColor = [UIColor clearColor];
  [container insertSubview:backdrop belowSubview:toVC.view];
  
  [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
    toVC.view.alpha = 1.0;
    backdrop.alpha = 1.0;
    toVC.view.frame = finalFrame;
    // use the unconverted image frame for the placeholder final position
    placeholder.frame = toImageFrame;
  } completion:^(BOOL finished) {
    fromImageView.hidden = NO;
    toImageView.hidden = NO;
    [placeholder removeFromSuperview];
    toVC.view.backgroundColor = backdrop.backgroundColor;
    [backdrop removeFromSuperview];
    [transitionContext completeTransition:finished];
  }];
}