Animation in transitionFromViewController:toViewController: not showing - ios

Question

I am trying to build a custom container view controller and am trying to create my own animation. I'm starting with something simple of moving the tapped cell to the center of the screen before making it zoom in by scaling and showing the new view. I'm starting with this:
- (void)showDetailViewController:(UIView *)viewToAnimate {
UIImageView *anImageView = [[UIImageView alloc] initWithImage:[self screenShot:_homeViewController.view]];
CGRect windowBounds = [[UIScreen mainScreen] bounds];
_detailViewController.view.frame = windowBounds;
[self addChildViewController:_detailViewController];
// get a screenshot of the old view before pushing the new view controller
_detailViewController.view.alpha = 0.0;
NSLog(#"center: %#", NSStringFromCGPoint(viewToAnimate.center));
[self transitionFromViewController:_homeViewController toViewController:_detailViewController duration:0.5 options:0 animations:^{
CGPoint windowCenter = CGPointMake(CGRectGetMidX(windowBounds), CGRectGetMidY(windowBounds));
viewToAnimate.center = windowCenter;
viewToAnimate.layer.transform = CATransform3DMakeScale(2, 2, 2);
NSLog(#"center 2 : %#", NSStringFromCGPoint(viewToAnimate.center));
_detailViewController.view.alpha = 1.0;
}completion:^(BOOL finished) {
[anImageView removeFromSuperview];
}];
[_detailViewController didMoveToParentViewController:self];
}
What I don't understand is, if I comment out the _detailViewController.view.alpha = 0 and _detailViewController.view.alpha = 1, I don't get any animation. It just jumps to the new childViewController. But with those two lines, I do see an animation. Why is that? Thanks.
Edit:
This is how I set up a container controller:
In viewDidLoad of ContainerViewController:
UIStoryboard *storyboard = [UIStoryboard storyboardWithName:#"MainStoryboard_iPad" bundle:nil];
_homeViewController = [storyboard instantiateViewControllerWithIdentifier:#"HomeViewController"];
_homeViewController.homeViewControllerDelegate = self;
_detailViewController = [storyboard instantiateViewControllerWithIdentifier:#"DetailViewController"];
[self setSubViewControllers:#[ _homeViewController, _detailViewController]];
_selectedViewController.view.frame = [[UIScreen mainScreen] bounds];
[self addChildViewController:self.selectedViewController];
[self.view addSubview:_selectedViewController.view];
[self didMoveToParentViewController:self];

Answer 1

In the code you posted, you show yourself adding _detailViewController as a child view controller, but don't show the step of adding _homeViewController as a child view controller of self. At the point when the code you posted is executed, is _homeViewController a child of self (the view controller that is running the transition)? It needs to be.