UIViewController setView nil automatically removes from superview - ios

Question

Calling setView on a UIViewController removes the current view from its superview automatically. I couldn't find this documented. In my case I want to dynamically swap a UIViewController object for another while maintaining my view structure. I was planning to just relink the view to the new controller but alas, that doesn't work.
In general, automatically removing the view from its superview seems like a sensible decision. The documentation should reflect this though.
(For anyone thinking it's a really bad idea to swap a view controller object in this way, let me add that the controller I'm swapping in is a subclass of the existing controller. And this method is great for adding functionality to a view that is an extension of another.)

Answer 1

This is how I solved it:
UIView *viewToKeep = self.viewController.view;
UIView *superview = viewToKeep.superview;
self.viewController.view = nil; // removes the view from its superview
UIViewController *swapInViewController = [[UIViewController alloc] init];
swapInViewController.view = viewToKeep;
[superview addSubview: viewToKeep];
[viewToKeep applyConstraintsToFillSuperview]; // a helper to add auto layout constraints that make the view always fill it's parent
self.viewController = swapInViewController;

Answer 2

Doing this is definitely going against the grain in UIKit, and therefore very likely a bad idea.
That said, here's something which prevents the view from being automatically removed, which means you don't need to use addSubview to put it back where it was.
Only tested as far as you see below, caveat emptor, etc. In Swift:
class View: UIView {
var preventRemovalFromSuperView = false
override func removeFromSuperview() {
if !preventRemovalFromSuperView {
super.removeFromSuperview()
}
preventRemovalFromSuperView = false
}
}
let vc1 = UIViewController()
let vc2 = UIViewController()
let sv = UIView()
let v = View()
// Existing hierarchy
sv.addSubview(v)
vc1.view = v
// Swap view controllers
v.preventRemovalFromSuperView = true // Prevent automatic removal
vc1.view = nil // Prevent UIViewControllerHierarchyInconsistency exception
vc2.view = v
// Check that view was not automatically removed
v.isDescendantOfView(sv) // true