How to use view controller containment

Paul Hudson    February 12th 2018    @twostraws

View controller containment allows you to embed one view controller inside another, which can simplify and organize your code. It takes four steps:

1. Call addChildViewController() on your parent view controller, passing in your child.
2. Set the child’s frame to whatever you need, if you’re using frames.
3. Add the child’s view to your main view, along with any Auto Layout constraints.
4. Call didMove(toParentViewController:) on the child, passing in your main view controller.

In Swift code it looks like this:

addChildViewController(child)
child.view.frame = frame
view.addSubview(child.view)
child.didMove(toParentViewController: self)

When you’re finished with it, the steps are conceptually similar but in reverse:

1. Call willMove(toParentViewController:), passing in nil.
2. Remove the child view from its parent.
3. Call removeFromParentViewController() on the child.

In code, it’s just three lines:

willMove(toParentViewController: nil)
view.removeFromSuperview()
removeFromParentViewController()

Just for convenience you might want to consider adding a small, private extension to UIViewController to do these tasks for you – they do need to be run in a precise order, which is easily done incorrectly.

Something like this ought to do it:

@nonobjc extension UIViewController {
    func add(_ child: UIViewController, frame: CGRect? = nil) {
        addChildViewController(child)

        if let frame = frame {
            child.view.frame = frame
        }

        view.addSubview(child.view)
        child.didMove(toParentViewController: self)
    }

    func remove() {
        willMove(toParentViewController: nil)
        view.removeFromSuperview()
        removeFromParentViewController()
    }
}

That’s marked @nonobjc so it won’t conflict with any of Apple’s own code, now or in the future.

Available from iOS 5.0 – learn more in my book Swift Design Patterns

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