RxSwift; ViewController never deinitialized

Question

I have a ViewController (called MainViewController ) which is backed by a ViewModel (called MainViewModel ).

The ViewModel has a variable that defines which child ViewController MainViewController should present as its child.

My problem is that, when a child is removed in favor of another child, it never gets deinit 'ed.

Here's the code:

MainViewController:

class MainViewController: UIViewController {

    var viewModel: MainViewModel!
    private let disposeBag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()

        viewModel.viewController
            .subscribe(onNext: { [weak self] vc in
                self?.newVc(vc)
            })
            .disposed(by: disposeBag)
    }

    static func instantiate(viewModel: MainViewModel) -> MainViewController {
        let vc = MainViewController()
        vc.viewModel = viewModel
        return vc
    }

    private func newVc(_ vc: UIViewController) {
        remove(childViewController: children.first)
        addFullScreen(childViewController: vc)
    }
}

MainViewModel:

class MainViewModel {

    lazy var viewController: Observable<UIViewController> = {
        return Observable.just(ColorViewController(.green))
            .delay(RxTimeInterval.seconds(3), scheduler: MainScheduler.instance)
            .startWith(ColorViewController(.yellow))
    }()

}

You see in MainViewModel s viewController variable, that it first emits a yellow ColorViewController, and 3 seconds later a green one. ColorViewController is a basic subclass of UIViewController, with a colored view, and a the deinit -method overwritten. This methods isn't called, when the yellow ColorViewController is removed..

Who holds the reference to that yellow ColorViewController, and how to fix it?

Bonus code:

extension UIViewController {

    public func addFullScreen(childViewController child: UIViewController) {
        guard child.parent == nil else { return }

        addChild(child)
        view.addSubview(child.view)

        child.view.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            view.leadingAnchor.constraint(equalTo: child.view.leadingAnchor),
            view.trailingAnchor.constraint(equalTo: child.view.trailingAnchor),
            view.topAnchor.constraint(equalTo: child.view.topAnchor),
            view.bottomAnchor.constraint(equalTo: child.view.bottomAnchor)
        ])

        child.didMove(toParent: self)
    }

    public func remove(childViewController child: UIViewController?) {
        guard let child = child else { return }
        guard child.parent != nil else { return }

        child.willMove(toParent: nil)
        child.view.removeFromSuperview()
        child.removeFromParent()
    }
}

UPDATE:

So I changed the viewController -variable to this:

lazy var viewController: Observable<UIViewController> = {
        return Observable<Int>.interval(RxTimeInterval.seconds(3), scheduler: MainScheduler.instance)
            .scan(0, accumulator: { (prev, next) -> Int in return prev + 1 })
            .map { index -> UIViewController in
                let modul = index % 3
                print("Index: \(index): \(modul)")
                switch modul {
                case 0: return ColorViewController(.yellow, tag: "Yellow")
                case 1: return ColorViewController(.blue, tag: "Blue")
                case 2: return ColorViewController(.green, tag: "Green")
                default: return ColorViewController(.red, tag: "Red")
                }
            }.startWith(ColorViewController(.cyan, tag: "Initial 1"),
                        ColorViewController(.purple, tag: "Initial 2"))
            .take(10)
    }()

Now I see that all the ColorViewController 's generated in the .map are deinitialized as expected. But the two passed into .startWith , are never deinitialized, not even after .take(10) causes the Observable to complete. Does that make sense to anyone?

Answer 1

Open up the visual memory debugger to find out who is holding on to the view controller that you want to have released. Here is an article about it: https://useyourloaf.com/blog/xcode-visual-memory-debugger/

And a video from Apple: https://developer.apple.com/videos/play/wwdc2018/416