Based on http://holko.pl/2017/06/26/checking-uiviewcontroller-deallocation/

DeallocationChecker.swift

extension UIViewController {
    public func dch_checkDeallocation(afterDelay delay: TimeInterval = 2.0) {
        let rootParentViewController = dch_rootParentViewController

        // We don’t check `isBeingDismissed` simply on this view controller because it’s common
        // to wrap a view controller in another view controller (e.g. in UINavigationController)
        // and present the wrapping view controller instead.
        if isMovingFromParentViewController || rootParentViewController.isBeingDismissed {
            let type = type(of: self)
            let disappearanceSource: String = isMovingFromParentViewController ? "removed from its parent" : "dismissed"

            DispatchQueue.main.asyncAfter(deadline: .now() + delay, execute: { [weak self] in
                assert(self == nil, "\(type) not deallocated after being \(disappearanceSource)")
            })
        }
    }

    private var dch_rootParentViewController: UIViewController {
        var root = self

        while let parent = root.parent {
            root = parent
        }

        return root
    }
}

private let swizzling: (UIViewController.Type) -> () = { viewController in
    let originalSelector = #selector(viewController.viewDidDisappear(_:))
    let swizzledSelector = #selector(viewController.proj_viewDidDisappear(animated:))

    let originalMethod = class_getInstanceMethod(viewController, originalSelector)
    let swizzledMethod = class_getInstanceMethod(viewController, swizzledSelector)

    method_exchangeImplementations(originalMethod, swizzledMethod)
}

extension UIViewController {
    open override class func initialize() {
        // make sure this isn't a subclass
        guard self === UIViewController.self else { return }
        swizzling(self)
    }

    // MARK: - Method Swizzling

    func proj_viewDidDisappear(animated: Bool) {
        self.proj_viewDidDisappear(animated: animated)

        dch_checkDeallocation()
    }
}