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func navigationController(_ navigationController: UINavigationController, animationControllerFor operation: UINavigationControllerOperation, from fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {}

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func navigationController(_ navigationController: UINavigationController, interactionControllerFor animationController: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning? {}

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final class TransitionAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {

    }

    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {

    }
}

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final class TransitionAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    // 1
    let presenting: Bool

    // 2
    init(presenting: Bool) {
        self.presenting = presenting
    }

    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {

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final class TransitionCoordinator: NSObject, UINavigationControllerDelegate {
    // 1
    var interactionController: UIPercentDrivenInteractiveTransition?

    // 2
    func navigationController(_ navigationController: UINavigationController, animationControllerFor operation: UINavigationControllerOperation, from fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        switch operation {
        case .push:
            return TransitionAnimator(presenting: true)
        case .pop:

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extension UINavigationController {
    // 1
    static private var coordinatorHelperKey = "UINavigationController.TransitionCoordinatorHelper"

    // 2
    var transitionCoordinatorHelper: TransitionCoordinator? {
        return objc_getAssociatedObject(self, &UINavigationController.coordinatorHelperKey) as? TransitionCoordinator
    }

    func addCustomTransitioning() {

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private func saveSomeArticles() {
    guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
        return
    }
}

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private func saveSomeArticles() {
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
            return
        }

        // 1
        let context = appDelegate.persistentContainer.viewContext

        // 2
        let mediumArticle = Article(context: context)

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private func loadArticles() -> [Article] {
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
            return []
        }

        let context = appDelegate.persistentContainer.viewContext

        // 1
        let request: NSFetchRequest<Article> = Article.fetchRequest()

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private func deleteAllArticles() {
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
            return
        }

        let context = appDelegate.persistentContainer.viewContext

        // 1
        let articles = loadArticles()