How to go back to the previous controller in one click after observing several controllers of the same type on iOS?

Question

We are using MvvmCross 4.4.0 on the our iOS project and I faced the following problem:

I need to implement "Item" page with reference to the other "Item" page;
I need an instant back navigation from any "Item" page to the previous controller ("Catalogue" controller).

Diagram:

Catalogue --ConcreteItem--> Item1 --MoreItems--> Item2 --MoreItems-->

Item3 --BackNavButton--> Catalogue.

I am doing the following thing in the Custom ViewPresenter:

                var topViewController = ParentRootViewController.TopViewController;
                ParentRootViewController.PushViewController(currentViewController, needAnimation);

                if (topViewController.GetType() == currentView.GetType()
                    && /*Logic to determine if its correct view types*/)
                {
                    topViewController.RemoveFromParentViewController();
                    topViewController.Dispose();
                }

And actually it works until I didn't return to the "Catalogue" page. The problem is that I need to click back button so many times I had clicked "More" button on "Item" page. Also if we use custom back button with such code in both "Catalogue" and "Item" pages:

  if (NavigationController?.NavigationBar?.BackItem != null)
        {
             var backbutton = new UIBarButtonItem(" ",
                                                 UIBarButtonItemStyle.Plain,
                                                 (sender, e) => { NavigationController?.PopViewController(true); })
            {
                Image = UIImage.FromBundle("BackButtonImage")
            };

            NavigationItem.LeftBarButtonItem = backbutton;
        }

then app crashes when clicking "Back" NavButton on "Catalogue" page with in lambda

(sender, e) => { NavigationController?.PopViewController(true);

The disposed object ItemPageViewController.

The question is : How to correctly implement "SingleTop" page in MvvmCross?

Or

How to fix this problem?

P.S.: If from MvxPresenter remove line

topViewController.Dispose();

then in custom lambda would throw NullReferenceException.

P.P.S.: I believe it the problem that I don't remove controller from navigation stack. I have tried to remove controllers in Custom View Presenter, but, firstly, it is null there sometimes, and even with null check nothing helped.

is Catalogue the starting point of your app ? – Mark Verkiel Jun 25 '18 at 09:02 — Mark Verkiel, Jun 25 '18 at 09:02
@Pilatus, No, it isn't. – Oleg Kosuakiv Jun 25 '18 at 09:23 — Oleg Kosuakiv, Jun 25 '18 at 09:23

score 2 · Accepted Answer · answered Jun 26 '18 at 08:26

UINavigationController has a function PopToViewController(UIViewController viewController, bool animated);

Instead of removing every ViewController when the views are of the same type, you could pop to the ViewController Catalogue when the backbutton is pressed.

UINavigationController has a property ViewControllers that we can use to find CatalogueViewController.

Since you're using MvvmCross we'll check for the ViewModel type.

var catalogueController = NavigationController.ViewControllers.First(c => 
    ((IMvxIosView)c).ViewModel.GetType() == typeof(CatalogueViewModel));

Now you can use the function PopToViewController to close all the views untill CatalogueController

CurrentNavigationController.PopToViewController(catalogueController, true);

Thanks, I did this and it worked. The only drawback is that I had do to use custom back button and specify the concrete type of parent controller which are dependencies. Bu I really wanted to have a generic solution. — Oleg Kosuakiv, Jun 26 '18 at 12:07
You could create an interface, implement it on the `ViewModel` and search for `ViewModel` that implements that interface and pop to that `ViewController`. Then this solution would be more generic. — Mark Verkiel, Jun 26 '18 at 12:15

How to go back to the previous controller in one click after observing several controllers of the same type on iOS?

1 Answers1