Adding a new page into PageViewController

Question

I am a Swift noob and am making a simple weather app. I used the Page-Based Application template.

The problem I have is the following:

When the user adds a city I call addCity and successfully append the new city name to my cities array. When I print that array in that function, it shows the new city at the end.

However, the function viewControllerAtIndex that creates a new page seems to use the old version of that array, without the new city appended. When I print the cities array, it is missing the new city name. Therefore, when the users swipes there is won't be a new page for the new city rendered. The user has to restart the app in order for the new city to show up.

I created a screen capture video to illustrate the problem.

https://youtu.be/DbMqgJ0lONk

(the cities array should also show "London", I think I just didn't restart the app)

I would appreciate any help here!

import UIKit

class ModelController: NSObject, UIPageViewControllerDataSource {

    var rootViewController = RootViewController()
    var cities = [""]
    let defaults = UserDefaults.standard

    override init() {
        super.init()

        self.cities = self.defaults.stringArray(forKey: "SavedStringArray") ?? [String]()

        if self.cities == [""] || self.cities.count == 0  {
            self.cities = ["Current Location"]
        }
    }

    func addCity(name:String) {
        self.cities.append(name)
        self.defaults.set(self.cities, forKey: "SavedStringArray")
        print ("cities from addCity:")
        print (self.cities)

    }


    func viewControllerAtIndex(_ index: Int, storyboard: UIStoryboard) -> DataViewController? {
        // Return the data view controller for the given index.
        if (self.cities.count == 0) || (index >= self.cities.count) {
            return nil
        }

        // Create a new view controller and pass suitable data.
        let dataViewController = storyboard.instantiateViewController(withIdentifier: "DataViewController") as! DataViewController

        //get city name
        dataViewController.dataObject = self.cities[index]
        print ("cities in viewControllerAtIndex:")
        print (self.cities)

        return dataViewController
    }



    func indexOfViewController(_ viewController: DataViewController) -> Int {
        // Return the index of the given data view controller.
        // For simplicity, this implementation uses a static array of model objects and the view controller stores the model object; you can therefore use the model object to identify the index.
        return self.cities.index(of: viewController.dataObject) ?? NSNotFound
    }


    // MARK: - Page View Controller Data Source

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        var index = self.indexOfViewController(viewController as! DataViewController)
        if (index == 0) || (index == NSNotFound) {
            return nil
        }

        index -= 1
        return self.viewControllerAtIndex(index, storyboard: viewController.storyboard!)
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        var index = self.indexOfViewController(viewController as! DataViewController)
        if index == NSNotFound {
            return nil
        }

        index += 1
        if index == self.cities.count {
            return nil
        }
        return self.viewControllerAtIndex(index, storyboard: viewController.storyboard!)
    }

}

Answer 1

The problem is, that you use two different instances of ModelController. One for the RootViewController and another in the TableViewController. They don't know each other.

A couple of options to address the problem:

1.) Hand over the same instance of ModelController to TableViewController when you segue into it.

E.g. by adding this prepare(for segue:) method toRootViewController`

func prepare(for segue: UIStoryboardSegue, sender: Any?) {

        if(segue.identifier == "Locations") {

            let destVC: TableViewController = segue.destination as! TableViewController;
            destVC.modelViewController = self.modelController;

        }
    }

This will ensure that the same ModelController will be handed over.

Note: you have to add this identifier ("Locations") to the segue going from Edit-button to the TableViewController scene.

Note 2: this code is untested and doesn't probably even compile. I'm not having Xcode available right now.

2.) Ensure that there cannot be more than one instance of ModelController (Singleton)

One random web link: https://thatthinginswift.com/singletons/