data-source.md

Using the data source

Let’s start by defining an array that contains the information we need to display our menu items:

class ViewController: UIViewController {
    let citites = [
        "Oslo",
        "Stockholm",
        "Tokyo",
        "Barcelona",
        "Vancouver",
        "Berlin"
    ]
}

Then we initialize our PagingViewController:

class ViewController: UIViewController {
    ...
    
    override func viewDidLoad() {
        super.viewDidLoad()
        let pagingViewController = PagingViewController<PagingIndexItem>()
        pagingViewController.dataSource = self
    }
}

Here we need to specify the generic PagingItem type. PagingItem is just an empty protocol that is used to generate the menu items without having to allocate the view controllers. The only requirement is that the type conforms to Hashable and Comparable. Since we’re just going to display a title in the menu items we’re just using PagingIndexItem, which is one of the default types provided by Parchment.

In our data source implementation we set the number of view controllers equal to the number of items in our cities array, and return an instance of PagingIndexItem with the title of each city:

extension ViewController: PagingViewControllerDataSource {
    func numberOfViewControllers<T>(in pagingViewController: PagingViewController<T>) -> Int {
        return cities.count
    }

    func pagingViewController<T>(_ pagingViewController: PagingViewController<T>, pagingItemForIndex index: Int) -> T {
        return PagingIndexItem(index: index, title: cities[index]) as! T
    }

    func pagingViewController<T>(_ pagingViewController: PagingViewController<T>, viewControllerForIndex index: Int) -> UIViewController {
        return CityViewController(city: cities[index])
    }
}

The viewControllerForIndex method will only be called for the currently selected item and any of its siblings. This means that we only allocate the view controllers that are actually needed at any given point.

Parchment will automatically set the first item as selected, but if you want to select another you can do it like this:

pagingViewController.select(index: 3)

This can be called both before and after the view has appeared.

Check out the DelegateExample target for more details.