Demonstrating protocol extensions on UIViewController.

ViewControllerProtocolExtensionExample.swift

import UIKit

public protocol CellContainer {
    var visibleCells: [NSIndexPath] { get }
}

public protocol CollectionViewContainer {
    var collectionView: UICollectionView? { get }
}

extension CellContainer where Self: CollectionViewContainer {
    public var visibleCells: [NSIndexPath] {
        get {
            return self.collectionView?.indexPathsForVisibleItems() ?? []
        }
    }
}

// seems like these need to be delcared explicitly? 
extension UICollectionViewController: CollectionViewContainer { }
// without this, compiler complains that UICollectionViewController has no member "visibleCells"
extension UICollectionViewController: CellContainer { }

let collectionVC = UICollectionViewController(collectionViewLayout: UICollectionViewFlowLayout())

collectionVC.visibleCells