import Preheat
import Nuke

class PreheatDemoViewController: UICollectionViewController {
    let preheater = Nuke.Preheater()
    var controller: Preheat.Controller<UICollectionView>?

    override func viewDidLoad() {
        super.viewDidLoad()

        controller = Preheat.Controller(view: collectionView!)
        controller?.handler = { [weak self] addedIndexPaths, removedIndexPaths in
            self?.preheat(added: addedIndexPaths, removed: removedIndexPaths)
        }
    }

    func preheat(added: [IndexPath], removed: [IndexPath]) {
        func requests(for indexPaths: [IndexPath]) -> [Request] {
            return indexPaths.map { Request(url: photos[$0.row]) }
        }
        preheater.startPreheating(with: requests(for: added))
        preheater.stopPreheating(with: requests(for: removed))
    }

    override func viewDidAppear(animated: Bool) {
        super.viewDidAppear(animated)

        controller?.enabled = true
    }

    override func viewDidDisappear(animated: Bool) {
        super.viewDidDisappear(animated)

        // When you disable preheat controller it removes all preheating 
        // index paths and calls its handler
        controller?.enabled = false
    }
}

import Preheat