Preheat 4.0

Preheat 4.0

LangLanguage SwiftSwift
License MIT
ReleasedLast Release Oct 2017
SwiftSwift Version 4.0
SPMSupports SPM

Maintained by kean.

Automates preheating (prefetching) of content in UITableView and UICollectionView.

This library is similar to UITableViewDataSourcePrefetching and UICollectionViewDataSourcePrefetching added in iOS 10

One way to use Preheat is to improve user experience in applications that display collections of images. Preheat allows you to detect which cells are soon going to appear on the display, and prefetch images for those cells. You can use Preheat with any image loading library, including Nuke which it was designed for.

The idea of automating preheating was inspired by Apple’s Photos framework example app.

Getting Started


Here is an example of how you might implement preheating in your application using Preheat and Nuke:

import Preheat
import Nuke

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

    override func 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 { Request(url: photos[$0.row]) }
        preheater.startPreheating(with: requests(for: added))
        preheater.stopPreheating(with: requests(for: removed))

    override func viewDidAppear(animated: Bool) {

        controller?.enabled = true

    override func viewDidDisappear(animated: Bool) {

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


  • iOS 8.0 / tvOS 9.0
  • Xcode 9
  • Swift 4



Import installed modules in your source files

import Preheat


Preheat is available under the MIT license. See the LICENSE file for more info.