A UIImageView subclass that loads images from your app bundle completely off the main thread.

AsyncImageView.swift

public class AsyncImageView: UIImageView {
  private class ImageLoadOperation {
    private(set) var isCancelled: Bool = false

    func cancel() {
      isCancelled = true
    }
  }
  private var imageLoadOperation: ImageLoadOperation?

  private func cancel() {
    if let imageLoadOperation = imageLoadOperation {
      imageLoadOperation.cancel()
      self.imageLoadOperation = nil
    }
  }

  override public var image: UIImage? {
    willSet {
      cancel()
    }
  }

  public func loadImageNamed(name: String) {
    cancel()

    let pathToImageOrNil = AsyncImageView.pathToImageNamed(name)
    guard let pathToImage = pathToImageOrNil else {
      super.image = nil
      return
    }

    let imageLoadOperation = ImageLoadOperation()
    self.imageLoadOperation = imageLoadOperation

    dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0)) {
      let imageOrNil = UIImage(contentsOfFile: pathToImage)
      guard let image = imageOrNil else {
        return
      }

      let decodedImage = AsyncImageView.decodeImage(image)

      dispatch_async(dispatch_get_main_queue()) {
        guard !imageLoadOperation.isCancelled else {
          return
        }

        super.image = decodedImage
      }
    }
  }

  private static func pathToImageNamed(name: String) -> String? {
    let screenScale = UIScreen.mainScreen().scale

    var resourceNames = [String]()
    switch screenScale {
    case 3:
      resourceNames.append(name + "@3x")
      fallthrough

    case 2:
      resourceNames.append(name + "@2x")
      fallthrough

    case 1:
      resourceNames.append(name)

    default:
      break
    }

    for resourceName in resourceNames {
      if let pathToImage = NSBundle.mainBundle().pathForResource(resourceName, ofType: "png") {
        return pathToImage
      }
    }

    return nil
  }

  private static func decodeImage(image: UIImage) -> UIImage {
    UIGraphicsBeginImageContextWithOptions(image.size, false, image.scale)
    defer {
      UIGraphicsEndImageContext()
    }

    image.drawAtPoint(CGPoint.zero)

    return UIGraphicsGetImageFromCurrentImageContext()
  }
}