Resize Download Image with AlamofireImage

You can use filters:

let url = URL(string: ...)!
let placeholder = UIImage(named: "app_default-resize")
let filter = AspectScaledToFillSizeFilter(size: imageView.frame.size)
imageView.af.setImage(withURL: url, placeholderImage: placeholder, filter: filter)

See https://github.com/Alamofire/AlamofireImage#image-filters-1.

For Swift 2 version, see previous revision of this answer.