UIImageView keep aspect ratio, but fit to width

I think you need to compare the image aspect ratio to the aspect ratio of the UIImageView itself:

private func updateUI() {
    guard let image = image else { return }
    let viewAspectRatio = self.bounds.width / self.bounds.height
    let imageAspectRatio = image.size.width / image.size.height
    if viewAspectRatio > imageAspectRatio {
        self.contentMode = .scaleAspectFill
    } else {
        self.contentMode = .scaleAspectFit
    }
}

override var image: UIImage? { didSet { updateUI() }}

override func layoutSubviews() {
    super.layoutSubviews()
    updateUI()
}

Note: this is aspect fit width

I think the best way to do it is to play with the mode of your imageView (Aspect Fill, Aspect Width, etc) and this is based on the ratio between the width and height of the image

if image.width > image.height {
    imageView.contentMode = UIViewContentModeScaleAspectFit
    //since the width > height we may fit it and we'll have bands on top/bottom
} else {
  imageView.contentMode = UIViewContentModeScaleAspectFill
  //width < height we fill it until width is taken up and clipped on top/bottom
}

UIViewContentModeScaleAspectFit

Scales the content to fit the size of the view by maintaining the aspect ratio. Any remaining area of the view’s bounds is transparent.

UIViewContentModeScaleAspectFill

Scales the content to fill the size of the view. Some portion of the content may be clipped to fill the view’s bounds.

I haven't tested it but off the top of my head this seems right