AutoscaleImageView.swift

import UIKit

class AutoscaleImageView: UIImageView {
    var aspectRatioConstraint: NSLayoutConstraint?

    override init(image: UIImage?) {
        super.init(image: image)
        recalculateAspectRatioConstraint()
    }

    override init(image: UIImage?, highlightedImage: UIImage?) {
        super.init(image: image, highlightedImage: highlightedImage)
        recalculateAspectRatioConstraint()
    }

    required init?(coder: NSCoder) {
        super.init(coder: coder)
        recalculateAspectRatioConstraint()
    }

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

    func recalculateAspectRatioConstraint() {
        if let image = image {
            let size = image.size

            let ratio = size.width / size.height

            if ratio != aspectRatioConstraint?.multiplier {
                aspectRatioConstraint?.isActive = false
                let aspectRatioConstraint = widthAnchor.constraint(equalTo: heightAnchor, multiplier: ratio)
                self.aspectRatioConstraint = aspectRatioConstraint

                aspectRatioConstraint.isActive = true
            }
        } else {
            aspectRatioConstraint?.isActive = false
        }
    }
}