Finding the size of a view adjusted by a UIStackView swift

The sizes are available after UIStackView.layoutSubviews() finishes. You can subclass UIStackView and override layoutSubviews:

class MyStackView: UIStackView {
    override func layoutSubviews() {
        super.layoutSubviews()
        print("arrangedSubviews now have correct frames")
        // Post a notification...
        // Call a method on an outlet...
        // etc.
    }
}