Why should not directly extend UIView or UIViewController?

Question

I saw this question, with this code:

protocol Flashable {}

extension Flashable where Self: UIView 
{
    func flash() {
        UIView.animate(withDuration:         


        

Answer 1

This approach is preferable to using UIView directly, as in

extension UIView {
    func flash() {
        ...
    }
}

because it lets programmers decide which UIView subclasses they wish to make Flashable, as opposed to adding flash functionality "wholesale" to all UIViews:

// This class has flashing functionality
class MyViewWithFlashing : UIView, Flashable {
    ...
}
// This class does not have flashing functionality
class MyView : UIView {
    ...
}

Essentially, this is an "opt in" approach, while the alternative approach forces the functionality without a way to "opt out".