为什么类型不符合协议？

Question

Let's have the following piece of code:

protocol ItemView {

    var image: UIImage? { get set }
}

 class BaseController<T: UIView where T: ItemView>: UIViewController {

    var itemView = T()
}

class ConcreteItemController: BaseController<UIImageView> {

}

For the BaseController I am getting an error message saying "Type "UIImageView" does not conform to protocol "ItemView"".

But why?

Same error message shows up when I do this instead:

 typealias ConcreteItemController = BaseController<UIImageView> 

I am really missing something here...why would the UIImageView not conform to ItemView protocol? It HAS an optional image property..so where's the problem?

Answer 1

UIImageView does not declare that it conforms to ItemView . Because swift is a strictly typed language, it doesn't infer conformance to protocols, as some types will only incidentally conform. You could however say something like:

protocol ItemView {

    var image: UIImage? { get set }
}

class BaseController<T: UIView where T: ItemView>: UIViewController {

    var itemView = T()
}

extension UIImageView: ItemView {}

class ConcreteItemController: BaseController<UIImageView> {

}