Hello, list!
I am trying to find my way around Swift’s protocol limitations. It appears that
in general, any protocol with declared associatedtype will break polymorphism?
Take the case below which does not compile. All "Thing” instances are
"Something<VC: UIViewController>” but they can’t be passed around or coerced as
so.
How is it that I can legally write the code:
class Thing: Something<UINavigationController> { }
and instantiate it, but it is not the very thing it implements?
All Thing instances conform to the public interfaces of
Something<UIViewController> so why can’t they be recognized as such and coerced
as such?
What is the work-around of this break in Polymorphism?
import UIKit
protocol Anything: class, NSObjectProtocol {
associatedtype ViewControllerType: UIViewController
var viewController: ViewControllerType { get }
init(viewController: ViewControllerType)
func addAnything(anything: Something<UIViewController>) -> Bool
}
class Something<VC: UIViewController>: NSObject, Anything {
typealias ViewControllerType = VC
private(set) var viewController: ViewControllerType
required init(viewController: ViewControllerType) { self.viewController
= viewController }
final private var things = [String: Something<UIViewController>]()
final internal func addAnything(anything: Something<UIViewController>)
-> Bool {
// implementation details...
return true
}
}
class Thing: Something<UINavigationController> { }
let firstThing = Thing(viewController: UINavigationController())
let secondThing = Thing(viewController: UINavigationController())
firstThing.addAnything(anything: secondThing)
// Playground execution failed: error: MyPlayground.playground:48:34: error:
cannot convert value of type 'Thing' to expected argument type
'Something<UIViewController>'
firstThing.addAnything(anything: secondThing as Something<UIViewController>)
// Playground execution failed: error: MyPlayground.playground:48:34: error:
cannot convert value of type 'Thing' to type 'Something<UIViewController>' in
coercion
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