Requiring Protocol and Class in Swift Properties

Question

In Objective-C you can require a class and additional protocol implementations for properties:

@property (nonatomic) UIViewController<UISplitViewDelegate> *viewController;

Is this possible in Swift? From the documentation it looks like you can only require either a class or a protocol.

Answer 1

There are actually two of ways to achieve this in Swift:

1. Using an empty "phantom" protocol. Create an empty protocol and make UIViewController conform to it. This is the most "Swift" method, it's safe and it's dynamic (doesn't require specifying a class at compile-time).

   protocol _UIViewControllerType {}
   extension UIViewController: _UIViewControllerType {}
   
   class MyClass {
       weak var viewController: protocol<UISplitViewControllerDelegate, _UIViewControllerType>?
   }
   

   You can also declare a typealias for this type (just to reduce the code mess).

   class MyClass {
       typealias ViewControllerType = protocol<UISplitViewControllerDelegate, _UIViewControllerType>
       weak var viewController: ViewControllerType?
   }
   

2. Using generic constraints. As mentioned by fnc12 and Konstantin Koval. This is safe, but doesn't allow you to "swap" the view controller instance at run-time.

   class MyClass<T: UIViewController where T: UISplitViewControllerDelegate> {
       weak var viewController: T?
   }
   

---

I hope the next Swift release adds a way to specify both constraints without using a "phantom protocol"...

typealias ViewControllerType = UIViewController: UISplitViewControllerDelegate // wish