How do I declare a variable that has a type and implements a protocol?

Question

My app has a protocol for detail view controllers, stating they must have a viewModel property:

protocol DetailViewController: class {     var viewModel: ViewModel? {get set} } 

I also have a few different classes that implement the protocol:

class FormViewController: UITableViewController, DetailViewController {     // ... }  class MapViewController: UIViewController, DetailViewController {     // ... } 

My master view controller needs a property that can be set to any UIViewController subclass that implements the DetailViewController protocol.

Unfortunately I can't find any documentation on how to do this. In Objective-C it would be trivial:

@property (strong, nonatomic) UIViewController<DetailViewController>; 

It appears that there isn't any syntax available in Swift to do this. The closest I've come is to declare a generic in my class definition:

class MasterViewController<T where T:UIViewController, T:DetailViewController>: UITableViewController {     var detailViewController: T?     // ... } 

But then I get an error saying that "Class 'MasterViewController' does not implement its superclass's required members"

This seems like it should be as easy to do in Swift as it is in Objective-C, but I can't find anything anywhere that suggests how I might go about it.

Answer 1

I think you can get there by adding an (empty) extension to UIViewController and then specifying your detailViewController attribute using a composed protocol of the empty extension and your DetailViewController. Like this:

protocol UIViewControllerInject {} extension UIViewController : UIViewControllerInject {} 

Now all subclasses of UIViewController satisfy protocol UIViewControllerInject. Then with that, simply:

typealias DetailViewControllerComposed = protocol<DetailViewController, UIViewControllerInject>  class MasterViewController : UITableViewController {   var detailViewController : DetailViewControllerComposed?   // ... } 

But, this is not particularly 'natural'.

=== Edit, Addition ===

Actually, you could make it a bit better if you define your DetailViewController using my suggested UIViewControllerInject. Like such:

protocol UIViewControllerInject {} extension UIViewController : UIViewControllerInject {}  protocol DetailViewController : UIViewControllerInject { /* ... */ } 

and now you don't need to explicitly compose something (my DetailViewControllerComposed) and can use DetailViewController? as the type for detailViewController.

Answer 2

As of Swift 4, you can now do this.

Swift 4 implemented SE-0156 (Class and Subtype existentials).

The equivalent of this Objective-C syntax:

@property (strong, nonatomic) UIViewController<DetailViewController> * detailViewController; 

Now looks like this in Swift 4:

var detailViewController: UIViewController & DetailViewController 

Essentially you get to define one class that the variable conforms to, and N number of protocols it implements. See the linked document for more detailed information.