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 v         


        

Answer 1

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;

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.