Swift: Property conforming to a specific class and in the same time to multiple protocols

Question

In Objective-C, it\'s possible to write something like that:

@property(retain) UIView *myView;

But how can

Answer 1

One and probably a bit ugly one of the ways to do that, is to create a wrapper protocol for UIView:

protocol UIViewRef {
    var instance: UIView { get }
}

Now it is possible to create a protocol which implements Protocol1, Protocol2 and UIViewRef, which is going to be used to get the UIView itself:

protocol MyUIViewProtocol: UIViewRef, Protocol1, Protocol2 { }

And last step will be implementing UIViewRef protocols for your UIViews, which, in you case, as I understand, already implement Protocol1 and Protocol2:

// SomeOfMyViews already implements Protocol1 and Protocol2
extension SomeOfMyUIViews: MyUIViewProtocol {
    var instance: UIView { return self }
}

As the result we have MyUIViewProtocol, implementers of which hold a reference to a UIView and each of them implement Protocol1 and Protocol2. One caveat though - to get the UIView itself, we need to ask it's reference from instance property. For example

// Lets say we're somewhere in a UIViewController
var views: [SomeOfMyUIView] = // Get list of my views
views.forEach { self.view.addSubview($0.instance) }

Answer 2

You can do this with a generic class using a where clause:

A where clause enables you to require that an associated type conforms to a certain protocol, and/or that certain type parameters and associated types be the same.

To use it, make the class your property is defined in a generic class with a type constraint to check if the type parameter for your property matches your desired base class and protocols.

For your specific example, it could look something like this:

class MyViewController<T where T: UIView, T: Protocol1, T: Protocol2>: UIViewController {
    var myView: T

    // ...
}

Answer 3

In Swift 4 it's finally possible. You can declare variable of some class conforming to protocol at the same time, like this:

class ClassA {
    var someVar: String?
}

protocol ProtocolA {}

class ClassB {
    var someOptional: (ClassA & ProtocolA)? // here is optional value
    var some: ClassA & ProtocolA // here is non-optional value; need to provide init though :)
}