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Properties of types conforming to protocols in Swift

December 25, 2015

In Objective-C it’s very natural to have a property of type that also conforms to one or few protocols. In Swift that becomes tedious cause you can not simply combine types and protocols.

You can use generics:

class MyClass<T: MyOtherType where T: MyProtocol> {
  var property: T

But you will notice soon that this approach has more drawbacks then advantages. First if you need to have another such property it will become a mess. Second is that you can not use this type in Interface Builder anymore, as well as you can not see it from Objective-C environment. Third is that interface always looks more complex if there are generics.

The other way could be to define a protocol that adds some traits of class that you want to extend. Let’s say you want to have a UIView property that also conforms to Animatable protocol and you want to access it’s animatable trait and view trait. But you don’t need to access all of the UIView properties, let’s say you need only frame for now. Then you can simply add this property to protocol that extends Animatable protocol, or use protocols composition:

protocol Animatable {
  func startAnimating()
  func stopAnimating()

protocol AnimatableView: Animatable {
  var frame: CGRect { get }

class ViewController: UIViewController {
  var view: AnimatableView

You probably already noticed some problems here too. Every time you will need to access new property of UIView you will need to add it to AnimatableView property. Also you can not pass view property as argument to methods that accept UIView. You can of course force cast it to UIView but pony will be hurt then.

There is much cleaner yet easy solution:

protocol Animatable {
  func startAnimating()
  func stopAnimating()

protocol AnyView {
  var view: UIView { get }

protocol AnimatableView: AnyView, Animatable {}
//or using protocols composition
typealias AnimatableView = protocol<AnyView, Animatable>

extension UIView: AnyView {
  var view: UIView { return self }

Instead of having protocol that exposes some properties of UIView we expose the whole UIView type via property and access it when we need to work with instance as with UIView.

This approach has all advantages of previous one - you can easily access your property by its different traits. But it does not have any of drawbacks of previous solutions. You don’t ever need to add any property from UIView class to your protocols if you want to access it. You can use the property in methods that accept UIView simply by passing in its view property. And of course you don’t loose Objective-C interoperability and Interface Builder support.

There are different examples of such approach. For instance Alamofire uses URLRequestConvertible protocol:

public protocol URLRequestConvertible {
  var URLRequest: NSMutableURLRequest { get }
extension NSURLRequest: URLRequestConvertible {
  public var URLRequest: NSMutableURLRequest { get }

Personally I would not use Convertible name for such protocols cause it doest not mean the same as Convertible protocols in Swift standard library where usually they mean that you can create an instance of protocol type from some other type. For instance you can create StringLiteralConvertible instance from String. Or you can create instance of ArrayLiteralConvertible from array literal. But you can not get array literal back from ArrayLiteralConvertible. So it is conversion only in one direction - from external type to self type. But here we need conversion in other direction - from self type to external type.

There are though CustromStringConvertible and ConstomDebugStringConvertible that look more like URLRequestConvertible from Alamofire. But for me they are kind of special and I think I liked original Printable name more.

Personally I would prefer to use names like AnyView or AnyURLRequest or even ViewRepresentable or URLRequestRepresentable. The former is shorter when the latter I think better describes the direction of conversion that happens.

Ilya Puchka
iOS developer @Babylon
Twitter | Github


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