Using Swift protocols in Objective-C

Published 04 Sep 2014

I am currently creating two new games for iOS. One is made in Swift and SpriteKit, while the other is made in Objective-C and UIKit.

The good part

The apps include a custom core lib that is written in Objective-C. Each app also uses CoreMeta - an IoC library written in Objective-C - to bootstrap the app and select which implementation to use for a certain protocol.

All Objective-C protocols in the library work great in Swift. To implement these protocols in Swift, I just add them to my bridging header. I’m then good to go.

One protocol that I reuse in every game is an Animal protocol. For Objective-C and UIKit, it is implemented by AnimalView, which inherits UIView. In Swift and SpriteKit, however, it is implemented by AnimalNode, which inherits SKNode.

This works absolutely great.

The bad part

All in all, Swift is a really nice experience. Less code, easier to read - gotta love it. However, as I created the Swift protocol and tried to register it with CoreMeta, I ran into problems.

Basically, CoreMeta is written in Objective-C, which lets you pass in protocols as method parameters. You do this by writing @protocol(ProtocolName). In Swift, on the other hand, you call ProtocolName.self.

This works great for Objective-C protocols, but does not work at all for Swift protocols. Since Swift protocols cannot be cast to Protocol *, they cannot be used as method parameter together with CoreMeta.

The solution

Searching for solutions to more advanced Swift problems often results in nothing, since Swift is so new. Still, lucky as I am, I finally found the solution to how to use Swift protocols in Objective-C: Add @objc before the protocol keyword.

After adding @objc to my Swift protocols, I could use them with CoreMeta and bootstrap them as I do with the Objective-C protocols. Hopefully, Swift evolves in a way that makes this not required.