Using Swift protocols in Objective-C

Sep 3, 2014 · Follow on Twitter and Mastodon swiftdi

I’m currently creating two new games for iOS. One is made in Swift and SpriteKit, while the other is made in Objective-C and UIKit. To share logic, I need to use Swift protocols in my Objective-C code.

The good part

The apps share an Objective-C core library and use the CoreMeta IoC library to bootstrap each app and select which implementation to use for a certain protocol.

Objective-C protocols in the library work great in Swift. To implement these protocols in Swift, I just add them to my bridging header.

One protocol that I reuse in every game is Animal. For Objective-C and UIKit, it’s implemented by an AnimalView that inherits UIView. For Swift and SpriteKit, it’s implemented by an AnimalNode that inherits SKNode. This works absolutely great.

The bad part

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

CoreMeta is written in Objective-C, which lets you pass in protocols as parameters, which you with the @protocol(ProtocolName) syntax. In Swift, on the other hand, you use ProtocolName.self.

This works great for Objective-C protocols, but doesn’t work at all for Swift protocols. Since Swift protocols can’t be cast to Protocol *, they can’t be used as method parameter with CoreMeta.

The solution

Searching for solutions to more advanced Swift problems often results in nothing, since Swift is so new. Still, I found 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.

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