Swift 4 is almost upon us, and I thought I would explore one of its features that I haven’t had the opportunity to that much: KeyPaths. There’s a lot of interesting nuance in here that I previously didn’t realize existed which I’d love to share.
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With Swift 4 you can rewrite the last two lines above like so:
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There are more KeyPath types than just KeyPath and WritableKeyPath as well. If you look at the Swift Evolution proposal for KeyPaths or the generated interface for the KeyPath classes in Xcode, you’ll see that the KeyPath types are actually a 5-deep linear class hierarchy:
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However as of the swift snapshot in Xcode 9 beta 6, this isn’t working (with an actually useful error message saying that it’s not implemented yet).
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However this also appears to not be implemented yet (with a much less useful error message). I expect it will make it into Swift 4.1 if it’s too late for it to get into Swift 4.0 (as of this article’s publishing).
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