[swift-evolution] [Review] SE-0070: Make Optional Requirements Objective-C only

[Douglas Gregor]

> On Apr 26, 2016, at 3:33 AM, James Froggatt <conductator at ntlworld.com> wrote:
> 
> Fair enough. Upon reflection, I think my real issue is somewhat different to what I suggested previously.
> 
> I wasn't intending to suggest such a thing would be practical, just that it would be a decent alternative to optional protocol requirements. The alternative given in the proposal seems to be more of a way to remove optional protocol requirements on the surface, while actually helping to make them a native feature, if you see what I mean. It's not a realistic alternative - it's a worse syntax for the exact same thing, which also comes with awful side-effects for Swift as a whole. No-one would ever seriously consider this as an alternative, yet it's listed as under the heading ‘Alternatives Considered’.

If you follow the swift-evolution discussion links in the proposal, you’ll note that a number of people have proposed exactly what is listed in “Alternatives Considered”. The only truly wacky idea in there is my caller-side default implementations idea, which I covered simply because it was my last stab at eliminating optional requirements before giving up and sequestering them permanently behind “@objc”.

> 
> You say the arguments given against optional closure properties are strong, but I don't they would be nearly as relevant to the case I suggested. By making them properties of the table view, the tableView parameter would be eliminated, meaning the property names could be unique.
> 
> EG:
> var numberOfRows: (inSection: Int) -> Int
> var cellForRow:: (at: NSIndexPath) -> UITableViewCell
> var moveRow: (from: NSIndexPath, to: NSIndexPath)
> 
> This removes the need to add the mentioned workarounds, since a function could be assigned to the closure property just as easily as an inline closure. I feel this is much more worthy of being considered as an alternative. The idea of these proposals is to document why we do things, so at least for someone wondering why we require all this @objc syntax rather than support optional protocol requirements natively, this would actually present them with a viable alternative which could be applied in their APIs.

Doing this implies creating a potentially large number of stored closure properties, which is not as storage-efficient as storing a single delegate reference. Moreover, it makes it harder to set up your customization points: instead of implementing one protocol, you’re writing assignments into some number of stored closure properties. Imaging trying to change the delegate to some other delegate temporarily: you would have to manually store each of the closures into some local structure and introduce your own, except that you can’t get them all because some new version of the platform would add new stored closure properties. Finally, Cocoa just doesn’t work like this, so you would require some massive re-architecture to get there. I don’t see how this is a better design.

	- Doug