[swift-evolution] [Mini-proposal] Require @nonobjc on members of @objc protocol extensions

Félix Cloutier felixcca at yahoo.ca
Mon Jan 4 22:49:08 CST 2016

For the folks who don't mix Swift and Objective-C that much, extensions on @objc classes are exposed to the Objective-C runtime, so there is a discrepancy here. I'm not passionate about the outcome, just dropping the info.


> Le 4 janv. 2016 à 23:32:25, Douglas Gregor via swift-evolution <swift-evolution at swift.org> a écrit :
> Hi all,
> We currently have a bit of a surprise when one extends an @objc protocol:
> @objc protocol P { }
> extension P {
>   func bar() { }
> }
> class C : NSObject { }
> let c = C()
> print(c.respondsToSelector("bar")) // prints "false"
> because the members of the extension are not exposed to the Objective-C runtime. 
> There is no direct way to implement Objective-C entry points for protocol extensions. One would effectively have to install a category on every Objective-C root class [*] with the default implementation or somehow intercept all of the operations that might involve that selector. 
> Alternately, and more simply, we could require @nonobjc on members of @objc protocol extensions, as an explicit indicator that the member is not exposed to Objective-C. It’ll eliminate surprise and, should we ever find both the mechanism and motivation to make default implementations of @objc protocol extension members work, we could easily remove the restriction at that time.
> 	- Doug
> [*] Assuming you can enumerate them, although NSObject and the hidden SwiftObject cover the 99%. Even so, that it’s correct either, because the root class itself might default such a method, and the category version would conflict with it, so...
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