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

Andrey Tarantsov andrey at tarantsov.com
Tue Jan 5 05:51:27 CST 2016

I'm against this, because I often write extensions on Apple classes (like, say, UIColor) that are only intended to be used from Swift, in a pure-Swift project, and I need no stinking' @nonobjc in there.

How much of a problem can this surprise be? You call a method, the compiler tells you it's not there, you look up the reason, no harm done.


> On Jan 5, 2016, at 11:32 AM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
> 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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