[swift-users] Cannot subclass a class with objc_subclassing_restricted attribute

[Joe Groff]

> On Apr 17, 2017, at 10:11 PM, Rick Mann <rmann at latencyzero.com> wrote:
> 
>> 
>> On Apr 17, 2017, at 08:54 , Joe Groff <jgroff at apple.com> wrote:
>> 
>> 
>>> On Apr 14, 2017, at 7:41 PM, Rick Mann via swift-users <swift-users at swift.org> wrote:
>>> 
>>> I'm refactoring some Objective-C code to inherit from a new Swift super class. This has been going okay, and I've been cleaning up build errors as I spot them (some auxiliary enums caused enum name changes, etc.).
>>> 
>>> But my last error seems to be that I can't subclass the Swift class: "Cannot subclass a class with objc_subclassing_restricted attribute". I didn't notice it before, and all the online references that say you can't subclass a Swift class point to a Swift document that no longer mentions that. They also don't say what they mean by "Swift" class (e.g. is it not marked with @objc?).
>>> 
>>> In any case, my Swift class looks like:
>>> 
>>> @objc
>>> class
>>> Camera : NSObject
>>> {
>>>  ...
>>> }
>>> 
>>> And my ObjC class looks like:
>>> 
>>> @interface
>>> MCPCamera : Camera
>>> 
>>>  ...
>>> 
>>> @end
>>> 
>>> I feel like this is a reasonable thing to try to do. Is it just not possible?
>> 
>> Subclassing Swift classes from Objective-C is not supported. If the documentation no longer mentions that restriction, we should fix that. Where is that document that you're referring to?
> 
> This is one document that doesn't clearly state it:
> 
> 	https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/MixandMatch.html#//apple_ref/doc/uid/TP40014216-CH10-ID136
> 
> It has nice sections like "Using Swift from Objective-C," and "Declaring a Swift Protocol That Can Be Adopted by an Objective-C Class," and "Adopting a Swift Protocol in an Objective-C Implementation," but no "Inheriting a Swift Class in Objective-C" (the discussion in such a section should explain that it can't be done and why). Searching the document for "derive", "inherit", "subclass" doesn't turn up any indication this can't be done.
> 
> Given that you can do all the other things, and there's such a thing as marking a Swift class as @objc, and you can subclass in Swift, it's quite unintuitive that you cannot go the other way.

Thanks. I'll file a bug with our documentation team to try to clarify that.

-Joe