[swift-users] Problem with Access Control and Extensions

Geordie Jay geojay at gmail.com
Thu Sep 21 06:44:05 CDT 2017


Adrian Zubarev via swift-users <swift-users at swift.org> schrieb am Mi. 20.
Sep. 2017 um 22:13:

> I don’t get your problem here. If you don’t want to debate the correctness
> of your code, why are you asking for help or even showing error messages
> for a code snippet that cannot work?
>
> 1. Drop the access modifier from the extension itself, because this is
> only for convenience, which may or may not rule over the members of the
> extension members. If you’re already explicitly setting the access modifier
> on the extension members then the convenience access modifier makes no
> sense.
> 2. The code cannot work, because you cannot override `viewDidLoad` on a
> class that you don’t own, on a subclass of `UISplitViewController` that
> would be possible.
>

I think the point is that this is possible in ObjcC. So the question is
really whether you can override methods in Swift from classes that are not
your own.

The answer, as far as I’ve seen, is no. Whenever I’ve needed this feature
I’ve used ObjC.

- Geordie



> ```
> class MySplitViewController : UISplitViewController {}
>
> extension MySplitViewController {
> override open func viewDidLoad() {
> super.viewDidLoad()
> /* ... */
> }
> }
> ```
>
>
> Am 20. September 2017 um 21:41:31, Rick Aurbach via swift-users (
> swift-users at swift.org) schrieb:
>
> I am trying to write an extension to a UIKit class, but am running into a
> can’t-win situation:
>
> The code I ‘want’ to write looks like:
>
>
> public extension UISplitViewController {
> override public func viewDidLoad() {
> super.viewDidLoad()
> if UIDevice.current.userInterfaceIdiom == .pad {
> preferredDisplayMode = .automatic
> } else {
> preferredDisplayMode = .primaryOverlay
> }
> }
> }
>
> This generates the error message
>
> /Users/rlaurb/Projects/Cooks-Memory/Cooks-Memory/AppDelegate.swift:131:23:
> Overriding instance method must be as accessible as the declaration it
> overrides
> /Users/rlaurb/Projects/Cooks-Memory/Cooks-Memory/AppDelegate.swift:131:23:
> Overridden declaration is here (UIKit.UIViewController)
>
> But I can’t change the access control of the function to ‘open’, because I
> get the warning that the function can’t be “more” accessible than the
> extension.
>
> And I can’t change the extension’s access to ‘open’ because apparently
> extensions can’t be open.
>
> Now I don’t want to get into a debate about whether this code works — it’s
> just an experiment — but is it even possible to express this idea?? I.e.,
> is it possible to express this idea without subclassing?
>
> Cheers,
>
> Rick Aurbach
>
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