[swift-evolution] [Idea] Expression to retrieve the Objective-C selector of a method

[Douglas Gregor]

Sent from my iPhone

> On Dec 27, 2015, at 12:07 AM, Jacob Bandes-Storch <jtbandes at gmail.com> wrote:
> 
> This is a neat idea. Here are some of my thoughts after initial readthrough:
> 
> - For symmetry with Obj-C code, how about using "@selector", such as @selector(UIView.`insertSubview(_:at:)`) ?

@ means at-tribute in Swift, whereas this is a specific expression. 

> - Or, why bother with a new expression? Could the compiler just do this automatically when it encounters an @objc function being passed as a Selector? So, you'd simply be able to say "let sel1: Selector = UIView.`frame.get`"

It could, but I don't think it should: the operation is not common enough that making it implicit would reduce overall syntactic noise, and it would introduce ambiguities between selector- and closure-based APIs. 

> - Should the migrator offer to convert string-constant selectors to this form?

Yes, absolutely.

> - It might be worth considering this in the context of the "type-safe selectors" idea that was floating around a while back.

Yes, I should have referenced that. Apologies!

> - Would it be valid to qualify a function with a subclass's name, when it's really only defined on the superclass? That is, would "objc_selector(MyView.`frame.get`)" work even if MyView doesn't override the `frame` property?

Yes. MyView still has that property even if it doesn't override it. 
> 
> I could see this last one as a potential source of user confusion, because naming a particular class wouldn't actually tell you which implementation gets called when performing the selector (that's just the nature of the Obj-C runtime).

To some extent, that's the nature of overriding. But objective-c allows one to use a selector with an unrelated class, which can certainly be confusing. I feel like that comes from the runtime itself, and isn't something we can avoid with any syntax we pick. 

> Jacob Bandes-Storch
> 
>> On Sat, Dec 26, 2015 at 11:48 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
>> Hi all,
>> 
>> Currently, producing an Objective-C selector in Swift is an error-prone operation. One effectively just writes a string literal and uses it in a context where an ObjectiveC.Selector is expected:
>> 
>>         control.sendAction(“doSomething:”, to: target, forEvent: event)
>> 
>> There are many points of failure here:
>> 
>> 1) The compiler doesn’t syntax-check at all to make sure it’s a valid spelling for a selector
>> 2) The compiler doesn’t look for existing methods with this selector anywhere
>> 3) The mapping from a Swift method name to an Objective-C selector isn’t always immediately obvious (especially for initializers), and will be getting significantly more complicated with the renaming work for Swift 3 (https://github.com/apple/swift-evolution/blob/master/proposals/0005-objective-c-name-translation.md).
>> 
>> I suggest that we add an expression ‘objc_selector(method-reference)` that produces the Objective-C selector for the named method, and produces an error if the method does not have an Objective-C entry point. For example:
>> 
>>         control.sendAction(objc_selector(MyApplication.doSomething), to: target, forEvent: event)
>> 
>> “doSomething” is a method of MyApplication, which might even have a completely-unrelated name in Objective-C:
>> 
>>         extension MyApplication {
>>                 @objc(jumpUpAndDown:)
>>                 func doSomething(sender: AnyObject?) { … }
>>         }
>> 
>> By naming the Swift method and having objc_selector do the work to form the Objective-C selector, we free the programming from having to do the naming translation manually and get static checking that the method exists and is exposed to Objective-C.
>> 
>> This proposal composes with my “Generalized Naming for Any Function” proposal, which lets us name methods fully, including getters/setters:
>> 
>>         let sel1: Selector = objc_selector(UIView.`insertSubview(_:at:)`) // produces the Selector “insertSubview:atIndex:"
>>         let sel2: Selector = objc_selector(UIView.`frame.get`) // produces the Selector “frame"
>> 
>> I don’t like the `objc_selector` syntax at all, but otherwise I think this functionality is straightforward.
>> 
>>         - Doug
>> 
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> 
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