[swift-evolution] Pitch: @objc attribute for top-level functions

[Charlie Monroe]

+1, missed this on a number of occasions.


> On Mar 31, 2017, at 5:00 PM, Charles Srstka via swift-evolution <swift-evolution at swift.org> wrote:
> 
> MOTIVATION:
> 
> Sometimes, it’s necessary to write a top-level C function in order to interact with some C-based code. This can come up, for example, when making a new port for a cross-platform app that implements OS-dependent functionality via C functions. More urgently, though, it also pops up in certain Apple APIs, such as the standard QuickLook plug-in template. In order to write a QuickLook plug-in, developers are required to write implementations for certain predefined C functions, such as GeneratePreviewForURL(), GenerateThumbnailForURL(), CancelPreviewGeneration(), and CancelThumbnailGeneration(). Unfortunately, this API contract cannot be met in Swift. Currently, Swift can only expose class members to C/Objective-C, which means that implementing a QuickLook plug-in involves a rather awkward series of hoops to jump through:
> 
> GeneratePreview.swift:
> 
> class QLGlue: NSObject {
> 	@objc static func generatePreview(_: UnsafeMutableRawPointer, request preview: QLPreviewRequest, url: URL, contentTypeUTI: String, options: [String : Any]) -> OSStatus {
> 		// generate the preview
> 	}
> }
> 
> GeneratePreview.m:
> 
> #import “MyProject-Swift.h"
> 
> OSStatus GeneratePreviewForURL(void *thisInterface, QLPreviewRequestRef request, CFURLRef url, CFStringRef contentTypeUTI, CFDictionaryRef options) {
>    return [QLGlue generatePreview:thisInterface request:request url:(__bridge NSURL *)url contentTypeUTI:(__bridge NSString *)contentTypeUTI options:(__bridge NSDictionary *)options];
> }
> 
> PROPOSED SOLUTION:
> 
> Allow the @objc on top-level functions. This will cause the functions to be exposed as C functions, satisfying contracts such as Apple’s QuickLook plug-in contract, and allowing developers to just write:
> 
> @objc func generatePreview(_: UnsafeMutableRawPointer, _ preview: QLPreviewRequest, _ url: CFURL, _ contentTypeUTI: CFString, _ options: CFDictionary) -> OSStatus {
> 	// generate the preview
> }
> 
> This would eliminate an entire source file of glue code.
> 
> IMPACT ON EXISTING CODE:
> 
> None, since @objc is not currently allowed on top-level functions. This is purely additive.
> 
> Charles
> 
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