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

[Charles Srstka]

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