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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