Copy multiple NSImage to NSPasteboard - swift

Question

I'm trying to copy multiple NSImages to the Pasteboard using Swift 4 - like this:
private func putItemsOnPasteboard (_ items: [Int]) {
if let images = mainImageController?.getNSImages(for: items) {
NSPasteboard.general.clearContents()
NSPasteboard.general.writeObjects(images)
if let c = NSPasteboard.general.pasteboardItems?.count {
print (c)
}
}
}
NSImage implements NSPasteboardWriting, and it works fine. The snag is, if I then paste into, say, Mail, it only pastes the first image, and I'm struggling to figure out why.
For instance, if I call the function with three images, it all appears to work, and my diagnostic 'print (c)' correctly shows '3'. But if I paste into Mail (or OneNote etc.) it only paste the first image.
I know Mail supports pasting multiple images, because I can select three thumbnails in Apple Photos, copy them to the pasteboard, and it correctly pastes all three into Mail.
Any clues would be gratefully appreciated!

Answer 1

If I copy a group of photos in Photos, and then inspect the pasteboard types, I get:
import Cocoa
print(NSPasteboard.general.types?.map { $0.rawValue } ?? [])
outputs:
["dyn.ah62d4rv4gu8ywyc2nbu1g7dfqm10c6xekr1067dwr70g23pw", "IPXPasteboardController", "com.apple.PhotoPrintProduct.photoUUID", "public.file-url", "CorePasteboardFlavorType 0x6675726C", "dyn.ah62d4rv4gu8y6y4grf0gn5xbrzw1gydcr7u1e3cytf2gn", "NSFilenamesPboardType", "dyn.ah62d4rv4gu8yc6durvwwaznwmuuha2pxsvw0e55bsmwca7d3sbwu", "Apple URL pasteboard type"]
Of those types, the one that looks interesting to me is public.file-url, which suggests that Photos is copying a group of URLs onto the pasteboard. Let's test that hypothesis:
import Cocoa
print(NSPasteboard.general.readObjects(forClasses: [NSURL.self], options: nil) ?? [])
outputs:
[file:///Users/*deleted*/Pictures/Photos%20Library.photoslibrary/resources/proxies/derivatives/1e/00/1e03/UNADJUSTEDNONRAW_thumb_1e03.jpg, file:///Users/*deleted*/Pictures/Photos%20Library.photoslibrary/resources/proxies/derivatives/1e/00/1e04/UNADJUSTEDNONRAW_thumb_1e04.jpg, file:///Users/*deleted*/Pictures/Photos%20Library.photoslibrary/resources/proxies/derivatives/1e/00/1e05/kOBCUhzGRcyeVfBCC8VfvQ_thumb_1e05.jpg]
The output is a list of URLs to JPEG files, suggesting that Photos is indeed providing a list of URLs and putting those on the pasteboard. So perhaps you could save the TIFF representations of your NSImages to a temporary location, and then write the URLs to those temporary files onto the pasteboard, and it would work.

Answer 2

Charles Srstka's answer was very helpful here. It appears that though you can put multiple NSImages on the Pasteboard, you can only paste one.
Swift's URL object doesn't comply with NSPasteboardWriting, but NSURL does, and it doesn't have the restriction of being only able to paste one.
So this worked
private func putImagesOnPasteboard (_ images: [Image], folderURL: URL) {
let imageURLs = images.map() {
image in
return NSURL (fileURLWithPath: folderURL.appendingPathComponent(image.fileName).path)
}
NSPasteboard.general.clearContents()
NSPasteboard.general.writeObjects(imageURLs)
}