How to Allow Users To Drag Files Out of Cocoa App

Question

I am working on a macOS application that involves creating a folder in a temporary directory.

How can I allow users to drag a folder icon off of the Cocoa app and onto their desktop (or into Finder and other apps) so that dropping it will actually paste the folder created by the app?

I have so far been able to write the finished folder that I want to save to a temporary directory. How do I place a folder icon on the application that can be dragged into the Finder? Thanks!

The complete code I'm currently using is this:

    class draggableFolder: NSImageView {
    override func mouseDown(with theEvent: NSEvent) {
        let pasteboardItem = NSPasteboardItem()
       // pasteboardItem.availableType(from: [NSPasteboard.PasteboardType.fileURL])
        pasteboardItem.setDataProvider(self, forTypes: [kUTTypeURL as NSPasteboard.PasteboardType])
        let draggingItem = NSDraggingItem(pasteboardWriter: pasteboardItem)
        draggingItem.setDraggingFrame(self.bounds, contents:self.image)

        beginDraggingSession(with: [draggingItem], event: theEvent, source: self as NSDraggingSource)
    }


}

extension draggableFolder: NSDraggingSource {
    func draggingSession(_ session: NSDraggingSession, sourceOperationMaskFor context: NSDraggingContext) -> NSDragOperation {
        return NSDragOperation.generic
    }
}

extension draggableFolder: NSPasteboardItemDataProvider {
    func pasteboard(_ pasteboard: NSPasteboard?, item: NSPasteboardItem, provideDataForType type:
        NSPasteboard.PasteboardType) {
        print("dataprovider")
        if let pasteboard = pasteboard, type.rawValue == String(describing: kUTTypeURL) {
           let folder = currentIconsetURL
            print("dataprovider2")
            print(NSURL.init(fileURLWithPath: currentIconsetURL))
            pasteboard.clearContents()
            pasteboard.declareTypes([NSPasteboard.PasteboardType.fileNameType(forPathExtension: "appiconset")], owner: nil)
            pasteboard.writeObjects([(URL.init(fileURLWithPath: currentIconsetURL).absoluteURL as NSURL) as NSPasteboardWriting])


        }
    }
}

Neale

What is the question, how to start a drag, how to create a folder or something else? Tell us what you've done so far. – Lillie

Thank you@Willeke. I have updated the question. – Neale

@Lillie I have updated the question to contain the code I'm using. – Neale

Take a look at NSFilePromiseProvider, the documentation is missing but you can read about it in AppKit Release Notes (macOS 10.12 and Earlier), Dragging File Promise Source. – Lillie

Thanks @willeke. I've read that NSFilePromiseProvider is used when you need to create a file that doesn't exist on the filesystem. Is that correct? I the I want to appear in Finer already exists in my app's temporary directory, and I want the user to be able to move it elsewhere. – Neale

You can "create" a file by moving it from the temporary directory. – Lillie

Answer 1

Sadly, the drag and drop guide Apple has is based entirely on the old deprecated API: https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/DragandDrop/DragandDrop.html

The rough outline for what you're asking, using the modern API is:

• You need something (ex, your view) to implement the NSDraggingSource protocol to return that it supports the Copy operation only (draggingSession(_:sourceOperationMaskFor:)).
• From within mouseDown (with a tracking loop) or mouseDragged, once the user's drag has moved a certain distance from its original location (3 or 4 points is all), then you should start a dragging session (NSView beginDraggingSession)
• The session should contain one NSDraggingItem where you pass in the URL for the folder (as an NSURL, not a Swift URL, because only the former conforms to NSPasteboardWriter).

Eternize

Thank you @seth. I'm running into the issue where the dragged file just writes the folder's path to a clipping file. The issue seems to be in the file type. I'm using kUTTypeFolder currently. Is this correct? – Neale

Where are you using that UTI? In the most basic form, you'd literally just hand over the URL and nothing else: let draggingItem = NSDraggingItem(pasteboardWriter: fileURL as NSURL); ...; beginDraggingSession(with: [draggingItem], event: currentEvent, source: self) – Eternize

If I use the code in the above comment, I cannot run seem to specify the data provider. How would I do this? I suspect this is the issue because when I attempt to drag the folder onto the desktop a grey prohibitory sign appears on the folder image. – Neale

Just as an update, I nearly got everything working. The only issue is that when I drag the file to the desktop, the file isn't written to the location.. – Neale

Hi @Jake3231, did you get a solution for dragging file outside the cocoa app? If yes, can you please share the solution? – Diplomate