Use NSItemProvider in combination with NSFilePromiseProvider - swift

Question

How can I use a NSFilePromiseProvider in combination with .onDrag in SwiftUI? It only allows me to return an NSItemProvider, but for this, the file must already exist on the computer as the callback of registerFileRepresentation is called eagerly.
I want to drag a remote file displayed in my application to my computer desktop. With AppKit I used file promises for this use case, but for SwiftUI I found no solution so far.
struct ContentView: View {
var body: some View {
Text("Draggable Text")
.onDrag {
let provider = NSItemProvider()
provider.registerFileRepresentation(
forTypeIdentifier: UTType.fileURL.identifier,
fileOptions: [],
visibility: .all
) {
// This file must already exists. How to provide a file promise here?
let fileUrl = URL(fileURLWithPath: "file:///path/to/file.txt")
$0(fileUrl, true, nil)
return nil
}
return provider
}
}
}

Answer 1

Here's a limited workaround. It uses a FileHandle to write to the created file even after the drop has finished.
It reliably works for drop targets that move the file (e.g. Finder).
It fails for drop targets that read from the file (e.g. Apple Mail) if the file hasn't been completely written when the drop event occurs. In this situation, the empty placeholder content is read by the target app.
import SwiftUI
struct ContentView: View {
var body: some View {
Text("Drag me")
.onDrag {
let provider = NSItemProvider()
provider.registerDataRepresentation(for: .fileURL, visibility: .all, loadHandler: loadFileForItemProvider)
return provider
}
}
#Sendable
func loadFileForItemProvider(onComplete callback: #escaping (Data?, Error?) -> Void) -> Progress? {
let progress = Progress(totalUnitCount: 100)
Task.detached {
do {
let temporaryDirectory = try FileManager.default.url(for: .itemReplacementDirectory, in: .userDomainMask, appropriateFor: .documentsDirectory, create: true)
let url = temporaryDirectory.appendingPathComponent("some_file.pdf")
try Data().write(to: url) // create temporary, emptyfile
let handle = try FileHandle(forWritingTo: url)
callback(url.dataRepresentation, nil)
let fileData = try await ... // load/generate file data here
try handle.write(contentsOf: fileData)
try? handle.close()
progress.completedUnitCount = 100
} catch {
callback(nil, error)
}
}
return progress
}
}
Remember to clean up the temporary directory after a certain duration or when quittung/launching the app.