How to use Transferable in an asynchronous way in SwiftUI? - ios

Question

I'd like to drag a file in my application (representing a file on a network drive) to the user's Desktop or Finder window. Consequently the file should only be downloaded once the drag & drop operation ended.
Using AppKit I could achieve this behavior using File Promises. But for SwiftUI, there seems to be no equivalent. The only option is to use NSItemProvider or Transferable (or both in combination) (see this question). But neither NSItemProvider nor Transferable support this asynchronous behavior.
The main problem is that itemProvider closure and also the FileRepresentation closure is called when the dragging session starts, thus also starting the download of the files. This makes the feature unusable e.g. if you drag a large file or many files at once.
struct Item: Identifiable, Transferable {
let id = UUID()
let name: String
let description: String
let url: URL
static var transferRepresentation: some TransferRepresentation {
FileRepresentation(contentType: .fileURL) { (item: DragItem) in
let urlSession = URLSession(configuration: .default)
// This download is already started when the drag & drop session starts
let (tempURL, response) = try! await urlSession.download(from: item.url)
return SentTransferredFile(tempURL)
}
}
}
let items = [
Item(name: "Name 1", description: "Description 1", url: URL(string: "https://...")),
Item(name: "Name 2", description: "Description 2", url: URL(string: "https://...")),
Item(name: "Name 3", description: "Description 3", url: URL(string: "https://..."))
]
struct Test: View {
var body: some View {
Table {
TableColumn("Column 1") { item in
Text(item.name)
}
TableColumn("Column 2") { item in
Text(item.description)
}
} rows: {
ForEach(items) { item in
TableRow(item)
.itemProvider {
let provider = NSItemProvider()
provider.register(item)
return provider
}
}
}
}
}
How can I achieve such an asynchronous behavior in SwiftUI as it was possible in AppKit using NSFilePromiseProvider and NSFilePromiseProviderDelegate?