Transferable, drag & drop, sharing, pickers

How it works

• Representations (result builder, order = preference): Codable (JSON by default); Data (exporting:/importing: closures, whole blob in RAM); File (a file URL, lazy — export returns SentTransferredFile, import gets ReceivedTransferredFile whose .file is temporary — copy it before returning); Proxy (reuse String, URL, Data, Image). Modifiers: .suggestedFileName, .exportingCondition.
• Drag & drop: .draggable(item) { preview }; .dropDestination(for: T.self) { items, location in … return true } isTargeted: { }. Only offered types that match T arrive.
• Share: ShareLink(item:subject:message:preview: SharePreview(title, image:)).
• Paste: PasteButton(payloadType:) — the tap is consent, so no “Allow Paste” alert (iOS 16 alerts on programmatic reads of another app’s content; UIKit: UIPasteControl).
• PhotosPicker(selection:matching:) (16) runs out of process — no photo-library permission; the app sees only picked items. await item.loadTransferable(type:) (in .task(id: item)) returns T?: nil = type not offered, throws = load failed. Video → your own FileRepresentation type.
• Files: .fileImporter hands back security-scoped URLs outside your sandbox; .fileExporter writes a FileDocument (17 adds a Transferable form).
• UIKit bridge: UIDragItem(itemProvider:), UIDropInteraction, UIActivityViewController. NSItemProvider gained register(_:) for a Transferable and loadTransferable(type:) (16); older SwiftUI: .onDrag / .onDrop(of:).

Example — one type, every surface

extension UTType {
  static let note = UTType(exportedAs: "com.acme.note") }
struct Note: Codable, Transferable {
  var title: String; var body: String
  static var transferRepresentation: some TransferRepresentation {
    CodableRepresentation(contentType: .note)   // richest first
    ProxyRepresentation(exporting: \.body)      // fallback: text
  } }
Text(note.title).draggable(note)
Color.clear.dropDestination(for: Note.self) { notes, _ in
  inbox += notes; return true }
ShareLink(item: note, preview: SharePreview(note.title))

Security-scoped URLs

.fileImporter(isPresented: $open,
              allowedContentTypes: [.pdf]) { r in
  guard case .success(let url) = r,
        url.startAccessingSecurityScopedResource() else { return }
  defer { url.stopAccessingSecurityScopedResource() } // balance!
  let data = try? Data(contentsOf: url)
  saved = try? url.bookmarkData()  // persist THIS, not the path
}

Next launch: URL(resolvingBookmarkData:…, bookmarkDataIsStale:); stale ⇒ re-create the bookmark while you have access. Unbalanced start calls leak kernel resources.

Interview traps

• Silent drop rejection: dropDestination(for: URL.self) never fires for a photo from Photos — it offers image data, not a file URL. Accept what is actually offered.
• 200 MB video as DataRepresentation — materialised in RAM, memory spike / jetsam. Use FileRepresentation.
• ReceivedTransferredFile.file used after the closure — gone; copy it inside the closure.
• Everyone receives plain text: the custom UTType is not declared in Info.plist, or the text representation is listed first.
• Saving the imported path instead of a bookmark: works in the callback, fails next launch.

Which representation?

Remember

“Describe once, richest first, big things as files, borrowed files by bookmark.”

Likely questions

1. What did Transferable replace? — separate share / drag / pasteboard code per surface.
2. Why no permission for PhotosPicker? — out-of-process picker; you get only the picked items.
3. File from Files works once, then fails? — security scope: start/stop, persist a bookmark.
4. exportedAs vs importedAs? — you own the type vs you declare someone else’s.