Does an NSFilePromiseProviderDelegate need to be also an NSView or NSViewController? - swift

Question

I am going through the AppKit tutorial Supporting Drag and Drop Through File Promises. I downloaded the demo app.
I tried to extract the NSFilePromiseProviderDelegate functionality from the ImageCanvasController class (which is an NSViewController) into a separate class. (Please see before & after code snippets below.)
Before my changes, dragging an image from the app canvas out into Finder and Apple Notes worked fine. But after my changes, nothing happens when I drag into Notes, and I get this error when I drag into Finder:
2022-02-26 23:16:52.713742+0100 MemeGenerator[31536:1975798] *** CFMessagePort: dropping corrupt reply Mach message (0b000100)
Is there any undocumented protocol conformance that I need to add to the new class? Or is there some underlying logic for which NSFilePromiseProviderDelegate only works if it's also an NSView or an NSViewController? In all the guides I found online, it is always tied to a view, but I didn't find any warning that it has to be.
Note:
The reason I want to separate the promise-provider functionality from views is, this way I could provide multiple NSDraggingItem objects to beginDraggingSession. For example, when multiple items are selected, and there is a mouseDragged event on one of them, I could start a dragging session including all the selected items.
Code before
class ImageCanvasController: NSViewController, NSFilePromiseProviderDelegate, ImageCanvasDelegate, NSToolbarDelegate {
...
/// Queue used for reading and writing file promises.
private lazy var workQueue: OperationQueue = {
let providerQueue = OperationQueue()
providerQueue.qualityOfService = .userInitiated
return providerQueue
}()
...
func pasteboardWriter(forImageCanvas imageCanvas: ImageCanvas) -> NSPasteboardWriting {
let provider = NSFilePromiseProvider(fileType: kUTTypeJPEG as String, delegate: self)
provider.userInfo = imageCanvas.snapshotItem
return provider
}
// MARK: - NSFilePromiseProviderDelegate
/// - Tag: ProvideFileName
func filePromiseProvider(_ filePromiseProvider: NSFilePromiseProvider, fileNameForType fileType: String) -> String {
let droppedFileName = NSLocalizedString("DropFileTitle", comment: "")
return droppedFileName + ".jpg"
}
/// - Tag: ProvideOperationQueue
func operationQueue(for filePromiseProvider: NSFilePromiseProvider) -> OperationQueue {
return workQueue
}
/// - Tag: PerformFileWriting
func filePromiseProvider(_ filePromiseProvider: NSFilePromiseProvider, writePromiseTo url: URL, completionHandler: #escaping (Error?) -> Void) {
do {
if let snapshot = filePromiseProvider.userInfo as? ImageCanvas.SnapshotItem {
try snapshot.jpegRepresentation?.write(to: url)
} else {
throw RuntimeError.unavailableSnapshot
}
completionHandler(nil)
} catch let error {
completionHandler(error)
}
}
}
Code after
class CustomFilePromiseProviderDelegate: NSObject, NSFilePromiseProviderDelegate {
/// Queue used for reading and writing file promises.
private lazy var workQueue: OperationQueue = {
let providerQueue = OperationQueue()
providerQueue.qualityOfService = .userInitiated
return providerQueue
}()
/// - Tag: ProvideFileName
func filePromiseProvider(_ filePromiseProvider: NSFilePromiseProvider, fileNameForType fileType: String) -> String {
let droppedFileName = NSLocalizedString("DropFileTitle", comment: "")
return droppedFileName + ".jpg"
}
/// - Tag: ProvideOperationQueue
func operationQueue(for filePromiseProvider: NSFilePromiseProvider) -> OperationQueue {
return workQueue
}
/// - Tag: PerformFileWriting
func filePromiseProvider(_ filePromiseProvider: NSFilePromiseProvider, writePromiseTo url: URL, completionHandler: #escaping (Error?) -> Void) {
do {
if let snapshot = filePromiseProvider.userInfo as? ImageCanvas.SnapshotItem {
try snapshot.jpegRepresentation?.write(to: url)
} else {
throw RuntimeError.unavailableSnapshot
}
completionHandler(nil)
} catch let error {
completionHandler(error)
}
}
}
class ImageCanvasController: NSViewController, ImageCanvasDelegate, NSToolbarDelegate {
...
func pasteboardWriter(forImageCanvas imageCanvas: ImageCanvas) -> NSPasteboardWriting {
let delegate = CustomFilePromiseProviderDelegate()
let provider = NSFilePromiseProvider(fileType: kUTTypeJPEG as String, delegate: delegate)
provider.userInfo = imageCanvas.snapshotItem
return provider
}
}

Answer 1

Does an NSFilePromiseProviderDelegate need to be also an NSView or NSViewController?
No.
delegate is a local variable and is released at the end of pasteboardWriter(forImageCanvas:). Without a delegate the file promise provider doesn't work. Solution: keep a strong reference to the delegate.
class ImageCanvasController: NSViewController, ImageCanvasDelegate, NSToolbarDelegate {
let delegate = CustomFilePromiseProviderDelegate()
...
func pasteboardWriter(forImageCanvas imageCanvas: ImageCanvas) -> NSPasteboardWriting {
let provider = NSFilePromiseProvider(fileType: kUTTypeJPEG as String, delegate: delegate)
provider.userInfo = imageCanvas.snapshotItem
return provider
}
}