NSOutlineView drag-n-drop multiple items to single xml file Finder

Question

I have a NSOutlineView with drag and drop support to reorder rows. Now I would like to add an export of the selected rows to a opml file in Finder. I've managed to set up NSFilePromiseProvider during outlineView:pasteboardWriterForItem:, which is called for every single row.

My current testing environment includes dummy methods of all related drag-n-drop delegate methods: NSDraggingSource, NSFilePromiseProviderDelegate, NSPasteboardItemDataProvider, NSPasteboardTypeOwner, NSPasteboardWriting, and the source and destination delegate methods of NSOutlineViewDataSource.
With a minimal method body and a print out whenever they are called.
And most of the time delegate methods are only called for specific NSPasteboardType like NSPasteboardTypeString.

How can I have one promised file for all selected rows at once? In the end I want to drag e.g., 3 rows from my outline view to the Desktop with 1 file created 'export.opml'.

Answer 1

Finally found a composition that works. Turns out outlineView:pasteboardWriterForItem: is not the right place to instantiate the NSFilePromiseProvider. Here is what worked for me:

ThisClass <NSFilePromiseProviderDelegate>
...

- (BOOL)outlineView:(NSOutlineView *)outlineView writeItems:(NSArray *)items toPasteboard:(NSPasteboard *)pasteboard {
    NSFilePromiseProvider *prov = [[NSFilePromiseProvider alloc] initWithFileType:@"public.xml" delegate:self];
    [pasteboard writeObjects:@[prov]];
    return YES;
}

- (nonnull NSString *)filePromiseProvider:(nonnull NSFilePromiseProvider *)filePromiseProvider fileNameForType:(nonnull NSString *)fileType {
    return @"myfile.opml";
}

- (void)filePromiseProvider:(nonnull NSFilePromiseProvider *)filePromiseProvider writePromiseToURL:(nonnull NSURL *)url completionHandler:(nonnull void (^)(NSError * _Nullable))completionHandler {
    NSLog(@"%@ %@", url, filePromiseProvider.userInfo);
    // write to file ...
    completionHandler(nil);
}