How can I have an NSDocument-based app open Folders, but save associated doc to Application Support?

Question

In my NSDocument based app, my documents are to be associated with directories (they are an index of the directory)

I want to be able to open/save documents according to the following use case:

1. Opening a directory: a) if there is a .myapp file in the folder, open it b) else look if there is HASH-OF-PATH-basename.myapp file in ~/Library/Application Support/MyApp and open that b) else create a new document in memory

2. Opening an xxx.myapp file, as normal

3. Save / Autosave a) if source of current doc was 1b, 1c, or 2 save file to ~/Library/Application Support/MyApp/HASH-OF-PATH-basename.myapp b) if source of current doc was 1a, save to that .myapp file

4. Save As / Save To b) save an xxxxx.myapp to wherever the user specifies

The obvious thing seemed to be just to implement readFromURL:ofType:error:L and writeToURL:ofType:forSaveOperation:originalContentsURL:error: and switch URL as necessary based on what the saveoperation was.

This works in that I can open files and folders, and when I save an open folder my file is saved to Application Support, but then after writeToURL:ofType:forSaveOperation:originalContentsURL:error an error is raised and a dialog shown with the following error:

NSDocument could not delete the temporary item at
file://localhost/private/var/folders/6P/6PNpIB-6HreGE+Ikqf5dWU+++TI/
-Tmp-/TemporaryItems/(A%20Document%20Being%20Saved%20By%20MyApp%203)
/SomeDirectory. 
Here's the error:
Error Domain=NSCocoaErrorDomain Code=4 UserInfo=0x200583a00 
"“SomeDirectory” couldn’t be removed."

Answer 1

This turned out to be fairly easy to resolve.

I overrode saveToURL:ofType:forSaveOperation:error: and did the url & type swapping based on SaveOperation type there.

Now things are working as expected.