On Dec 31, 2008, at 01:32, Kyle Sluder wrote:
For safe save operations, AppKit writes the data to a temporary file on the same volume, and then swaps the old file with the new, which is an atomic operation. If it can't do that, it will rename the original file and write the new one with the old name. This is why absoluteURL or absoluteOriginalContentsURL won't necessarily jive with -fileURL (see the comment header for -[NSDocument writeSafelyToURL:ofType:forSaveOperation:error:] for more details). The upshot is that absoluteOriginalContentsURL will be a URL which you can use to access your existing on-disk data, whether or not AppKit has temporarily renamed it.
I don't see how trying to do this in - writeToURL:ofType:forSaveOperation:originalContentsURL:error: is ever going to work.
If you are given (basically) an old and new package location, then you're forced to copy everything:
-- Trying to write changed files in the old location would be a really bad idea.
-- Moving the unchanged files from the old location to the new location would probably work (if the save operation is a pure save, not a save-as or save-to), but would be a really bad idea if there was an error during the save (because the contents of the new location would presumably get thrown away).
The *real* question here is: what's a *safe* strategy for saving a package by changing parts of it? You really need a single atomic operation to commit the changes, and most file systems don't provide this for an arbitrary set of files. NSDocument's answer is that there isn't a safe strategy, so it always saves by creating a copy. (And even that's not perfectly safe if the file system doesn't provide the equivalent of FSSwapFiles.)
Using Core Data as the storage mechanism for blobs of data is a possibility, but it's also a PITA because:
-- you likely need to turn off NSPersistentDocument's undo handling and provide your own
-- Core Data doesn't have save-to, and its save-as sucks (does a store migration)
-- if you have a lot of data, Core Data is going to keep copies of much of it in internal caches
My suggestion would be to go ahead and use a package document format, and to copy the unchanged files, and see how long it takes. If the save times are unacceptable, then a database solution (not Core Data) is probably the next step.
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