The description for NSApplications finishLaunching says:

Activates the receiver, opens any files specified by the NSOpen user default […]

I have tried both storing a string and an array of strings in my user defaults (with the name NSOpen), but neither works. Does anyone know how to use this feature?

hmm… it seems that it isn’t really using that default – if I launch my app with “-NSOpen �someFile�”, then NSOpen will take the string value of �someFile�, and NSApp will correctly open it, but if I set that same string value in my user defaults, NSApp will not open it.

NSUserDefaults works with command-line arguments, environment variables, and the persistent defaults database. The system apparently doesn’t search the persistent defaults database (~/Library/Preferences/reverse.domain.identifier.product.plist) for the NSOpen key, just the argument and environment databases. It’s a feature, not a bug.

Well, it seems to do more than that, cause if one provide several -NSOpen arguments, only one will be in the defaults, but NSApp will open all the files.

Have you checked if the defaults system automatically coalesces multiple arguments with the same key into e.g. an array? I’m not sure either way but it’d be interesting.

Yes, the first argument is in the defaults (as a string) – the GNUStep implementation of finishLaunching also only check for a string. I don’t know how one would normally acces launch parameters?!?

The docs for NSUserDefaults (specifically, +sharedUserDefaults) note that the first domain that is searched is NSArgumentDomain, “consisting of defaults parsed from the application’s arguments”.