{"id":50486,"date":"2025-12-17T14:55:16","date_gmt":"2025-12-17T19:55:16","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=50486"},"modified":"2025-12-17T14:55:16","modified_gmt":"2025-12-17T19:55:16","slug":"userdefaults-registerdefaults-footgun","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2025\/12\/17\/userdefaults-registerdefaults-footgun\/","title":{"rendered":"UserDefaults.register(defaults:) Footgun"},"content":{"rendered":"

Jeff Johnson<\/a> (Mastodon<\/a>):<\/p>\n

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“Every instance of UserDefaults shares the contents of the argument and registration domains.” In other words, the result of calling registerDefaults<\/code> on the object returned by [NSUserDefaults initWithSuiteName:]<\/code> is the same<\/em> as calling registerDefaults<\/code> on the object returned by [NSUserDefaults standardUserDefaults]<\/code>! Yet the documentation for registerDefaults<\/code> does not mention this fact.<\/p>\n

How did this become a Link Unshortener bug? In the NSApplicationDelegate method applicationWillFinishLaunching<\/code>, I call [NSUserDefaults initWithSuiteName:]<\/code> and registerDefaults<\/code> to register the default values of Link Unshortener settings. Then I check whether the app container settings need to be migrated. If migration is necessary, then I call [NSUserDefaults setObject: forKey:]<\/code> on the group defaults, using [NSUserDefaults objectForKey:]<\/code> from the app defaults. If the default key has never been set in the app defaults, then [NSUserDefaults objectForKey:]<\/code> should<\/em> return nil. Or so I thought! But at that point registerDefaults<\/code> has already been called on the group defaults object, and the app defaults object shares the registration domain with the group defaults object, so [NSUserDefaults objectForKey:]<\/code> returns a non-nil value, which gets saved in the group defaults.<\/p>\n<\/blockquote>\n\n

Previously:<\/p>\n