UserDefaults/NSUserDefaults removeObject(forKey:) mysteriously failing in app and between apps - swift

Question

I am working on an "uninstaller" for an macOS app we've had for several years now. The purpose for the uninstaller is to allow us to put a given system into a nascent state as if the original app had never been installed so that we can more reliably test the install process.
The original app has an extensive array of preferences stored in UserDefaults. In the original app there is a resetToDefaults() method which works just fine resetting all the defaults however for the uninstaller we'd wanted to remove the values completely. It looks to be straight-forward and this is what I came up with...
func flushPreferences() {
let defaults = getDefaultPreferences()
for preferenceName in defaults.keys.sorted() {
UserDefaults.standard.removeObject(forKey: preferenceName)
}
UserDefaults.standard.synchronize()
}
... which does not work at all.
I read in the documentation
Removing a default has no effect on the value returned by the objectForKey: method if the same key exists in a domain that precedes the standard application domain in the search list.
and I don't really understand what "domain" relates to and thought it might be app so tried the code as a test in the original app and that does nothing either.
Someone else suggested this, which also does nothing
let appDomain = Bundle.main.bundleIdentifier!
UserDefaults.standard.removePersistentDomain(forName: appDomain)
I also found some test code which works absolutely fine... which looks to be nearly identical to what I'm doing. I even tried using it with hard-coding one of our pref keys and that fails as well.
func testRemoveObject() {
let myKey:String = "myKey"
UserDefaults.standard.set(true, forKey: myKey)
let beforeVal = UserDefaults.standard.value(forKey: myKey)
print("before: \(beforeVal ?? "nil")")
UserDefaults.standard.removeObject(forKey: myKey) // Note: This is the only line needed, others are debugging
let afterVal = UserDefaults.standard.value(forKey: myKey)
print("after: \(afterVal ?? "nil")")
}
What am I missing? It looks like this one (based on what I've been able to find on the web) can be somewhat mysterious but I'm thinking it must be something obvious that I'm not seeing.

Answer 1

Well, thanks to red_menace's suggestion I found one article that led to another that suggested that the following command will reset the user preferences cache:
killall -u #USER cfprefsd
which seemed to work (yay) but upon further investigation it appears that simply closing the app is what updates the actual preference in the .plist and that changing it in the app will not show up until you exit.
This makes sense as it explains why you can create a preference on the fly save it, confirm it saved, delete it and confirm it deleted but cannot delete a previously saved preference — as similar to the persistent prefs perhaps the new preference is not added to the cache until the application exits.
This could also explain the various odd behaviors that other posters were finding (only worked every other time, had to do it asynchronously, etc.). As for NSUserDefaults.synchronize() has been depreciated and developer.apple.com indicates that it is unneeded and does nothing.
So one problem solved...
As it turns out my initial instinct was accurate as well and you cannot access prefs from one app in another using the removeObject(forKey: preferenceName)
// Will not work cross-application, though will work locally (inter-app)
UserDefaults.standard.removeObject(forKey: preferenceName)
To get it to work cross applications you have to use CFPreferencesSetAppValue(_ key:, value:, applicationID:) which is part of the "Preferences Utilities" section of the Core Library which requires that you know the appDomain of the initial app. So, the final solution is:
In the source app:
let appDomain = Bundle.main.bundleIdentifier! // Note, needed by uninstaller
will give you the domain for the stored preference in the source app.
And in the app doing the changing — the final working code:
func flushPreferences() {
let defaults = getDefaultPreferences()
let sourceAppDomain = "{THE_BUNDLE_ID_FROM_SOURCE_APP}"
for preferenceName in defaults.keys {
print("Preference name: \(preferenceName)")
CFPreferencesSetAppValue(preferenceName as CFString,
nil,
sourceAppDomain as CFString)
}
}
Hope this helps someone save some time at some point - thanks to everyone who contributed. This one was a BEAR!