NSUserDefaults written in -applicationWillTerminate: do not take effect; Why?

Question

I have put the code below inside my AppDelegate, but when I start the app again I notice that the values are still saved (Not NULL). Why is that?

The code:

    - (void)applicationWillTerminate:(UIApplication *)application
{       [[NSUserDefaults standardUserDefaults]
         setObject:NULL forKey:@"roomCat"];
        [[NSUserDefaults standardUserDefaults]
         setObject:NULL forKey:@"TFA"];
        [[NSUserDefaults standardUserDefaults]
        setObject:NULL forKey:@"comments"];

}

Thank you.

Answer 1

You should be using -removeObjectForKey: instead of setting NULL. The former is the official way to remove values, while the latter is undocumented behavior.

In any case, if using -removeObjectForKey: doesn't work, then you can add a call to

[[NSUserDefaults standardUserDefaults] synchronize];

at the end. But only do that if it doesn't work without it. The reason being because calling -synchronize is (relatively) expensive, so it should only be done when required to ensure correctness.

---

After taking another look, I suspect your real problem is this method isn't being called at all. On iOS 4 and later, when apps enter the background, they don't call this method, instead they call -applicationDidEnterBackground:. You should try putting this code there instead.

Answer 2

In this case, I would expect that you would need to manually call the synchronize method on NSUserDefaults as the last line in this method to ensure it syncs before the application terminates.