Delete NSUserDefault key value when app quits

Question

I have saved an NSString in my NSUserDefault with key @"userKey" .
I want to be able to reset this value to @"" when the app is totally shut down but not when it enters the background.

I have tried:

[[NSUserDefaults standardUserDefaults] setObject:@"" forKey:@"opponentFile"];

in my appdelegate's -applicationWillTerminate: but nothing happens.

When I open the app after completely closing it the default value is still there.
How do I do this?

Answer 1

Permanent NSUserDefaults changes are scheduled at intervals.
You can instead force this by adding the following to your -applicationWillTerminate: .

[[NSUserDefaults standardUserDefaults] synchronize];

^{Don't force this too frequently as it tends to reduce your app performance.}

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Or... you can just do

[[NSUserDefaults standardUserDefaults] setObject:@"" forKey:@"opponentFile"];

in the -didFinishLaunchingWithOptions: instead.
Seems like the same thing to me.

Answer 2

请尝试：

- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application;