NSUserdefaults在iOS应用中无法正常工作

Question

I have an app which shows the settings page ONCE! This is when it is first download from the app store. After that then it goes to the main page only.

but when you swipe the app when you double click the iPhone button and remove the app then it goes back to the settings page.

Here is some code from my app

didfinishwithOptions

if (![[NSUserDefaults standardUserDefaults] boolForKey:@"SettingsShown"])
{
    [[NSUserDefaults standardUserDefaults] setBool:YES forKey:@"SettingsShown"];
    [[NSUserDefaults standardUserDefaults] setBool:NO forKey:@"ShowBackButton"];
    [[NSUserDefaults standardUserDefaults] synchronize];
    self.window.rootViewController = [self.window.rootViewController.storyboard instantiateViewControllerWithIdentifier:@"SettingsViewController"];

}



if ([[NSUserDefaults standardUserDefaults] boolForKey:@"SettingsShown"] == NO)
{
    [[NSUserDefaults standardUserDefaults] setBool:NO forKey:@"SettingsShown"];
}

then I have put

- (void)applicationWillTerminate:(UIApplication *)application
{
    [[NSUserDefaults standardUserDefaults] setBool:NO forKey:@"SettingsShown"];
    NSLog(@"Application is terminated");
}

this should run when you swipe the application when you double click on the iPhone button. but it isn't setting the user default to 0 because it is running the settings page again from didfinishwithoptions. Can anyone advise ?

Answer 1

Any value put into NSUserDefaults is not necessarily written instantly. If you app, for instance, terminates in some unusual way, there is no guarantee, the data will be written.

You can force the system to write to the NSUserDefaults , using synchronize:

 [[NSUserDefaults standardUserDefaults] synchronize]

From the documentation :

Writes any modifications to the persistent domains to disk and updates all unmodified persistent domains to what is on disk. Because this method is automatically invoked at periodic intervals, use this method only if you cannot wait for the automatic synchronization (for example, if your application is about to exit) or if you want to update the user defaults to what is on disk even though you have not made any changes.

There is a minor performance penalty doing so.