Question

I need my app to remember which was the last UIViewController opened so when the app is loaded out of memory, I substitute a rootViewController property in the AppDelegate with one saved in NSUserDefaults:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    UIWindow *window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window = window;

    frontViewController = [[SGLoginScreenViewController alloc] init];

    UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:frontViewController];

    self.window.rootViewController = navController;
    [self.window makeKeyAndVisible];
    return YES;
}

What I want to do is to put some code in the ViewDidLoad method of each ViewController and remember its name in NSUserDefaults to later use it in didFinishLaunchingWithOptions, something like this:

[[NSUserDefaults standardUserDefaults] setObject:self forKey:@"currentViewController"];
[[NSUserDefaults standardUserDefaults] synchronize]; 

The problem is, however, that XCode is giving me this warning:

*** -[NSUserDefaults setObject:forKey:]: Attempt to insert non-property value '<SGMainScreenFirstTimeViewController: 0x8e1fea0>' of class 'SGMainScreenFirstTimeViewController'.  Note that dictionaries and arrays in property lists must also contain only property values.

Any ideas on how to implement this whole thing right? Thanks in advance

Answer 1

You can only save in user defaults objects that can conform to the Key-Value coding (NSString, NSnumber, etc). Converting class to string and recreating class knowing class name would be the way to go for what you wish to achieve.
This would be a good approach for saving in NSUserDefaults

[[NSUserDefaults standardUserDefaults] setObject:NSStringFromClass([self class]); forKey:@"currentViewController"];
[[NSUserDefaults standardUserDefaults] synchronize]; 

And for loading back you recreate the class based on the saved string, as below:

NSString *savedClassName = [[NSUserDefaults standardUserDefaults] objectForKey:@"currentViewController"];
UIViewController *controller = [(UIViewController *)NSClassFromString(savedClassName) alloc] init];