iOS 6 - State Preservation and Restoration

Question

I have implemented iOS 6 API for state saving, it works - after I quit the app and launch back in for some milliseconds the restored view controller fly in, but then it's replaced by the main view controller I display at launch.

I'm setting every time the app launch the root view of the main window, so this must be the issue.

Here is my code:

- (BOOL)application:(UIApplication *)application willFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [self commonInitializationLaunching:launchOptions];
    return YES;
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [self commonInitializationLaunching:launchOptions];
    return YES;
}

- (void)commonInitializationLaunching:(NSDictionary *)launchOptions
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{

        self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
        // Override point for customization after application launch.
        static NSString *const kKeychainItemName = @"OAuthGoogleReader";
        self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
        self.navController = [[UINavigationController alloc] initWithRootViewController:self.viewController];

        GTMOAuth2Authentication *auth;
        auth = [GTMOAuth2ViewControllerTouch authForGoogleFromKeychainForName:kKeychainItemName
                                                                     clientID:kClientID
                                                                 clientSecret:kClientSecret];

        self.window.rootViewController = self.navController;

        [self.window makeKeyAndVisible];

        BOOL isSignedIn = [auth canAuthorize];
        if (isSignedIn) {
            NSLog(@"Signed");
        }else{
            NSString *scope = @"https://www.google.com/reader/api/";

            GTMOAuth2ViewControllerTouch *viewController;
            viewController = [[GTMOAuth2ViewControllerTouch alloc] initWithScope:scope
                                                                        clientID:kClientID
                                                                    clientSecret:kClientSecret
                                                                keychainItemName:kKeychainItemName
                                                                        delegate:self
                                                                finishedSelector:@selector(viewController:finishedWithAuth:error:)];
            [self.navController pushViewController:viewController animated:YES];
            //        self.window.rootViewController = viewController;
        }
    });
}

You can see that in -(void)commonInitializationLaunching:(NSDictionary *)launchOptions I'm setting my window's root view. I don't know what to put in there. Perhaps check if there is saved state and then load this method? But how?

Thanks!

Here is what I've tried following Rob's advice:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    if (!self.isRestored) {
        self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    }
    [self commonInitializationLaunching:launchOptions];
    [self.window makeKeyAndVisible];
    return YES;
}

with nothing in willFinishLaunching... I also removed by window code from my commonInitializationLaunching method.

Eklund

Has my answer helped any? Some feedback would be great. – Dian

Answer 1

Storyboards will do most of the heavy lifting for you, such as restoring the window. Using code, however, will not restore the window. You will need to hold on to your root view controller using the encoder. Your code will look something like this:

NSString * const AppDelegateRootVCKey = @"AppDelegateRootVCKey";

- (void)application:(UIApplication *)application willEncodeRestorableStateWithCoder:(NSCoder *)coder {
    [coder encodeObject:self.window.rootViewController forKey:AppDelegateRootVCKey];
}

- (void)application:(UIApplication *)application didDecodeRestorableStateWithCoder:(NSCoder *)coder {

    // Grabs the preserved root view controller.
    UIViewController * vc = [coder decodeObjectForKey:AppDelegateRootVCKey];

    if (vc) {
        UIWindow * window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
        window.rootViewController = vc;
        window.restorationIdentifier = NSStringFromClass([window class]);

        // The green color is just to make it obvious if our view didn't load properly.
        // It can be removed when you are finished debugging.
        window.backgroundColor = [UIColor greenColor];

        self.window = window;
    }
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    if (!self.window) {

        UIWindow *window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

        // The blue color is just to make it obvious if our view didn't load properly.
        // It can be removed when you are finished debugging.
        window.backgroundColor = [UIColor blueColor];

        UIViewController *root = // However you create your root.

        window.rootViewController = root;
        window.restorationIdentifier = NSStringFromClass([window class]);

        self.window = window;
    }

    [self commonInitializationLaunching:launchOptions];
    [self.window makeKeyAndVisible];

    return YES;
}

Another gotcha to watch out for is to make sure that your UINavigationControllers and UITabBarControllers have restoration identifiers.

Dian

you answer seem really logical, though I couldn't implement it in my test program, and leaved it for some other time. Thanks for the great answer! – Eklund

Answer 2

State restoration is generally integrated with storyboards. If you're using a storyboard, you should not be creating your own window, view controllers, etc. You should let the storyboard do this for you. What's happening is that the storyboard is doing all the state restoration, and then you're creating a new window and laying it on top of all that. If that's the case, you're probably creating two copies of your UI on every launch. You're just not noticing it.

---

If you are constructing your entire interface in code (not a recommended approach, but it does work), then you need to determine whether state restoration happened before creating your UI. This is fairly simple:

• In your commonInitializationLaunching:, initialize only non-UI elements (things that wouldn't ever be in state-preservation). This is the place to handle things that the UI elements might rely on during state restoration. You don't have any of these in your current code.

• In application:didDecodeRestorableState:, set an app delegate ivar to indicate that state was restored.

• In application:didFinishLaunchingWithOptions:, after running commonInitializationLaunching:, check your ivar. If state wasn't restored, create a UI.

Do remember that the commonInitializationLaunching: pattern only exists for backward compatibility with iOS 5. If you don't need that, then just put non-UI in willFinish and UI in didFinish (if state wasn't restored).

Orfinger

Really nice approach, but all I get now is black screen - when I move my [self.windows makeKeyAndVisible] to depend on the iVar isRestored I even tried it with simple Master-Detail app with Storyboard, and didn't worked.. Is there something that I'm missing? – Eklund

You need to call -makeKeyAndVisible in all cases. You just should only create a UIWindow when not restoring. – Orfinger

It still doesn't work - a black screen. Check the edit of my question – Eklund