Correct way to create and restore UIViewControllers during state restoration?

Question

I'd like to do state restoration in my app that doesn't use story boards. I'm seeing my primary app ViewController instantiated twice during state restoration - how do you ensure that it's only created once?

The way I understand the flow, application:willFinishLaunchingWithOptions and application:didFinishLaunchingWithOptions would use a commonInit method which would setup the applications UIWindow and its rootViewController. In my case, the rootViewController is a UINavigationController with a class named 'MyMainViewController' serving as UINavigation's rootViewController.

Along side this I'm also handling willEncodeRestorableStateWithCoder and didDecodeRestorableStateWithCoder, respectively. But it seems that by the time I get to my didDecodeRestorableStateWithCoder, I see two separate instances of MyMainViewController created.

What's the way to ensure that only one UIViewController is created during restoration?

Order of calls during restoration:

• Create new instance MyMainViewController (#1) via application:willFinishLaunchingWithOptions:
• MyMainViewController's viewControllerWithRestorationIdentifierPath:coder invoked and MainViewController is restored (#2)
• application:didDecodeRestorableStateWithCoder: is called and UINavigationController is decoded and assigned to self.window

Here's what I'm doing in my AppDelegate:

NSString * const kRootViewControllerKey = @"RootViewControllerKey";

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

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

- (void)commonInitWithOptions:(NSDictionary *)launchOptions {

    static dispatch_once_t predicate;
    dispatch_once(&predicate, ^ {

        // While this will be called only once during the lifetype of the app, when the process is killed 
        // and restarted, I wind up with an instance of MyMainViewController created first from here
        // and then once again, during MyMainViewController's viewControllerWithRestorationIdentifierPath:coder 
        // that's invoked later on. 

        UIViewController *rootViewController = [MyMainViewController alloc] init];
        UINavigationController *aNavController = [[UINavigationController alloc] initWithRootViewController:rootViewController];

        aNavController.navigationBarHidden = YES;
        aNavController.restorationIdentifier = NSStringFromClass([aNavController class]);

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

        self.window = aWindow;
    });
}

// Encode app delegate level state restoration data
- (void)application:(UIApplication *)application willEncodeRestorableStateWithCoder:(NSCoder *)coder {
    [coder encodeObject:self.window.rootViewController forKey:kRootViewControllerKey];
}

// Decode app delegate level state restoration data
- (void)application:(UIApplication *)application didDecodeRestorableStateWithCoder:(NSCoder *)coder {

    // Find the preserved root view controller and restore with it
    UINavigationController *navControlller = [coder decodeObjectForKey:kRootViewControllerKey];

    if (navControlller) {
        self.window.rootViewController = navControlller;
    }

}

Trudge

did you ever find a fix for this? I'm running into the exact same problem, seeing my view controller being init'd twice. – Oquinn

Nope - never did. I'm not sure how to work around it as I cannot move to using storyboards. – Trudge

Answer 1

There's only ever supposed to be one instance of my root view class, so I solved it by adding a class method to alloc and init the class only once and return the value otherwise:

+ (id) initOnce {
    static id view_ref;

    if(!view_ref)
        view_ref = [[UIViewController alloc] init];

    return view_ref;
}

Now when the class is initialized via [UIViewController initOnce], the same view reference always gets returned, whether during willFinishLaunchingWithOptions or viewControllerWithRestorationIdentifierPath.

Perfect

Also, you can just not set the .restorationClass on the root view and UIKit state restoration seems to do "The Right Thing". – Perfect