在状态恢复期间创建和恢复 UIViewControllers 的正确方法？

Question

我想在不使用故事板的应用中进行状态恢复。我看到我的主应用 ViewController 在状态恢复期间被实例化了两次 - 你如何确保它只创建一次？

按照我理解流程的方式，application:willFinishLaunchingWithOptions 和pplication:didFinishLaunchingWithOptions 将使用commonInit 方法来设置应用程序 UIWindow 及其 rootViewController。在我的例子中，rootViewController 是一个 UINavigationController，其中一个名为“MyMainViewController”的类用作 UINavigation 的 rootViewController。

除此之外，我还分别处理willEncodeRestorableStateWithCoder 和didDecodeRestorableStateWithCoder。但似乎当我到达我的didDecodeRestorableStateWithCoder 时，我看到创建了两个独立的 MyMainViewController 实例。

如何保证恢复时只创建一个UIViewController？

恢复期间的调用顺序：

• 通过应用程序创建新实例 MyMainViewController (#1):willFinishLaunchingWithOptions:
• MyMainViewController 的 viewControllerWithRestorationIdentifierPath:coder 调用和 MainViewController 恢复（#2）
• application:didDecodeRestorableStateWithCoder: 被调用，UINavigationController被解码并分配给self.window

这是我在 AppDelegate 中所做的：

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;
    }

}

Answer 1

我的根视图类只应该有一个实例，所以我通过添加一个类方法来解决它，只分配一次并初始化该类，否则返回值：

+ (id) initOnce {
    static id view_ref;

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

    return view_ref;
}

现在，当通过 [UIViewController initOnce] 初始化类时，始终返回相同的视图引用，无论是在 willFinishLaunchingWithOptions 还是 viewControllerWithRestorationIdentifierPath 期间。