Programming iOS: clarifications about Root View Controller

Question

Through this question I would like to know if I understand well the notion of Root View Controller.

In iOS application, the Root View Controller (RVC) is the controller whose view gets added to the UIWindow application at startup, isn't true?

[window addSubview:rvcController.View];
[window makeKeyAndVisible];

Now, an UIWindow has also a rootViewController property. When running the previous snippet of code, does that property gets populated with the rvcController or do I have to set it explicitly?

Then, in a UINavigationController it is possible to set a RVC that is different from the previous RVC set for the entry point.

In this case, the first time I add a controller to the navigationController stack (pushing a new controller on it), does the framework set that controller as the RVC for the navigationController or do I have to set it explicitly through initWithRootViewController method?

Answer 1

Ya.. when I began iPhone dev.. the rootViewController thing threw me for a loop too. But it’s really straight forward.

when the app starts, I create a UIWindow object in my app delegate class. Also, in that class, I have a property of type UIWindow called window;

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

    UIWindow *w = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen]bounds]];
    self.window=w;
    [w release];
    // other code here...
}

I then create a UIViewController whose view will be the first view in the window hierarchy, this could be called the "root view controller".

The confusing part is...that often we create a UINavigationController as the "root view controller" and that navigation controller has an init method that asks for a "RootViewController", which is the first viewcontroller it will place on its stack.

So, the window gets a "root view controller", which is the UINavigationController, which also has a RootViewController, which is the first view controller you want to show.

once you sort that out, its all makes sense.. I think :-)

here is some code that does it all.. (taken from a project I have open in front of me)

//called with the app first loads and runs.. does not fire on restarts while that app was in memory
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    


    //create the base window.. an ios thing
    UIWindow *w = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen]bounds]];
    self.window=w;
    [w release];

    // this is the home page from the user's perspective
    //the UINavController wraps around the MainViewController, or better said, the MainViewController is the root view controller
    MainViewController *vc = [[MainViewController alloc]init];

    UINavigationController *nc = [[UINavigationController alloc]initWithRootViewController:vc];
    self.navigationController=nc;  // I have a property on the app delegate that references the root view controller, which is my navigation controller.

    [nc release];
    [vc release];

    //show them
    [self.window addSubview:nc.view];
    [self.window makeKeyAndVisible];

    return YES;
}

Answer 2

Now, an UIWindow has also a rootViewController property. When running the previous snippet of code, does that property gets populated with the rvcController or do I have to set it explicity?

You have to set it explicitly, and if you do, you can remove the addSubview line, because that's handled automatically when you set a root view controller.

Then, in a UINavigationController it is possible to set a RVC that is different from the previous RVC set for the entry point.

Of course, a navigation controller's root view controller has nothing to do with that of the window.

In this case, the first time I add a controller to the navigationController stack (pushing a new controller on it), does the framework set that controller as the RVC for the navigationController or do I have to set it explicity through initWithRootViewController method?

initWithRootViewController is just a shortcut for initializing an empty navigation controller and pushing the first (root) view controller onto the stack. Note that rootViewController is not a property of UINavigationController, you would access it via [navController.viewControllers objectAtIndex:0].