Why is root view set to zero size after dismissing a modal VC, when Auto Layout constraints are installed on UIWindow? - ios

Question

I try to use Auto Layout for my application's root view, i.e. I install Auto Layout constraints in the application's UIWindow and enable Auto Layout on the root VC's view.
The problem: When I dismiss a modally presented VC, the view hierarchy "collapses" and only the UIWindow remains visible. I assume that the root VC's view is resized to zero.
If I do not use Auto Layout on the application's root view everything seems to work fine.
My question: Is it forbidden to use Auto Layout for an application's root view? If not, what am I doing wrong? If yes, is this restriction documented somewhere in the official Apple docs, or is it merely "common knowledge"?
The following code is a minimal sample application that demonstrates the problem. You can simply copy&paste the code into a new Xcode project (use the "empty application" template).
#pragma mark Interface declarations
#interface AppDelegate : UIResponder <UIApplicationDelegate>
#property (strong, nonatomic) UIWindow* window;
#end
#interface PresentingViewController : UIViewController
#end
#interface PresentedViewController : UIViewController
#end
#pragma mark AppDelegate implementation
#implementation AppDelegate
- (BOOL) application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Set this to false and the problem goes away
bool useAutoLayout = true;
self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
self.window.backgroundColor = [UIColor whiteColor];
PresentingViewController* pvc = [[PresentingViewController alloc] init];
UINavigationController* nc = [[UINavigationController alloc] initWithRootViewController:pvc];
self.window.rootViewController = nc;
[self.window addSubview:nc.view];
if (useAutoLayout)
{
nc.view.translatesAutoresizingMaskIntoConstraints = NO;
NSDictionary* viewsDict = [NSDictionary dictionaryWithObjectsAndKeys:
nc.view, #"ncView",
nil];
NSArray* hConstraintsWindow = [NSLayoutConstraint constraintsWithVisualFormat:#"H:|-0-[ncView]-0-|"
options:0
metrics:nil
views:viewsDict];
[self.window addConstraints:hConstraintsWindow];
NSArray* vConstraintsWindow = [NSLayoutConstraint constraintsWithVisualFormat:#"V:|-0-[ncView]-0-|"
options:0
metrics:nil
views:viewsDict];
[self.window addConstraints:vConstraintsWindow];
}
nc.view.backgroundColor = [UIColor redColor];
[self.window makeKeyAndVisible];
return YES;
}
#end
#pragma mark PresentingViewController implementation
#implementation PresentingViewController
- (void) loadView
{
self.view = [[UIView alloc] initWithFrame:CGRectZero];
self.view.backgroundColor = [UIColor greenColor];
self.title = #"presenting vc";
self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAdd target:self action:#selector(add:)];
}
- (void) add:(id)sender
{
PresentedViewController* pvc = [[PresentedViewController alloc] init];
UINavigationController* navigationController = [[UINavigationController alloc]
initWithRootViewController:pvc];
navigationController.modalTransitionStyle = UIModalTransitionStyleCoverVertical;
[self presentViewController:navigationController animated:YES completion:nil];
}
#end
#pragma mark PresentedViewController implementation
#implementation PresentedViewController
- (void) loadView
{
self.view = [[UIView alloc] initWithFrame:CGRectZero];
self.view.backgroundColor = [UIColor yellowColor];
self.title = #"modal vc";
self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemDone target:self action:#selector(done:)];
}
- (void) done:(id)sender
{
[self dismissViewControllerAnimated:YES completion:nil];
}
#end
EDIT: In response to trojanfoe's answer, here are my thoughts why I believe I should be allowed to use Auto Layout for the root view:
UIWindow is derived from UIView, so I see no reason why I should not be allowed to install constraints in it.
It's true that there is no UIViewController instance that manages a UIWindow - but if we talk about the roles of the MVC design pattern, from my point of view the application delegate clearly takes the controller role, and therefore should be allowed to set up constraints.
To make this clear: If it's not possible to use Auto Layout on the root view for some technical reason, I will, of course, accept that. In fact, I want to be convinced not to use Auto Layout, but I prefer rational argument to blind coding.

Answer 1

I believe you are over-complicating the set-up of the views:
Constraints should be set on views only, not on the window.
The view controller is responsible for managing the view hierarchy and you don't need to be involved at all.
I think removing your constraints/view manipulation code will solve your issues:
PresentingViewController* pvc = [[PresentingViewController alloc] init];
UINavigationController* nc = [[UINavigationController alloc] initWithRootViewController:pvc];
self.window.rootViewController = nc;
nc.view.backgroundColor = [UIColor redColor];
[self.window makeKeyAndVisible];
Also shouldn't that be PresentedViewController given the navigation view controller will do the presenting?