iPad App Crashes on Orientation change

Question

I am developing an iPad app. I allow both landscape and portrait mode. My UI is fine in portrait mode but when I change it to landscape mode, my UI gets messed up. I saw some SO posts related to this and I added following code in initWith... in my UIView.

[[UIDevice currentDevice] beginGeneratingDeviceOrientationNotifications];
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(abc)
                                             name:UIDeviceOrientationDidChangeNotification
                                           object:nil];

My UI is working fine in portrait mode after doing this. When I change it to landscape mode, my UI is fine. But after I change it back into portrait mode, my app crashes. I read some posts on SO related to app crashing an got to know about instruments. I enabled zombies and found that a message is being sent to already released object and this message is coming from NSNotificationCenter.

Is there something else that I need to handle apart from registering my device ? Also, is there any way where in I can change the implementation from UIView to UIViewController and implement the methods that UIViewController has regarding device orientation ? Please let me know the steps I need to follow in order to get this done. Thanks!

Answer 1

Where are you registering for the notifications? You need to remove the observer when you are about to change orientations (either in prepForSegue or willAnimateRotationToInterfaceOrientation depending on however you've got your setup) in order prevent messaging a no longer valid object. You also don't want to pile up several notifications if your registering in viewDidAppear/viewWillAppear.

Remove the observer using:

[[NSNotificationCenter defaultCenter] removeObserver:self];//removes all notifications for that object (the way I've used it before)

or if you want to be specific, do something like:

[[NSNotificationCenter defaultCenter] removeObserver:self name:UIDeviceOrientationDidChangeNotification object:[UIDevice currentDevice];//remove just that notification

Answer 2

The UIViewController class has several methods that deal with changes in orientation. See the docs for a discussion of those methods.

One method you should look into is viewWillLayoutSubviews. This is a common place to perform manual view layout. This is called anytime the view controller's orientation changes.

Using these methods is much more common than registering for device orientation change notifications. Based on your statements about the crash, a possible issue is that you never remove the observer that you add. For every call to addObserver there must be a corresponding call to removeObserver. Otherwise the observer is called even if it has long gone away. And this results in the crash you describe.