Question

I’m using the following code to detect/listen for when the iPad changes device orientation.

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

This calls my didRotate: method when something changes. In my didRotate: method, I use UIDeviceOrientation orientation = [[UIDevice currentDevice] orientation]; to figure out what the orientation is, and apply my code accordingly.

However, in real life testing, I noticed that didRotate: gets called every second if the iPad is in a person’s hand. It appears the listener is literally listening for every little tilt and shift in the iPad, which obviously happens a lot in a person’s hands (as opposed to flat on a desk).

Any ideas on how I can fix this? I could change my code to use interface orientations, but I’ve been having trouble with it for whatever reason. Thank you.

*UPDATE: This listener is created in my UIImageView subclass. There are about a dozen or more on the screen. I have added [[UIDevice currentDevice] endGeneratingDeviceOrientationNotifications]; to my removal methods for when I remove an instance. That helps explain why didRotate kept showing up so much (my error).

However, I have narrowed down crashes: whenever I remove this an instance of this subclass, and rotate my iPad, I crash. Sometimes I get weird messages like [__NSArrayM didRotate]: is unrecognized selector (and other wierd objects like UIPanVelocity…something). Is my listener still listening after the instance is removed?

FIXED: Thank you for your help. I finally noticed what was wrong. I was removing the instance without removing the observer and ending notifications. Adding following code into my instance removal methods fixes my problem:

[[UIDevice currentDevice] endGeneratingDeviceOrientationNotifications];
[[NSNotificationCenter defaultCenter] removeObserver:self];

Answer 1

I’m guessing that you add observers repeatedly and never remove them. This means that the same entry point may get invoked multiple times for a single event. It also means that when the “self” object goes away you get a crash.