Device orientation vs interface orientation

Just today I got bit by confusing device orientation and interface orientation. I really should know better. Device orientation is of course the orientation that the device is currently being held in, while interface orientation is the orientation of the running app’s user interface.

What I was trying to do was to hide the status bar while in landscape mode and show it in portrait mode for an iPhone app that operates in the 3 principal orientations: portrait, landscape left, and landscape right.

To achieve that I was using code like this:

#pragma mark - Status Bar

- (BOOL)prefersStatusBarHidden
    return (UIDeviceOrientationIsLandscape([[UIDevice currentDevice] orientation]));

#pragma mark - Orientation

- (void)willAnimateRotationToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration
    [super willAnimateRotationToInterfaceOrientation:toInterfaceOrientation duration:duration];
    [self setNeedsStatusBarAppearanceUpdate];

This works fine at first blush, especially in the simulator, but not so well in practice. First off, the app doesn’t even support all 4 possible interface orientations (the fourth being upside down portrait), so what happens when the phone is held upside down? Well the interface orientation doesn’t change from its previous orientation (most likely landscape) but the device orientation is not landscape and so the status bar appears. Bug.

But even worse, there are additional device orientations (namely face up and face down) that are neither portrait nor landscape and have no matching interface orientation. If the phone was last in landscape interface orientation and then gets laid flat on the desktop, the device orientation is no longer landscape (it is flat), and so the status bar appears. Bug again.

Just for the record, here’s the definition of UIDeviceOrientation:

typedef NS_ENUM(NSInteger, UIDeviceOrientation) {
    UIDeviceOrientationPortrait,            // Device oriented vertically, home button on the bottom
    UIDeviceOrientationPortraitUpsideDown,  // Device oriented vertically, home button on the top
    UIDeviceOrientationLandscapeLeft,       // Device oriented horizontally, home button on the right
    UIDeviceOrientationLandscapeRight,      // Device oriented horizontally, home button on the left
    UIDeviceOrientationFaceUp,              // Device oriented flat, face up
    UIDeviceOrientationFaceDown             // Device oriented flat, face down

And here’s the definition for UIInterfaceOrientation:

typedef NS_ENUM(NSInteger, UIInterfaceOrientation) {
    UIInterfaceOrientationPortrait           = UIDeviceOrientationPortrait,
    UIInterfaceOrientationPortraitUpsideDown = UIDeviceOrientationPortraitUpsideDown,
    UIInterfaceOrientationLandscapeLeft      = UIDeviceOrientationLandscapeRight,
    UIInterfaceOrientationLandscapeRight     = UIDeviceOrientationLandscapeLeft

It’s interesting that UIInterfaceOrientation is defined in terms of UIDeviceOrientation.


So what I should have been doing was this:

#pragma mark - Status Bar

- (BOOL)prefersStatusBarHidden
    return (UIInterfaceOrientationIsLandscape([[UIApplication sharedApplication] statusBarOrientation]));

This works as expected when the device is upside down or flat.

A potential pitfall of CGRectIntegral

This morning while I was checking an app for misaligned elements, I happened upon a misaligned button. (If you’re not using either the iOS Simulator or Instruments to check your app for misaligned images, you should be, but that’s a post for another day.)

Checking the code it was obvious to me where the problem was.

backButton.frame = CGRectMake(5, (navigationBar.bounds.size.height
    - imageBack.size.height)/2, imageBack.size.width,

Centering code is especially prone to pixel misalignment. In this case imageBack has a size of (50, 29) while the navigationBar has a height of 44 points. The code above generates a rect with origin = (5, 7.5) and size = (50, 29). So the image ends up vertically misaligned, which in turn makes the child text label inside also misaligned, and hence they show up painted in magenta when the Color Misaligned Images option is checked in the iOS Simulator Debug menu.

This looks like a job for CGRectIntegral, right? But when I change the code to this:

backButton.frame  = CGRectIntegral(CGRectMake(5, 
    (navigationBar.bounds.size.height - imageBack.size.height)/2,
    imageBack.size.width, imageBack.size.height));

I end up with this:

The button is no longer misaligned, but it is now being stretched (hence the yellow wash). Debugging shows that CGRectIntegral has converted the input rect of (5, 7.5) x (50, 29) into (5, 7) x (50, 30). So now the image is being stretched vertically by 1 point. That might be fine for UILabel but not for an image.

The other issue with using CGRectIntegral is that the original rect is actually fine for retina devices because they have 2 pixels per point, so a value of 7.5 actually falls on a pixel boundary, and is the optimal centering for this image. If we adjusted it to origin.y = 7 (without stretching) then it would be 2 pixels closer to the top than to the bottom on a retina device.

I’ve written some helper functions to correctly pixel align rectangles (not point align) for both retina and non-retina screens, and posted them in this gist.

Under non-retina it would convert the rectangle to (5, 7) x (50,29) to pixel align it without stretching, while under retina it would leave the rectangle unmodified at (5, 7.5) x (50, 29).

This finally clears the magenta (alignment) and yellow (stretch) washes from the button:


According to the Apple Documentation for CGRectIntegral:

A rectangle with the smallest integer values for its origin and size that contains the source rectangle. That is, given a rectangle with fractional origin or size values, CGRectIntegral rounds the rectangle’s origin downward and its size upward to the nearest whole integers, such that the result contains the original rectangle.

The fractional origin of (5, 7.5) is rounded downward to (5, 7), but I initially thought the size would be left unmodified (not rounded up) because it already comprises 2 whole integers. But that wouldn’t contain the original rectangle, whose lower right corner is positioned at (55, 36.5). In order to contain the original rectangle, the height has to be increased by 1 point from 29 to 30.

Don’t rename your .xcdatamodeld file

Ok, you can rename it but be prepared for weird bugs with Xcode 4.3 (and possibly earlier versions).  In my case I created a new Core Data model file and added it to an existing project.  Then I renamed the model file before attempting to build the project.  Yet even with a clean build and install (deleting the app from the device first), NSManagedObjectModel -initWithContentsOfURL returned nil even though my NSURL pointer was non-nil.

NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"ModelName"
__managedObjectModel = [[NSManagedObjectModel alloc] 

Short answer: quit and reopen Xcode to fix the problem.

(Solution via Stack Overflow)

Just a black screen on the iPod touch

The other day I ran into this with a universal app. It ran fine on the iPad and on the iPhone, but produced the above when run on an iPod touch. The app would briefly show its splash screen, and then go black. My first thought was that the app had hung during initialization or it had crashed. The truth (as usual) was much simpler. As outlined in this StackOverflow post (don’t look at the answers, look at the author’s own comment on his question), the trouble is that we had specified a nib for both iPhone and iPad but not for iPod touch (this is more likely to happen if you’ve been manually messing around with your app’s plist).

You can fix it by editing the plist directly or by changing the settings under the Info tab of your target’s settings.  If your settings look like this:

Either change “Main nib file base name (iPhone)” to “Main nib file base name” (this will use the same nib for both iPhone and iPod touch) or create a new entry with “Main nib file base name” (this will allow you to use a different nib for iPhone and iPod touch, although I’ve yet to notice an app that has done so).

If you already have a valid entry for “Main nib file base name”, then you have another issue altogether…