Wednesday, February 23, 2011

Repeat after me: enumerations should be type-safe!

We're doing some iPhone code maintenance at Oxen. It is a legacy application, created by another company.

As you may know, iPhone applications are built using Objective C and the Apple platform, Cocoa.

The customer requested me fixing an error that was related with the device orientation. I never have had to deal before with orientation handling, so I started looking at the code:

UIInterfaceOrientation currentOrientation = [[UIDevice currentDevice] orientation];

The UIInterfaceOrientation enumeration has 4 different values:

typedef enum {
    UIInterfaceOrientationPortrait           = UIDeviceOrientationPortrait,
    UIInterfaceOrientationPortraitUpsideDown = UIDeviceOrientationPortraitUpsideDown,
    UIInterfaceOrientationLandscapeLeft      = UIDeviceOrientationLandscapeRight,
    UIInterfaceOrientationLandscapeRight     = UIDeviceOrientationLandscapeLeft
} UIInterfaceOrientation;

Objective C treats enumeration as integers. The problem was that, when the device was face up, the orientation returned by UIDevice was 5. This didn't seem a valid value.

After hour struggling with this issue I realized that the enumeration wasn't correct. The correct one was UIDeviceOrientation:

typedef enum {
    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
} UIDeviceOrientation;

which included all the possible values.

Some code used this enumeration, so I discovered it accidentally (not reading the documentation was my responsibility). At first time I thought I was crazy: I didn't noticed that there were two different enumerations. Magically, the missing options appeared. Finally I realized about the error and I felt better about my mental health.

My point here is that type-unsafe enumerations (such as the ones provided by Objective C) are very error prone. Java guys solved this problem with enums in Java 5 release. It would be nice to have something similar in Objective C. A silly error like this can make you loss valuable time.

Yes, I know. This is not the best example, because the first enumeration was defined using the second one, so the values were interchangeable. But it can be confusing when you're fixing bugs at 2:00 PM :)


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