Update: It's an oversight on Apple's part. Now you have to read it all, no more tl;dr
Originally, I created this project to get help on a question, but a nice side effect of preparing sample code, and explaining it to an audience is that you carefully reexamine what you did — and you find the solution yourself.
The key question was:
Why does
if (&UIPageViewControllerOptionSpineLocationKey != NULL)
cause dyld to fail on iOS 4 (effectively crashing the app on launch), whereas
if (&UIPageViewControllerOptionInterPageSpacingKey != NULL)
does not?
The Apple Documentation says that this should work, if, well if:
- "The base SDK for your Xcode project must be iOS 4.2 or newer.": Check
- "The deployment target for your project must be iOS 3.1 or newer.": Check
- "The compiler for your project must be the LLVM-GCC 4.2 compiler or newer": Check
- "You must ensure that any frameworks not available in your project’s deployment target are weakly linked, rather than required": Che… oh, wait!
The framework in question is actually UIKit.framework. While it is most certainly available on any iOS, it is not available in the version we are looking for.
In order to allow weak linking for UIPageViewControllerOptionSpineLocationKey, it is necessary to go to the target's Build Phases, and in the "Link Binary With Libraries" section set the UIKit.framework to "Optional".
This will fix the issue.
What still puzzles me is why UIPageViewControllerOptionInterPageSpacingKey does not require this step. Any hints are appreciated.
I got the answer to my last question from David Duncan, and it seems to be an oversight on Apple's part. If we look at the headers, we see
UIKIT_EXTERN NSString * const UIPageViewControllerOptionSpineLocationKey;
UIKIT_EXTERN NSString * const UIPageViewControllerOptionInterPageSpacingKey NS_AVAILABLE_IOS(6_0);
Because of the NS_AVAILABLE macro, the linker knows that UIPageViewControllerOptionInterPageSpacingKey needs to be weak linked. It's missing for UIPageViewControllerOptionSpineLocationKey, and that requires the developer to designate UIKit.framework as Optional in the project.
So another solution is to redeclare the identifier with the appropriate NS_AVAILABLE_IOS(5_0) macro. This makes weak linking UIKit.framework unnecessary.