I have some Objective C classes that I've built for use in a project. They are model and utility classes, and have no direct UI responsibility. To aid in automated testing and debugging, I've built them as a project that creates a static library. The project has a test target that runs suites of automated unit tests, and a library target that builds a C-style static library archive binary. The install configuration of this target copies the library to $HOME/lib/ , and the class headers to $HOME/include/$LIBNAME.

This way when I use these classes in another project, I can just #import the headers in the sources, add the static library to the project frameworks list, add the include and lib directories to the compiler and linker search paths in the XCode target inspector, and build as normal. Build times are reduced, base classes are frozen in a stable, well tested implementation, code re-use is easier, everybody wins.

Recently I broke this happy pattern, a little perplexingly, with what I thought was a fairly innocuous piece of refactoring. I noticed that one of my classes was rather a simple set container, and its implementation really little more than a thin wrapper around NSMutableArray , with only a trivial specialisation of behaviour. As it was only used within a parent class structure, with no interface outside the library innards, it seemed a bit of overkill to have it implemented as a sizeable class. The special behaviour really boiled down to maybe two additional methods on top of the normal array interface.

I first refactored it to be a subclass of NSMutableArray, but that actually introduced more complexity. NSMutableArray is implemented as a class cluster , with an abstract API around a private hidden shadow class. In order to subclass it, you are expected to provide your own implementations of a subset of its interface. In my case, this would have made for more code than the class I was trying to replace.

Of course, Objective-C allows you to define categories on any existing classes. Categories allow you to formally define and implement additional methods onto an existing class definition at compile time. I could re-implement my class as a tiny category on NSMutableArray, removing lots of my code, and reducing the size of my library footprint and perhaps add some value by introducing NSMutableArray's extensive interface.

Surprisingly, it wasn't plain sailing. Coding up the category, and tweaking the library to use NSMutableArray in place of the now-redundant class was straightforward. Once the updated code passed the original test suite, it was deployed as a library. The first time I built a project using it, it crashed on startup, with an unhandled exception. I cleaned all targets and rebuilt. Same problem. I checked the library headers to confirm that the new data structures were properly defined on include. No problems there, but still a hard crash on initialisation.

The system logs had an entry for the crash; selector not recognized attached to symbols that were recognisably the new array methods from my category. Running 'nm' against the library file showed the symbols present, and correctly defined as a category on NSMutableArray. I was stumped. After a bit of googling, I came up with the correct solution.

It turns out that in order to link against a static library that contains Objective C categories, you need to pass the linker a special flag, '-ObjC'. Adjusting the build settings of my project to include this flag in the 'Other linker flags' entry of the target inspector fixed it so that the symbols are correctly resolved at runtime. Here is the official word, Technical Q&A QA1490.

posted by cms on 2008-09-24 12:03:31