iPhone Dev Tip - Blocks are great, er.. kind of.

I found myself confronted by this rather vague and unexpected error today when dropping a new build onto a test device.

dyld: Symbol not found: __NSConcreteStackBlock

Vague because the symbol didn't reference any library that I was expecting to have been added and unexpected because the build was working on the other dev and test devices.

The only difference was the test device was running iOS 3.1 rather than the 4.x variants of the others, which was in fact the root of the problem.

In an animation block I had used the nice new method

[UIView transitionWithView:duration:options:animations:completion] 

Which takes a set of animation operations as a block, which is an iOS4 only feature. Thought it worth leaving a reminder as the usual stack overflow/google trawl didn't throw me any bones.