Wrapping C Function Callbacks to Swift (Part 2)

In part 1 I discussed bridging a Swift reference through a C void pointer. This was for the purpose of wrapping C libraries but maintaining an object oriented approach that ties the callback to a specific instance.  However if you are not fortunate enough to have a C API that offers a void pointer to pass a context, then this approach will work as long as the callback function is called synchronously. That is to say the callback function is invoked before the C function returns and on the same thread as the call to the C function. While this is less common it can crop up from time to time. I found an occurrence of this when I wanted a B-Tree implementation and tried calling the DBM module of BSD.

Since the callback will be made synchronously on the calling thread. We can make use of a capability of dispatch queues, that lets us set a specific key/value pair on a dispatch queue and fetch it within the callback function. The name specific is likely taken from pthreads for thread specific data. To do this you need to create an instance of DispatchSpecificKey and then set the context you wish to pass to the callback function prior to calling the C function.