Threads, tasks, and the Objective-C SDK

Stephen,

Having worked a little more with the SDK and gotten more familiar with it, I don't think it needs to be changed.  Having API that block carries its own problems of interruptibility, etc.  It is only a couple of lines of code to add a custom wait for completion.

As far as the locked flags, I see why your implementation needs them. But mine is simpler.  I only have one single task that performs all the downloads sequentially.  I realize this probably means lower performance, but I think it is worth it for the simplicity.  Each download can potentially create an error requiring user response to "Skip or Abort".  The most logical thing for the user to expect if he Aborts is that none of the files after that file have been transferred.  But if there are simultaneous downloads, there may be several successful downloads that occur after the one that had an error, and by the time the user decides to Abort, those downloads have already completed.  So with only one task that is manually started, there is no question about which tasks need to be cancelled.