Threads, tasks, and the Objective-C SDK

Thank you, Stephen, for all that work.  I will study this and incorporate as much of this into my design as I can understand.  There are a few simplifications I will make, though.  In my application the download only takes place when a certain view controller is active and the user is involved, if only as an observer.  Instead of a 5-second time-out on operations, there will be a "Cancel" button in the UI that the user can use to terminate anything that seems to be taking too long.  Also, I found that I don't really need a separate NSResponseQueue.  The main UI queue serves me just fine, and it makes it possible for me to access UI elements directly from within a response block.

I am a little curious why you found it necessary to put locks around accesses to boolean flags that are only set by one thread.  I think you need to have both threads modifying the variable to need a lock.