ReactiveCocoa/ReactiveCocoa — pull request #540 by jspahrsummers (2013-05-30 06:20:39+00:00)
Title: Refactor RACCommand to work solely on the main thread

Fixes #533. Although I mentioned in the issue:

> It'll still be usable from the background, just less efficient.

… it turns out that supporting _partial_ background operation opens the class up to a whole host of concurrency issues. It's simpler and less error-prone to just require it to be used on the main thread.