I’ve been doing a ton of thinking about data persistence and message passing recently. Your main iOS app runs in a separate process with its own sandbox to your WatchKit extension. Which makes it quite tricky to keep your App and the WatchKit extension in sync without deadlocks, data conflicts, etc.
I had been starting to work on a mechanism to work around this by building a messaging mechanism between the two. The fine folks at Mutual Mobile just saved me the effort. Their MMWormhole library does exactly what I want. It is a dead simple messaging scheme for sending basic commands back and forth (in real-time if both are running).
I haven’t integrated with it yet but reading through its code it is 99% what I would have built so it seems extremely promising.