In most cases the WatchKit App you develop has to exchange data with its iOS App. In earlier versions of watchOS it was enough to define an app group to access the same data from the iOS app and from the WatchKit Extension. But since watchOS 3 things have changed. The apps for watchOS became native, i.e. they are natively executed at the Apple Watch. This implies of course changes in the way how the data is exchanged.
Watch Connectivity Framework is the way Apple provides to synchronize data with WatchOS Apps.
If your existing Watch app and iOS app use a shared group container or iCloud to exchange data, you must change the way you exchange that data in watchOS 2. Because the WatchKit extension now runs on Apple Watch, the extension must exchange data with the iOS app wirelessly. You can do that using an NSURLSession object or using the Watch Connectivity framework, which supports bidirectional communication between your iOS app and WatchKit extension.
To see a corresponding session click here: WWDC 2015 – Session 713 – watchOS
I could also recommend an excellent tutorial/overview for beginners: watchOS 2: How to communicate between devices using Watch Connectivity
Short summary of the most important things about Watch Connectivity Framework:
- Two separate stores have to be maintained. There is no automatic way for Core Data data to be synchronized with the watch app
- There are two communication categories: Background transfers and Interactive messaging
- Background transfers can be: Application context, User Info Transfer and File transfer. Application context is always overridden by the latest data when waiting in the transfer queue. User Info realizes FIFO principle, i.e. all data in the queue will be delivered. File transfer is self explaining.