I’m building a WatchKit extension for an existing iOS app and have been experimenting with the “plumbing” between the two. Watch extensions are different animals than other iOS 8 extensions, but there is a lot of overlap in this area. Today I made a diagram to show the options of getting data to the iPhone app from the Watch and vice-versa.

'a flowchart diagram'

This is simply a graphical representation of the summary presented in a Developer Forums thread.

The real gem in this is that Darwin notifications (CFNotificationCenterGetDarwinNotifyCenter) can be used to notify the other processes that there is new data available in the shared container. I haven’t worked with Darwin notifications before, but the big difference from NSNotifications is that you can’t pass any data along with it, just an identifier. Oh, and they are inter-process, which is essential for this type of communication.

Note that “write data to shared container” includes any API that writes out files - just point it inside the NSURL returned -[NSFileManager containerURLForSecurityApplicationGroupIdentifier:].


Added in beta 3, this method and the corresponding application:handleWatchKitExtensionRequest:reply: UIApplicationDelegate method provides a unique option for WatchKit extensions. It is the only way to explicitly wake up the containing iOS app from an Apple Watch if it is suspended or not running, but note that the iOS app is awoken in the background. The reply block allows the iOS app to return a small amount of data to the WatchKit extension, but care must be taken with it. You’ll get a warning if the logic on the iOS app side doesn’t call the reply block. Also, ensure that the previous call to openParentApplication:reply: has finished before making another. I’ve had calls ignored because a previous one was in flight.


If you’re using NSUserDefaults, it’s just an API on top of a plist file. When you stand up an NSUserDefaults instance using -initWithSuiteName: the data ends up in a file <SHARED_CONTAINER>/Library/Preferences/<APP_ID>.plist (where SHARED_CONTAINER is something like ~/Library/Developer/CoreSimulator/Devices/1A8DF360-B0A6-4815-95F3-68A6AB0BCC78/data/Container/Data/Application/). That file will be written to every time you call +synchronize or at “periodic intervals”.

Other Tidbits

MMWormhole is a very neat library which handles a lot of these details for you providing a simple API for passing messages between an iOS app and extensions.

Tom Harrington has some great extension development tips.

Remember to stay away from the file coordination APIs.