Activates the session asynchronously.
- iOS 9.0+
- watchOS 2.0+
This method executes asynchronously and calls the
session(_: method of your delegate object upon completion. Call this method when your app is ready to communicate with its counterpart. Your cannot send or receive messages until you call this method. If the
delegate property is
nil, calling this method logs an error.
In watchOS 2.1 and earlier, this method activates the session synchronously and always results in an active session.