The delegate assigned when this object was created.
- iOS 7.0+
- macOS 10.9+
- tvOS 9.0+
- watchOS 2.0+
This delegate object is responsible for handling authentication challenges, for making caching decisions, and for handling other session-related events. The session object keeps a strong reference to this delegate until your app exits or explicitly invalidates the session. If you do not invalidate the session, your app leaks memory until it exits.