Sharing code between original iOS App and App Extension
Nothing should be copied. Just add required files to compile sources for your extension:
Just make sure that the files you want to share gets a tick in the section Target Membership.
Select your file, open the utility panel on the right and tick the appropriate target under Target Membership.
Embedded frameworks are a great way to reuse code between your application and your extension. Having moved some of the code to embedded frameworks will provide us with a great opportunity to write some unit tests around it.
See section Using an Embedded Framework to Share Codehttps://developer.apple.com/library/ios/documentation/General/Conceptual/ExtensibilityPG/ExtensionScenarios.html#//apple_ref/doc/uid/TP40014214-CH21-SW1