I've added a Today Extention Widget Project in my Xcode workspace and I want to use some source file from the main app, so I've flagged the "target membership" on this files. The problem is that the Xcode linker fail with duplicate symbol error when compiling the main app. So what is the correct way to share source file between main app and widget?
You can use Frameworks to share code/files between your Host App and App Extension.
You can create an embedded framework to share code between your app extension and its containing app. For example, if you develop an image filter for use in your Photo Editing extension as well as in its containing app, put the filter’s code in a framework and embed the framework in both targets.
For more on sharing scenarios between Host App and App Extension refer to: https://developer.apple.com/library/content/documentation/General/Conceptual/ExtensibilityPG/ExtensionScenarios.html#//apple_ref/doc/uid/TP40014214-CH21-SW1