Static Library and Swift
As mentioned, Apple does allow Swift in static libraries as of Xcode 9 Beta 4.
We attempted to do this on an existing project with an Objective-C-based target and "child" static library projects and kept running into a linking error
ld: library not found for -lswiftSwiftOnoneSupport for architecture x86_64
ld: library not found for -lswiftDispatch for architecture x86_64
This is because the main target (app) is trying to build solely against Objective-C and isn't told by the static library that it needs to include Swift libraries. This was because there weren't any Swift files in the
Compile Sources section of our
Build Phases for the app target.
So basically all you have to do is add at least one
.swift file to that compile list and it will include the Swift libraries for you. It doesn't even need to have any code or values in it, it can be an empty file.
Then you can start adding Swift files to your "child" static library project. I would let it generate the bridging header for you at first then you can move it around and change what gets imported (make sure the project points to the right file in the build settings if you move it).
You should still keep in mind that using Swift and Objective-C within the same static library may have issues of its own. I suggest reading the Apple developer doc "Swift and Objective-C in the Same Project" on how to address importing Objective-C into Swift (using a bridging header) and how to use the Swift files in your Objective-C code (importing the generated
-Swift.h for your library).