It is possible to use swift files in cmake projects. In my case, I added a swift based UI test to an objc project. Here is the snippet that worked:

set(CMAKE_TRY_COMPILE_PLATFORM_VARIABLES "CMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_ALLOWED")
set(CMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_ALLOWED "NO")

set(CMAKE_Swift_LANGUAGE_VERSION 5.1)
enable_language(Swift)

unset(CMAKE_TRY_COMPILE_PLATFORM_VARIABLES)
unset(CMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_REQUIRED)

xctest_add_bundle(myapp-test myapp
  tests/test.swift
)

initialize_ios_target(myapp-test)

When I used pthread in iOS I ran into a problem where cmake is trying to compile a small tiny program to see if the toolchain is working. Same problem here, it failed for me in the code signing step and therefor I had to turn it off for the swift compiler detection.

I had to upgrade to CMake 3.16 for this to work.