A work in progress
Fitsy is a pure Swift library that can read and write FIT files. A couple of other libraries are available that offer Swift APIs to read and write FIT files. Those may be more complete, and depending on your needs, may be better suited to your needs.
Fitsy's advantage is that it's 100% Swift, and doesn't interoperate with the official C++ FIT SDK https://www.thisisant.com/resources/fit-sdk-beta/, which means Fitsy is fully cross-platform, and does not rely on the Obj-C runtime, which is only available on Apple platforms.