It dawned on me trying to setup speed/cadence on my indoor trainer since it is fixed single gear without freewheel that cadence can be easily calculated from speed sensor.
Except both the garmin speed2 sensor and my Fenix are unwilling to do that natively.
There is an app for Edge bicycle computers to kind of do what I want but I need it for Fenix and I really would like it to save/inject into the FIT file and replace/simulate the native cadence/rpm field if possible? (can the api do that?)
vaguely similar app for Edge:
https://apps.garmin.com/en-US/apps/d1b5300f-73f1-45ee-982c-631496844381
I guess this app could calculate the field but won't save it as a native field replacement?
https://apps.garmin.com/en-US/apps/639f34d0-62d5-425a-a459-4b1a93c87626
I remember there is another custom calculation datafield that can write to FIT files under it's own name, can't remember which one at the moment.
aha! Appbuilder - still has the problem of not writing the native cadence field
https://apps.garmin.com/en-US/developer/8545c609-c030-4c0c-b3d8-4bc63ebdc901/apps?sort=name