is this possible using their NFC? and if so can it be done continuously as a background running app that wont require the user to constantly launch?
I found this solution : Passing data to Apple Watch app but wasnt sure if it was just from Watch -> to non-watch Devices only.
Currently there is no API to do this, what is more there is no API that expose hardware like Bluetooth, NFC, heart-rate sensors etc.
I assume that the Watches are from different people and hence do not have the same apple id used to sign in to their companion iPhone. This would preclude a use of hand-off to mediate data transfer.
What you could do is have a web server that holds the data you want to communicate, and use iPhones to get notifications from it. Then post those to the Apple Watches.
