So I've had a look around and I cant seem to find an answer anywhere so here goes. Is it possible with the MS Band SDK to run a function within my app when the user taps a button?
Currently (at the time of writing) there is no way for the user to directly interact with a tile-app and thus pass a response to the application installed on the phone*
Your options are (as I see it):
To use the sensors to define 'gestures'**
Guide the user to use Cortana to provide speech commands ***
*This might change, but due to the very little storage capacity on the band if this was added I would assume only very basic interaction such as yes/no/cancel dialogos etc. and simpler responses using the keyboard when/if it becomes available for third party tiles.
**There is currently a bug with background work so you might have to prevent the lock screen from locking while receiving and interpreting sensor data on the phone which will impact the battery on the phone. This is expected to be fixed soon.
*** Speech commands are well supported on Windows Phone but I'm unsure how well supported they are on iOS and Android