As a proof of concept of API client library, here you can find the android EET client. It registers predefined sale to playground for now. You can try the OpenEET app on Google Play
Current features:
- silly (but working) persistence !
- online/offline registration
- show sales registered using the app
- registers to playground v3
Planned features:
- register sale in several modes (simple, simple dph, advanced)
- send text/image/pdf receipt via android sharing
- export sales in csv format
- print recepit