As noted elsewhere (on slack channel):
“There’s no SDK for Dart/Flutter for integrating with ODK. If you want to build an app that communicates with ODK servers (like Central, Kobo etc) the Open Rosa standard documents an HTTP API: https://docs.getodk.org/openrosa/.”
Basically, ODK Collect mobile app (which KoboCollect is just a custom fork of) is a native mostly-Java Android app. There’s also a native iOS mobile client in the works (watch this space…) but AFAIK nobody has written a Flutter ODK-compatible XForms client yet.
As noted above, if you want to perform data collect on a mobile device with a Flutter app, and push the data up into the ODK Central or KoboToolbox backends, your best bet is probably to write a flutter wrapper around the existing Enketo web forms XForms web client. Enketo can operate offline BTW.
Or else write a new Flutter XForms client that can talk to these backends via the requisite Openrosa API standard.
