Currently, Titanium's ButtonBar is an iOS only feature. We should add support on Android for parity.
On iOS, a Titanium ButtonBar works just like a Titanium TabbedBar. Only one button/tab can be selected at a time. The only difference is the appearance. On iOS, a ButtonBar is implemented via UISegmentedControl.
Implement it as a radio button group. This seems to be the most appropriate solution. And I think this would be the solution on Windows as well.
Alternatively, implement it just like TabbedBar. Have the Android ButtonBar proxy derive from the TabbedBar proxy and have them use the same implementation. Ideally implemented via the Java TabLayout or BottomNavigationView.