tested on sapui5 version 1.28.6
In SmartFilterBar, if I define the a custom control, say checkbox. When press "search" button of filterbar. The filter value of the checkbox is not included as the filter of odata request. Thus the custom filter control has no use in query.
I think the problem maybe is that there's no documentation to describe, for a custom control, how to pass the value back to smartFilterBar to form a filter. Since custom control can be any type of control, there's no way for smartFilterBar to know how to get the value from the custom control and perform search with it.
If there's a way to do that, please advise.