Creating a sub select list from parent select list in flutter

Im creating an application in flutter, I want to display a selected list - which is composed from:

1. text(string)
2. textField
3. list when I pressed in list element- display another selected list which composed from the list's values. I began to write the code, using in DropDownButton but I got stuck at the part with 'sub-category select list'

Im new in flutter, I'll be happy for any help.. If necessary - I will share the code ...