onChanged property

Called when the user selects an item.

If the onChanged callback is null or the list of Combobox.items is null then the combobox button will be disabled, i.e. its arrow will be displayed in grey and it will not respond to input. A disabled button will display the Combobox.disabledHint widget if it is non-null. If Combobox.disabledHint is also null but Combobox.placeholder is non-null, Combobox.placeholder will instead be displayed.