Simple fetch data into String or parse JSON or maybe how to iterate on DataSource?

[bartosak]

SmartClient Version: v8.3_2012-11-20/LGPL Development Only (built 2012-11-20)

Hello,
I was trying to do apply ComboBoxItem validation functionality into TextItem. Giving DataSource into ComboBoxItem it acts like to have IsOneOfValidator. FormItem autovalidates to have value specified in dataSource.
I want only to have that validation without PickList and autocomplete.
My DataSource is:

public class SupervisorIdsDataSource extends DataSource {
    private static String TARGET_URL = "ds/get_supervisors";
    private static String DS_ID = "SUPERVISORS_DS";
    private static SupervisorIdsDataSource instance = null;
    private SupervisorIdsDataSource(String id) {
        super(id);
        setDataFormat(DSDataFormat.JSON);
        setDataURL(TARGET_URL);
        DataSourceIntegerField field = new DataSourceIntegerField(BasicDataFieldNames.SUPERVISOR_ID);
        setClientOnly(true);
        setFields(field);
        setCacheAllData(true);
    }
    public static SupervisorIdsDataSource getInstance(){
        if (instance == null) {
            instance = new SupervisorIdsDataSource(DS_ID);
        }
        return instance;
    }
}

and "ds/get_supervisors" is:

[
    {supervisorId: 100000},
    {supervisorId: 120000},
    {supervisorId: 134000},
    {supervisorId: 145220},
    {supervisorId: 524520}
]

Maybe other aproach exists ? How to add IsOneOfValidator to TextItem fetching JSON data from server?

PS. Curently I dont use any WebServices but only hardcoded static files to prepare proof of concept that this technology (SmartGWT) meets my client needs.