ControlContext :: value - Get Value handle
The function returns value handle that acts as data source for the control.
In case of user-defined data sources, controls may (temporarily) not refer to a data source. When no data source is available for the control, the function throws an exception. In order to avoid exceptions, one may call hasData() before calling value().
In case of combo-boxes, the value handle returns the value handle for the combo box data source.
Return value: Value handle ( odaba::Value & )
The value handle is a reference to an (usually) opened value. Invalid or not opened value handles my cause an exception.