Combobox populate from HTTPService query

Question

I got this working with static XML files, but now its not
working with HTTPService! I'm having problems getting a ComboBox to
fill up with dynamic data from an SQL query which spits back XML
data. I am basically filling one ComboBox based on the selection
from another ComboBox. On startup, I call this method:
public function loadSystems(serv:HTTPService, s:String) :
void {
var params:Object = {};
params[s] = s;
serv.send(params);
To fill in the first ComboBox, which is defined as:
<mx:ComboBox minWidth="130" maxWidth="130" id="mainSys"
dataProvider="{sysOps.lastResult.system.data}"
change="fillComboBox(mainSys, subSys, 'subsystem', subOps,
sysIDs)" rowCount="10" />
It fills up fine, but the fillComboBox method (below) comes
up with a null result and fails
// fill destination combobox based on selection from src
combobox
public function fillComboBox(src:ComboBox, dest:ComboBox,
arg:String,
serv:HTTPService, idArray:ArrayCollection) : void {
var ind:int = src.selectedIndex;
// choose ID from the ID array with the selecetedIndex
var select:String = idArray.getItemAt(ind) as String;
var params:Object = {};
params[select] = select;
params[arg] = arg;
serv.send(params);
dest.dataProvider = serv.lastResult.system.data; //
serv.lastResult is null
I know the server side is getting valid arguments in params
and is spitting out valid XML so the problem is on the Flex side.
Any ideas or help would be greatly appreciated. Thanks!

Answer 1

Although caching is a common problem, that is not the cause
of your problem. In Flex, all HTTPService calls are asynchronous,
which means you cannot call the service and set the result in the
same function like you are doing in your fillComboBox method. You
need to add an event listener and get the result in a different
method. However, in your function you are also passing the
destination which complicates things. Instead of passing the
destination, you could try passing a function that will assign the
result to the component you want dynamically:
change="fillComboBox(mainSys, setSubSys, 'subsystem', subOps,
sysIDs)"
public function fillComboBox(src:ComboBox,
resultFunction:Function, arg:String, serv:HTTPService,
idArray:ArrayCollection) : void
serv.addEventListener(ResultEvent.RESULT,resultFunction);
private function setSubSys(evt:ResultEvent):void {
subSys.dataProvider = evt.result.system.data;
In general, it makes debugging much easier if you avoid
binding HTTPService results directly to components and instead use
a listener function where you can examine the result data for any
problems.
Vygo