class UIInstance getElementById(id) returns a Generic object if the widget is a ListBox - listbox

Question

I have an app, which display a panel with fields filled from a document.
The user edit the fields, and the updated values updates the document.
Everything is working, with one exception.
The panel has several TextBox and a ListBox. When the panel is displayed, a function fills the values in the text boxes. It uses getElementById in order to get the widgets.
I would like to show the ListBox with the proper value already selected. Unfortunately, the getElementById on this widget returns a Generic, and the standard methods of ListBox aren't available.
Here is a fragment of code:
function fillSingleDocFields(meta, app) {
var eleTitolo = app.getElementById('idnomeDoc'); // OK!
eleTitolo.setText(meta.titolo);
var lbTipologia = app.getElementById('idtipologia');
Logger.log(lbTipologia); // KO
setListBoxSelected(lbTipologia, meta.tipologia);
return app;
}
the setListBoxSelected function fails on the first ListBox specific method:
function setListBoxSelected(lb, value) {
for (var i=0;i<lb.getItemCount();i++) // runtime error
{
if ( lb.getValue(i) == value)
lb.setSelectedIndex(i);
}
}
The error is (translated from italian): no getItemCount function in Generic object.
Here is the code execution transcript:
[13-07-08 12:13:01:691 CEST] (class).getElementById([idnomeDoc]) [0 secondi]
[13-07-08 12:13:01:692 CEST] (class).setText([mio titolone 2]) [0 secondi]
[13-07-08 12:13:01:692 CEST] (class).getElementById([idtipologia]) [0 secondi]
[13-07-08 12:13:01:692 CEST] (class).toString() [0 secondi]
[13-07-08 12:13:01:692 CEST] (class).toString() [0 secondi]
[13-07-08 12:13:01:692 CEST] Logger.log([Generic, []]) [0 secondi]
[13-07-08 12:13:01:693 CEST] (class).toString() [0 secondi]
[13-07-08 12:13:01:735 CEST] Esecuzione non riuscita: TypeError: Impossibile trovare la funzione getItemCount nell'oggetto Generic. (riga 29, file "Controller") [0.404 secondi di esecuzione totale]
A possible solution could be to set the selected value in the ListBox when I create it, but unfortunately, at that moment, that values is unknown.
My questions are:
is it possible to cast the result of getElementById? (I don't think so, looking to similar questions);
is there something wrong in my approach? Or is the getElementById always returning Generic objects (this contrast with other answers).
Thanks

Answer 1

Your issue is not with the generic widget returned by getElementById. Which, even though does not autocomplete specific methods anymore, still accepts that you call it.
The issue is that the ListBox object does not have a getItemCount method. Here's is a quick test:
function testList() {
var app = UiApp.createApplication().setTitle('Test');
var list = app.createListBox().addItem('test');
Logger.log(list.getItemCount());
return app.add(list);
}
And for that matter, it also does not have a getValue method.
Usually Apps Script widgets does not have such methods (for you to get things). You have to either know that beforehand or get it from a callback-element, by inspecting the parameter passed to your callback-handler function.

Answer 2

Unfortunately there is no implementation of this function. (see issue)
But you can pass values ​​through the callback in handler function. See example.
function ui(){
var app = UiApp.createApplication();
var lb = app.createListBox().setName('lb'); //not id
lb.addItem('first item').addItem('second item');
var hndlr = app.createServerHandler('send')
.addCallbackElement(lb); //be sure to add callback
var btn = app.createButton('SEND');
btn.addClickHandler(hndlr);
app.add(lb).add(btn);
SpreadsheetApp.getActiveSpreadsheet().show(app);
}
function send(e){
var app = UiApp.getActiveApplication();
var lbValue = e.parameter.lb; //selected value
//do somethink with lbValue
app.add(app.createLabel(lbValue));
return app;
}