Can't simulate SimpleDataField

Hello,

I'm new in Monkey C,
I'm Trying to copy the SimpleDataField example but when I try to simulate it the watch face is always black

And other question. What is the way to put several SImpleDataField together?

The code is very simple:

using Toybox.Application as App;
using Toybox.WatchUi as Ui;

class SDF_PoCApp extends App.AppBase {

function initialize() {
AppBase.initialize();
}

//! onStart() is called on application start up
function onStart() {
}

//! onStop() is called when your application is exiting
function onStop() {
}

//! Return the initial view of your application here
function getInitialView() {
return [ new SDF_PoCView() ];
}

}


using Toybox.WatchUi as Ui;

class SDF_PoCView extends Ui.SimpleDataField {

function initialize() {
label = "My Label";
}

function compute(info)
{
return 0 ;
}

}


Thanks