Update list of objects inside method Flutter / Dart - flutter

Question

The code reads a firebase database then add each read in a list of objects (SalesData class), chartdata.
The problem is when I update the list (chartdata) in the function readData() I can print it inside databaseReference.once().then((DataSnapshot snapshot) {}, but when it out of this bracket it is always empty. I an wondering how I could use this list in the chart constructor "dataSource: "
P.S if I hot reload the chart builds fine!!
class Home extends StatefulWidget {
#override
_HomeState createState() => _HomeState();
}
// próximo passo: colocar os valores lidos do firebase em uma lista que seja
class _HomeState extends State<Home> {
List<SalesData> chartdata = [];
#override
Widget build(BuildContext context) {
// here I try to update the list chartdata, in order to build the chart
readData();
return Scaffold(
appBar: AppBar(
title: Text('Home'),
),
body: Column(
children: <Widget>[
RaisedButton(
child: Text('Read Data'),
onPressed: () {
readData();
},
),
SfCartesianChart(
primaryXAxis: CategoryAxis(),
series: <LineSeries<SalesData, double>>[
LineSeries<SalesData, double>(
// Bind data source
dataSource: chartdata,
xValueMapper: (SalesData sales, _) => sales.year,
yValueMapper: (SalesData sales, _) => sales.sales)
])
],
));
}
final databaseReference =
FirebaseDatabase.instance.reference().child("UID/Esp_32");
void readData() {
databaseReference.once().then((DataSnapshot snapshot) {
Map<dynamic, dynamic> dados = snapshot.value;
dados.forEach((key, value) {
chartdata.add(SalesData(value['VoltageIn'], value['Sensor40']));
});
});
}
}
class SalesData {
SalesData(this.year, this.sales);
final double year;
final double sales;
}

Answer 1

First remove the method readData() at the beginning of the build method this could cause an infinite loop, you can call in the initState method.
So in your readData method you need to call setState to update the screen, like this:
void readData() {
databaseReference.once().then((DataSnapshot snapshot) {
Map<dynamic, dynamic> dados = snapshot.value;
dados.forEach((key, value) {
chartdata.add(SalesData(value['VoltageIn'], value['Sensor40']));
});
setState((){});
});
}
But I think that you should study some architecture to improve your code, like MVVM.