setState not updating variable when declared on initState - flutter

Question

I have run into an issue with Flutter: I have a navbar which, depending on the item selected, returns me an integer number called Index. This index is then passed through a List to get the content of the body of the Scaffold, an object of Widget() class.
The default body is an object of HomePage(), that has an integer parameter called rpm. By default, the HomePage() should be the body displayed, so, as it depends on the rpm parameter, I declare the rpm parameter on the initState. I also have a setState that changes dynamically the rpm.
The weird thing is: if I declare the List<Widgets> bodyList in the initState, the setState doesn't seem to work. However, if I declare List<Widgets> bodyList in the build method, I can see the content of the HomePage() change dynamically with the rpm.
An excerpt of the code. Not working:
class _ScreenTreeState extends State<ScreenTree> {
int _index;
int _rpm;
List<Widget> bodyList;
#override
void initState() {
_isPlaying = false;
_rpm = 0;
_index = 0;
bodyList = [
HomePage(rpm: _rpm),
StatisticsScreen(),
WeightScreen(),
SettingsPage()
];
super.initState();
}
#override
Widget build(BuildContext context) {
return Scaffold(...
Working:
class _ScreenTreeState extends State<ScreenTree> {
int _index;
int _rpm;
List<Widget> bodyList;
#override
void initState() {
_isPlaying = false;
_rpm = 0;
_index =
0;
super.initState();
}
#override
Widget build(BuildContext context) {
bodyList = [
HomePage(rpm: _rpm),
StatisticsScreen(),
WeightScreen(),
SettingsPage()
];
return Scaffold(...
My theory is that this may have to do with the fact that if _rpm is declared as an integer, Dart reads it as a primitive and not as the object of an integer class, so if declared on the initState, I'm not actually passing an object, but a primitive. But I don't understand then why is it working when declared in the build method.
I feel this is irrelevant, but if somebody needs it, here it is the code that updates the value of _rpm. Please, don't read too much into this because I feel it is actually not relevant and may distract and confuse more than help:
onPressed: () {
if (_isPlaying) {
setState(() {
_isPlaying = false;
});
Provider.of<MicrophoneEngine>(context, listen: false)
.stopRecording();
} else {
setState(() {
_isPlaying = true;
});
Provider.of<MicrophoneEngine>(context, listen: false)
.startRecording((rpmCall) {
setState(() {
_rpm = rpmCall;
});
});
}
},

Answer 1

It's a bad practice to keep references for the widgets(like in the first example), in fact, you don't need that List at all.
Every time you use setState, you rebuild the widgets and you pass the new value. In the first example you pass the HomePage(rpm: _rpm) from the init every time, that's why it is not updated. Meaning, you don't rebuild HomePage with the updated value, you just passing the HomePage from the initState which was instantiated with the initial _rpm value of 0.