Moving a dynamicaly generated Positioned Widget in Flutter - flutter

Question

How can I move an Positioned Widget in Flutter that was created at runtime?
Szenario:
Lets say I have a Stack Widget. The children are a variable of type List<Widget>[].
I receive the command to create a new Positioned Widget.
A second stream sends updated x and y positions, those I want to apply on left and top of the dynamically created Positioned Widget.
To do this I added two variables to my class and set them to left and top of the Positioned Widget.
While this works as along as the Positioned Widget is created during build time, it does not work when I dynamically add the Widget to the List<Widget> at runtime. (the Positioned Widget appears, but i can not change left/top through the variables of _MyClass)
Some pseudo code that shows the bits.
class _MyClass extends State<StatefulWidget> {
double x = 0.0;
double y = 0.0;
List<Widget> myList = [];
[...]
void _updateLocation(dx, dy) { // called by a stream...
setState(() {
x = dx;
y = dy;
});
}
[...]
void addHeart() {
myList.clear();
myList.add (
Positioned(
top: y,
left: x,
child:Icon(
Icons.favorite,
color: Colors.pink,
size: 24.0,
),
)
);
}
[...]
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text("Demo"),
),
body: Center(
child: Container (
child: Stack(
children: myList, //this does not update the top/left of the object
/* //THIS WORKS
children: <Widget>[
Positioned(
top: y,
left: x,
child:Icon(
Icons.favorite,
color: Colors.pink,
size: 24.0,
),
)
]
*/
)
)
),
);
}
I'm new to Flutter, and I propably don't get the mechanics right here, though, I tried to get this working with setState and Keys, but I feel like I'm missing a important bit of information here.
A possible hack I found is to provide the List<Widget> through a function to the Stack Widget and rebuilding the List<Widget> everytime I get an update on x and y. Which essentially deletes and adds the Widget from the stack everytime I get a new x and y... This seems like the worst way to achieve what I want.
How can I do this in a better way, what I'm missing here, why can I update top/left through x and y when creating the widget in build(..) but not when adding it to the list at runtime?

Answer 1

Ofcourse it would not work when you have a predefined list as children of stack. For your widget to move and change its location, flutter needs to actually rebuild the changed widgets, and note that it does call build method each time it re-renders.
So it is not a bad solution to call a function which creates the list each time (as it is exactly what flutter does for all widgets).
You do not need to worry about adding and deleting the items of stack each time the function is called, that is because flutter does not actually re-render anything on the page if it has not changed compared to current content of the page.