flutter, Why not render ListView<Widget> when build method called? - flutter

Question

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(home: Scaffold(body: Center(child: TestWidget())));
}
}
class TestWidget extends StatefulWidget {
TestWidgetState createState() => new TestWidgetState();
}
class TestWidgetState extends State<TestWidget> {
List<Widget> _bodyItems = [];
List<Widget> _topItems = [];
final Widget _boundary = Column(
children: <Widget>[
SizedBox(
height: 10,
),
SizedBox(
child: Container(
color: Colors.black12,
),
height: 1,
),
SizedBox(
height: 10,
),
],
);
#override
void initState() {
super.initState();
Widget e = GestureDetector(key: Key("0"), onTap: () {
onChangedFunction(Key("0"));
},child: Text("This text is on body range"));
_bodyItems.add(e);
_topItems = [];
}
void onChangedFunction(Key key) async {
setState(() {
_bodyItems.removeAt(0);
_topItems.add(Text("This text is on top range."));
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: SafeArea(
child: Column(
children: <Widget>[
SizedBox(
height: 10,
),
ConstrainedBox(
constraints: BoxConstraints(
maxHeight: 100,
),
child: ListView(
scrollDirection: Axis.horizontal,
children: _topItems,
),
),
_boundary,
ConstrainedBox(
constraints: BoxConstraints(
minHeight: 450,
maxHeight: 450,
),
child: Column(
children: _bodyItems,
),
),
_boundary,
],
)));
}
}
result
top items : [Text("This text is on top range.")]
This code deletes the body item when the widget of the body item is tapped and added item at the top item.
Looking at the result, you can see that the data disappeared from the body item widget and the data was added to the top item widget.
However, the data of the top item is not render.
I want to know what the reason is.

Answer 1

This happens because Children is constant, you can see that if you look into definition.
You will find following line in implementation of ListView.
List<Widget> children = const <Widget>[],
but you can change widget in side children, so you have to give list as a children of list.
As Foolowing.
children: [..._topItems],

Answer 2

Replace _topItems and _bodyItems with below
children: [..._topItems], & children: [..._bodyItems], --inside your body