How to lift state from a child widget up to another child widget - flutter

Question

How would I lift a data from a child widget up to another child widget that's directly above it.
import 'package:flutter/material.dart';
class Tree extends StatefulWidget {
#override
_TreeState createState() => _TreeState();
}
class _TreeState extends State<Tree> {
#override
Widget build(BuildContext context) {
return Container(
child: Column(
children: <Widget>[
bigTree(),
],
),
);
}
}
Widget bigTree() {
var fruit = data; //Undefined name 'data'.
return Container(
child: Row(
children: <Widget>[
Text(fruit),
smallTree(),
],
),
);
}
Widget smallTree() {
var data = 'apple'; //How to lift this data
return Container(
child: Text(data),
);
}
I tried adding a constructor `_TreeState({this.data}); but it's coming up as null

Answer 1

What you have aren't actually widgets. They're functions returning widgets. If you want the bigTree to "hold state" in flutter's traditional meaning, you'd need to make it into a class which extends StatefulWidget and implements create state as your Tree widget does. Currently, as functions, every time you'd hot reload, the variables in your widget returning functions would be reset and thus no "state" is present.
Here is how I'd change things
import 'package:flutter/material.dart';
class Tree extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Container(
child: Column(
children: <Widget>[
BigTree(),
],
),
);
}
}
class BigTree extends StatefulWidget {
#override
_BigTreeState createState() => _BigTreeState();
}
class _BigTreeState extends State<BigTree> {
final fruit = 'apple';
#override
Widget build(BuildContext context) {
return Container(
child: Row(
children: <Widget>[
Text(fruit),
SmallTree(fruit),
],
),
);
}
}
class SmallTree extends StatelessWidget {
SmallTree(this.data);
final String data;
#override
Widget build(BuildContext context) {
return Container(
child: Text(data),
);
}
}

Answer 2

I think Nolence answer is good. I want to give more general. I am begginer, however this is how I understand:
With simple setState.
You let parent widget to manage the state. You pass all usefull 'tools' to the stateless child widget where action occurs.
class ChildWidget extends StatelessWidget {
// used tools
final var inputData;
final Function changeData;
ChildWidget(this.inputData, this.changeData);
#override
Widget build(BuildContext context) {
return Container(
...
// action
onTap: () {
changeData(inputData);
},
// data usage
Text('My changed ${inputData.toString()}'),
Implement logic (define tools) in the parent widget:
class ParentWidget extends StatefulWidget {
#override
_ParentWidgetState createState() => _ParentWidgetState();
}
class _ParentWidgetState extends State<ParentWidget> {
// define tools and pass to child
var inputData = 0;
void changeData(var outputData) {
// Implement logic
print(outputData);
setState(() {
inputData += outputData;
});
}
#override
Widget build(BuildContext context) {
return Container(
...
ChildWidget(inputData, changeData),
When the changeData function will be used it will call statefull widget to change the state and rebuild the tree (render).