notify child that somewhere in parent onTap happened - flutter

Question

could you please show me how can I notify my statefull child widget that somewhere in parent user clicks on button?
I have two separate .dart files
in the first file I described main screen widget with FAB
and in the second one I have ListWidget (like RecyclerView)
If user tap on FAB I want notify my ListWidget about it so it can e.g. add one more item.
I have java/android background but it's quite hard for me to change my mind flow.

Answer 1

The first option would be to build the child widget each time you add an item to the list, passing the list as a parameter to the child.
But using streams is a nice way to avoid rebuilding the child widget each time. I think the following code is a good starting point (You could also use a StreamBuilder to build the list leveraging the stream).
In main.dart
import 'dart:async';
import 'package:base_test_project/expanding_list.dart';
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter Demo',
home: new MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
StreamController<int> _controller = StreamController<int>();
int _number = 0;
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
),
body: new Center(
child: new ExpandingList(stream: _controller.stream),
),
floatingActionButton: new FloatingActionButton(
onPressed: () {_controller.add(_number++);},
child: new Icon(Icons.add),
),
);
}
}
In expanding_list.dart
import 'dart:async';
import 'package:flutter/material.dart';
class ExpandingList extends StatefulWidget {
Stream<int> stream;
ExpandingList({this.stream});
#override
_ExpandingListState createState() => _ExpandingListState();
}
class _ExpandingListState extends State<ExpandingList> {
List<int> _myList = [];
#override
void initState() {
super.initState();
widget.stream.listen((number) {
setState(() { _myList.add(number); });
});
}
#override
Widget build(BuildContext context) {
return ListView.builder(
itemCount: _myList.length,
itemBuilder: (context, index) {
return Padding(
padding: EdgeInsets.all(15.0), child: Text("Item ${_myList[index]}"));
});
}
}