Updating Widget from external file - flutter

Question

For example I have a Widget in the first.dart file and another widget in the second.dart file. The first widget is a simple Button and the second Widget includes a Container with the color red. How can I change the color of this container when I tap the button?
If both widgets are in one file it would be of course very simple, just call the setState() method. But as I said each widget is on a separate file.
What I've tried? Honestly, nothing much. I'm not sure if this is even possible. My first idea was to use something like ValueListenable but the result was not really great.

Answer 1

First, you need to create a new dart file with a stateless widget, set it to receive arguments with final _color and ColoredContainer(this._color) as shown:
import 'package:flutter/material.dart';
class ColoredContainer extends StatelessWidget {
final _color;
ColoredContainer(this._color);
#override
Widget build(BuildContext context) {
return Container(
width: 100,
height: 100,
color: _color,
);
}
}
Then in the main.dart file, import the dart file and use ColoredContainer as a widget which you can pass the Colors to it, in my case I passed a different Color from a list of colors every time setState is called:
import 'package:flutter/material.dart';
import 'ColoredContainer.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
home: MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int index = 0;
final List<Color> _colorList = [
Colors.red,
Colors.green,
Colors.blue,
];
#override
Widget build(BuildContext context) {
return Scaffold(
body: Container(
child: Column(
children: <Widget>[
RaisedButton(onPressed: (){
setState(() {
index += 1;
});
}, child: Text('Tap me')),
ColoredContainer(_colorList[index]),
],
),
),
);
}
}