build.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          children: <Widget>[
            Extract(),
            const ExtractConst(),
            buildText(),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }

  /// To wywola za kazdym razem
  Text buildText() {
    print('buildText');
    return Text('You have pushed the button this many times:');
  }
}

/// To się zbuduje tylko raz jeśli uzyty jest const konstruktor
class ExtractConst extends StatelessWidget {
  const ExtractConst({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    print('ExtractConst.build');
    return Text('You have pushed the button this many times:');
  }
}

/// To się zbuduje za kazdym razem bez wzgledu na to, ze zostalo wydzielone do oddzielnego widgetu
class Extract extends StatelessWidget {
  Extract({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    print('Extract.build');

    return getText();
  }

  /// To wywola za kazdym razem
  Text getText() {
    print('Extract.getText');
    return Text(
      'You have pushed the button this many times:',
    );
  }
}