What is the difference between functions and classes to create reusable widgets?

Question

I have realized that it is possible to create widgets using plain functions instead of subclassing StatelessWidget. An example would be this:

Widget functio         


        

Answer 1

I've been researching on this issue for the past 2 days. I came to the following conclusion: it is OKAY to break down pieces of the app into functions. It's just ideal that those functions return a StatelessWidget, so optimisations can be made, such as making the StatelessWidget const, so it doesn't rebuild if it doesn't have to. For example, this piece of code is perfectly valid:

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 {
  int _counter = 0;

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
            const MyWidgetClass(key: const Key('const')),
            MyWidgetClass(key: Key('non-const')),
            _buildSomeWidgets(_counter),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }

  Widget _buildSomeWidgets(int val) {
    print('${DateTime.now()} Rebuild _buildSomeWidgets');
    return const MyWidgetClass(key: Key('function'));

    // This is bad, because it would rebuild this every time
    // return Container(
    //   child: Text("hi"),
    // );
  }
}

class MyWidgetClass extends StatelessWidget {
  const MyWidgetClass({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    print('${DateTime.now()} Rebuild MyWidgetClass $key');

    return Container(
      child: Text("hi"),
    );
  }
}

The use of function there is perfectly fine, as it returns a const StatelessWidget. Please correct me if I'm wrong.