How to add fade in/out on state transition

Question

By default, when there's a state change on a StatefulWidget, there's no animation on state transition. Is there any way to had some? For example, on transition, I want to fade out the old state and fade in the new state.

I know how to add transition animation between routes, but couldn't find a way to do it between state transitions.

Has an example, let's use the Flutter default app:

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @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(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

So, each time I click the button the number changes immediately. What I would like to see is the old number fade out and the new number fade in.

Answer 1

This answer, using AnimatedOpacity works ok. But I found using AnimatedSwitcher much simpler and cleaner.

(...)
    body: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Text(
            'You have pushed the button this many times:',
          ),
          AnimatedSwitcher(
            duration: const Duration(milliseconds: 500),
            transitionBuilder: (Widget child, Animation<double> animation) {
              return FadeTransition(child: child, opacity: animation);
            },
            child: Text(
              '$_counter',
              key: ValueKey<int>(_counter),
              style: Theme.of(context).textTheme.display1,
            ),
          ),
        ],
      ),
    ),
(...)

See here for more information about AnimatedSwitcher.