UI Codelab, Step8 (animations) https://codelabs.developers.google.com/codelabs/flutter#7

step8_ui.dart

// Copyright 2017, 2020 The Flutter team. All rights reserved.
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file.

import 'package:flutter/material.dart';

void main() {
  runApp(
    const FriendlyChatApp(),
  );
}

String _name = 'Your Name';

class FriendlyChatApp extends StatelessWidget {
  const FriendlyChatApp({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'FriendlyChat',
      home: ChatScreen(),
    );
  }
}

class ChatMessage extends StatelessWidget {
  const ChatMessage({
    required this.text,
    required this.animationController,
    Key? key,
  }) : super(key: key);
  final String text;
  final AnimationController animationController;

  @override
  Widget build(BuildContext context) {
    return SizeTransition(
      sizeFactor:
          CurvedAnimation(parent: animationController, curve: Curves.easeOut),
      axisAlignment: 0.0,
      child: Container(
        margin: const EdgeInsets.symmetric(vertical: 10.0),
        child: Row(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            Container(
              margin: const EdgeInsets.only(right: 16.0),
              child: CircleAvatar(child: Text(_name[0])),
            ),
            Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: [
                Text(_name, style: Theme.of(context).textTheme.headline4),
                Container(
                  margin: const EdgeInsets.only(top: 5.0),
                  child: Text(text),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

class ChatScreen extends StatefulWidget {
  const ChatScreen({
    Key? key,
  }) : super(key: key);

  @override
  State<ChatScreen> createState() => _ChatScreenState();
}

class _ChatScreenState extends State<ChatScreen> with TickerProviderStateMixin {
  final List<ChatMessage> _messages = [];
  final _textController = TextEditingController();
  final FocusNode _focusNode = FocusNode();

  void _handleSubmitted(String text) {
    _textController.clear();
    var message = ChatMessage(
      text: text,
      animationController: AnimationController(
        duration: const Duration(milliseconds: 700),
        vsync: this,
      ),
    );
    setState(() {
      _messages.insert(0, message);
    });
    _focusNode.requestFocus();
    message.animationController.forward();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('FriendlyChat')),
      body: Column(
        children: [
          Flexible(
            child: ListView.builder(
              padding: const EdgeInsets.all(8.0),
              reverse: true,
              itemBuilder: (_, index) => _messages[index],
              itemCount: _messages.length,
            ),
          ),
          const Divider(height: 1.0),
          Container(
            decoration: BoxDecoration(color: Theme.of(context).cardColor),
            child: _buildTextComposer(),
          ),
        ],
      ),
    );
  }

  Widget _buildTextComposer() {
    return IconTheme(
      data: IconThemeData(color: Theme.of(context).colorScheme.secondary),
      child: Container(
        margin: const EdgeInsets.symmetric(horizontal: 8.0),
        child: Row(
          children: [
            Flexible(
              child: TextField(
                controller: _textController,
                onSubmitted: _handleSubmitted,
                decoration:
                    const InputDecoration.collapsed(hintText: 'Send a message'),
                focusNode: _focusNode,
              ),
            ),
            Container(
              margin: const EdgeInsets.symmetric(horizontal: 4.0),
              child: IconButton(
                  icon: const Icon(Icons.send),
                  onPressed: () => _handleSubmitted(_textController.text)),
            ),
          ],
        ),
      ),
    );
  }

  @override
  void dispose() {
    for (var message in _messages) {
      message.animationController.dispose();
    }
    super.dispose();
  }
}