Flutter App 3B: making the Send button enabled only when there's text to send

main.dart

import 'package:flutter/material.dart';

const String _name = "YourName";

void main() {
  runApp(new FriendlychatApp());
}

class FriendlychatApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: "Friendlychat",
      home: new ChatScreen(),
    );
  }
}

class ChatScreen extends StatefulWidget {
  @override
  State createState() => new ChatScreenState();
}

class ChatMessage extends StatelessWidget {
  ChatMessage({this.text, this.animationController});
  final String text;
  final AnimationController animationController;

  @override
  Widget build(BuildContext context) {
    return new SizeTransition(
        sizeFactor: new CurvedAnimation(
          parent: animationController,
          // https://api.flutter.dev/flutter/animation/Curves-class.html
          //curve: Curves.easeOut
          curve: Curves.linear
        ),
        axisAlignment: 0.0,
        child: new Container(
          decoration: myBoxDecoration(),
          margin: const EdgeInsets.symmetric(vertical: 10.0),
          child: new Row(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              new Container(
                decoration: myBoxDecoration(),
                margin: const EdgeInsets.only(right: 16.0),
                child: new CircleAvatar(child: new Text(_name[0])),
              ),
              new Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  new Text(_name, style: Theme.of(context).textTheme.subhead),
                  new Container(
                    decoration: myBoxDecoration(),
                    margin: const EdgeInsets.only(top: 5.0),
                    child: new Text(text),
                  ),
                ],
              ),
            ],
          ),
        ));
  }

  BoxDecoration myBoxDecoration() {
    return BoxDecoration(
      border: Border.all(
        width: 3.0,
        color: Colors.blue,
      ),
      borderRadius: BorderRadius.all(Radius.circular(5.0)),
    );
  }
}

class ChatScreenState extends State<ChatScreen> with TickerProviderStateMixin {
  final List<ChatMessage> _messages = <ChatMessage>[];
  final TextEditingController _textController = new TextEditingController();
  bool _isComposing = false;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(title: new Text("Friendlychat")),
      body: new Column(
        children: <Widget>[
          new Flexible(
            child: new ListView.builder(
              padding: new EdgeInsets.all(8.0),
              reverse: true,
              itemBuilder: (_, int index) => _messages[index],
              itemCount: _messages.length,
            ),
          ),
          new Divider(height: 1.0),
          new Container(
            decoration: myBoxDecoration(),
            child: _buildTextComposer(),
          ),
        ],
      ),
    );
  }

  Widget _buildTextComposer() {
    return new Container(
      decoration: myBoxDecoration(),
      margin: const EdgeInsets.symmetric(horizontal: 8.0),
      child: new Row(
        children: <Widget>[
          new Flexible(
            child: new TextField(
              controller: _textController,
              onChanged: (String text) {
                setState(() {
                  _isComposing = text.length > 0;
                });
              },   
              onSubmitted: _handleSubmitted,
              decoration:
                  new InputDecoration.collapsed(hintText: "Send a message"),
            ),
          ),
          new Container(
            decoration: myBoxDecoration(),
            margin: new EdgeInsets.symmetric(horizontal: 4.0),
            child: new IconButton(
                icon: new Icon(Icons.send),
                onPressed: _isComposing
                  ? () => _handleSubmitted(_textController.text)
                  : null, 
                //onPressed: () => _handleSubmitted(_textController.text)
            ),
          ),
        ],
      ),
    );
  }

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

  void _handleSubmitted(String text) {
    _textController.clear();

    setState(() {
      _isComposing = false;
    });
    
    ChatMessage message = new ChatMessage(
      text: text,
      animationController: new AnimationController(
        duration: new Duration(milliseconds: 1500),
        vsync: this,
      ),
    );

    setState(() {
      _messages.insert(0, message);
    });

    message.animationController.forward();
  }

  BoxDecoration myBoxDecoration() {
    return BoxDecoration(
      border: Border.all(
        width: 3.0,
        color: Colors.blue,
      ),
      borderRadius: BorderRadius.all(Radius.circular(5.0)),
    );
  }
}