Test

main.dart

import 'package:flutter/material.dart';
void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Timer',
      theme: new ThemeData(
        primaryColor: Colors.grey.shade800,
      ),
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {

  final GlobalKey<_MyTextWidgetState> textGlobalKey = new GlobalKey<_MyTextWidgetState>();


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("title"),
      ),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.spaceAround,
        children: <Widget>[
          IconButton(
            icon: Icon(Icons.add_circle),
            onPressed: () { textGlobalKey.currentState._toggle(false);},
            iconSize: 150.0,
          ),
          MyTextWidget(key: textGlobalKey), //just update this widget
        ],
      ),
    );
  }
}

class MyTextWidget extends StatefulWidget{

  MyTextWidget({Key key}):super(key:key);

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

class _MyTextWidgetState extends State<MyTextWidget> {

  bool _isButtonDisabled = true;

  void _toggle(bool disabled) {
    setState(() {
      _isButtonDisabled = disabled;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new RaisedButton(
      child: new Text(
        _isButtonDisabled ? "Hold on..." : "I'm Active"
      ),
      onPressed: _isButtonDisabled ? null : () {_toggle(!_isButtonDisabled);},
    );
  }
}