dart-pad.dart

class _MyHomePageState extends State<MyHomePage> {
  bool _visibility = true;
  void _show() {
    setState(() {
      _visibility = true;
    });
  }
  void _hide() {
    setState(() {
      _visibility = false;
    });
  }
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('widget.title'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
              Text('이 글자를 안보이게 하고 싶습니다.',
                style: Theme.of(context).textTheme.headline4,  
              ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () => {_visibility? _hide() : _show()},
        tooltip: 'show/hide',
        child: Icon(Icons.add),
      ),
    );
  }
}