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double _size = 10;
@override
Widget build(BuildContext context) {
return Scaffold(
floatingActionButton: FloatingActionButton(
child: Icon(Icons.add),
onPressed: () => setState(() => _size += 30),
),
body: TweenAnimationBuilder<double>(
duration: Duration(milliseconds: 300),
tween: Tween<double>(begin: _size, end: _size),
builder: (_, size, __) => Text('A', style: TextStyle(fontSize: size)),
),
);
}