Animation Error in Flutter :

spinner_text.dart
import 'package:flutter/material.dart'; class SpinnerText extends StatefulWidget { final String text; SpinnerText({this.text = ''}); @override _SpinnerTextState createState() => _SpinnerTextState(); } class _SpinnerTextState extends State<SpinnerText> with SingleTickerProviderStateMixin { String topText = ''; String bottomText = ''; AnimationController _animationController; Animation<double> _spainAnimation; @override void initState() { super.initState(); bottomText = widget.text; _animationController = new AnimationController( duration: const Duration(milliseconds: 750), vsync: this) ..addListener(() => setState(() {})) ..addStatusListener((AnimationStatus status) { if (status == AnimationStatus.completed) { setState(() { bottomText = topText; topText = ''; _animationController.value = 0.0; }); } }); _spainAnimation = new CurvedAnimation( parent: _animationController, curve: Curves.elasticOut); } @override void dispose() { _animationController.dispose(); super.dispose(); } @override void didUpdateWidget(SpinnerText oldWidget) { super.didUpdateWidget(oldWidget); if (widget.text != oldWidget.text) { topText = widget.text; _animationController.forward(); } } @override Widget build(BuildContext context) { return ClipRect( clipper: new RectClipper(), child: new Stack(children: <Widget>[ new FractionalTranslation( translation: new Offset(0.0, _spainAnimation.value - 1.0), child: Text( topText, maxLines: 1, overflow: TextOverflow.ellipsis, style: TextStyle(color: Colors.black, fontSize: 16.0), ) ), FractionalTranslation( translation: new Offset(0.0, _spainAnimation.value), child: Text( bottomText, maxLines: 1, overflow: TextOverflow.ellipsis, style: TextStyle(color: Colors.black, fontSize: 16.0), )) ])); } } class RectClipper extends CustomClipper<Rect> { @override Rect getClip(Size size) { return new Rect.fromLTWH(0.0, 0.0, size.width, size.height); } @override bool shouldReclip(CustomClipper<Rect> oldClipper) { return true; } }
main.dart
import 'package:flutter/material.dart'; import 'spinner_text.dart'; void main() => runApp(MyApp()); class MyApp extends StatelessWidget { @override Widget build(BuildContext context) { return MaterialApp( title: 'Flutter Tutorial', theme: ThemeData( primarySwatch: Colors.blue, ), home: MyHomePage(title: 'Animation Error'), ); } } class MyHomePage extends StatefulWidget { MyHomePage({Key key, this.title}) : super(key: key); final String title; @override _MyHomePageState createState() => _MyHomePageState(); } class _MyHomePageState extends State<MyHomePage> { int _counter = 0; void _incrementCounter() { setState(() { _counter++; }); } @override Widget build(BuildContext context) { return Scaffold( appBar: AppBar( title: Text(widget.title), ), body: Center( child: Column( mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[ Text( 'You have pushed the button this many times:', ), SpinnerText( text: '$_counter', ) ])), floatingActionButton: FloatingActionButton( onPressed: _incrementCounter, tooltip: 'Increment', child: Icon(Icons.add), )); } }
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