AnimatedContainer Test

main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  double currentWidgetWidth = 0.0;
  double _newWidgetWidth;

  @override
  void initState() {
    _newWidgetWidth = MediaQuery.of(context).size.width;
    Future.delayed(Duration(seconds: 1)).then((value) => animate());
    super.initState();
  }

  void animate() {
    setState(() {
      currentWidgetWidth = _newWidgetWidth;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.grey,
      width: MediaQuery.of(context).size.width,
      height: 50,
      child: AnimatedContainer(
        duration: Duration(seconds: 1),
        width: currentWidgetWidth,
        height: 50,
        child: Text(
          "LABEL",
          textAlign: TextAlign.center,
        ),
        decoration: BoxDecoration(
          color: Colors.green,
        ),
      ),
    );
  }
}