Scanning Animation In Flutter

main.dart

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:scanning_aimation/scanner.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return CupertinoApp(
      debugShowCheckedModeBanner: false,
      title: 'Photo Scanner',
      home: ScannerScreen(),
      theme: CupertinoThemeData(
        brightness: Brightness.dark
      ),
    );
  }
}

scanner_screen.dart

import 'package:flutter/cupertino.dart';
import 'package:scanning_aimation/scanner_widget.dart';

class ScannerScreen extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return ScannerScreenState();
  }
}

class ScannerScreenState extends State<ScannerScreen>
with SingleTickerProviderStateMixin {

  AnimationController _animationController;
  bool _animationStopped = false;
  String scanText = "Scan";
  bool scanning = false;

  @override
  void initState() {
    _animationController = new AnimationController(
        duration: new Duration(seconds: 1), vsync: this);

    _animationController.addStatusListener((status) {
      if (status == AnimationStatus.completed) {
        animateScanAnimation(true);
      } else if (status == AnimationStatus.dismissed) {
        animateScanAnimation(false);
      }
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return CupertinoPageScaffold(
      navigationBar: CupertinoNavigationBar(middle: Text("Scanning Animation")),
      child: SafeArea(
        child: Container(
            height: double.infinity,
            width: double.infinity,
            child: Column(
                mainAxisSize: MainAxisSize.max,
                crossAxisAlignment: CrossAxisAlignment.center,
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  Stack(children: [
                    Padding(
                      padding: const EdgeInsets.all(16.0),
                      child: Container(
                        decoration: BoxDecoration(
                            border: Border.all(color: CupertinoColors.white),
                            borderRadius:
                                BorderRadius.all(Radius.circular(12))),
                        child: ClipRRect(
                          borderRadius: BorderRadius.all(Radius.circular(12)),
                          child: Image(
                              width: 334,
                              image: NetworkImage(
                                  "https://images.pexels.com/photos/1841819/pexels-photo-1841819.jpeg?auto=compress&cs=tinysrgb&dpr=3&h=750&w=1260")),
                        ),
                      ),
                    ),
                    ImageScannerAnimation(
                      _animationStopped,
                      334,
                      animation: _animationController,
                    )
                  ]),
                  Padding(
                    padding: const EdgeInsets.only(top: 32.0),
                    child: CupertinoButton(
                      color: CupertinoColors.activeGreen,
                      onPressed: () {
                        if (!scanning) {
                          animateScanAnimation(false);
                          setState(() {
                            _animationStopped = false;
                            scanning = true;
                            scanText = "Stop";
                          });
                        } else {
                          setState(() {
                            _animationStopped = true;
                            scanning = false;
                            scanText = "Scan";
                          });
                        }
                      },
                      child: Text(scanText),
                    ),
                  )
                ])),
      ),
    );
  }

  void animateScanAnimation(bool reverse) {
    if (reverse) {
      _animationController.reverse(from: 1.0);
    } else {
      _animationController.forward(from: 0.0);
    }
  }

  @override
  void dispose() {
    _animationController.dispose();
    super.dispose();
  }
}

scanner_widget.dart

import 'package:flutter/animation.dart';
import 'package:flutter/cupertino.dart';

class ImageScannerAnimation extends AnimatedWidget {
  final bool stopped;
  final double width;

  ImageScannerAnimation(this.stopped, this.width,
      {Key key, Animation<double> animation})
      : super(key: key, listenable: animation);

  Widget build(BuildContext context) {
    final Animation<double> animation = listenable;
    final scorePosition = (animation.value * 440) + 16;

    Color color1 = Color(0x5532CD32);
    Color color2 = Color(0x0032CD32);

    if (animation.status == AnimationStatus.reverse) {
      color1 = Color(0x0032CD32);
      color2 = Color(0x5532CD32);
    }

    return new Positioned(
        bottom: scorePosition,
        left: 16.0,
        child: new Opacity(
            opacity: (stopped) ? 0.0 : 1.0,
            child: Container(
              height: 60.0,
              width: width,
              decoration: new BoxDecoration(
                  gradient: new LinearGradient(
                begin: Alignment.topCenter,
                end: Alignment.bottomCenter,
                stops: [0.1, 0.9],
                colors: [color1, color2],
              )),
            )));
  }
}

For those whose animation effect is not started
add this line in initial Method in scanner_screen.dart

_animationController.forward(from: 0.0); // add this line