Progres indicator

main.dart

// Test at:
//   https://dartpad.dev/?id=a07e7f4ceefff225c9ed0b9d276ec5a0


import 'dart:async';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return OutlinedButton(
        child: const Text('Start!'),
        onPressed: () async {
          final a = StreamController<double?>();
          final b = StreamController<Widget?>();
          final c = Completer<void>();
          progress(
            context: context,
            title: const Text('Uploading files'),
            value: a.stream,
            message: b.stream,
            result: c.future,
          );
          // Start the progression at 0
          a.add(0);

          const count = 10;
          for (var file = 0; file < count; file++) {
            b.add(Text('Uploading photo $file of $count'));

            // Do some async work here
            await Future.delayed(const Duration(seconds: 1));

            // Increment the progress bar (from 0.0 to 1.0)
            a.add((file + 1) / count);
          }

          // The progress is done
          c.complete();
        });
  }
}

Future<T?> progress<T>({
  required BuildContext context,
  required Future<T> result,
  Stream<double?>? value,
  Stream<Widget?>? message,
  Widget? title,
}) async {
  BuildContext? innerContext;

  showDialog<void>(
    context: context,
    barrierDismissible: false,
    builder: (BuildContext context) {
      innerContext = context;
      return AlertDialog(
        title: title,
        content: SingleChildScrollView(
          child: _ProgressIndicator(value, message),
        ),        
      );
    },
  );

  try {
    return await result;
  } finally {
    if (innerContext != null) {
      Navigator.pop(innerContext!);
    }
  }
}

class _ProgressIndicator extends StatefulWidget {
  const _ProgressIndicator(this.value, this.message);

  final Stream<double?>? value;

  final Stream<Widget?>? message;

  @override
  State<_ProgressIndicator> createState() => _ProgressIndicatorState();
}

class _ProgressIndicatorState extends State<_ProgressIndicator> {
  StreamSubscription<double?>? _sub;
  StreamSubscription<Widget?>? _subMessage;
  double? _value;
  Widget? _message;

  @override
  void initState() {
    _sub = widget.value?.listen((event) {
      if (mounted) {
        setState(() {
          _value = event;
        });
      }
    });
    _subMessage = widget.message?.listen((event) {
      if (mounted) {
        setState(() {
          _message = event;
        });
      }
    });
    super.initState();
  }

  @override
  void dispose() {
    _sub?.cancel();
    _subMessage?.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return ListBody(
      children: <Widget>[
        LinearProgressIndicator(value: _value),
        const SizedBox(height: 16),
        if (_message != null) Center(child: _message!),
      ],
    );
  }
}