main.dart

import 'dart:async';
import 'dart:math';

import 'package:flutter/material.dart';

// Native

// Pretend this is the native side emitting progress.
Stream<double> get nativeProgressEmitter async* {
  var progress = 0.0;
  while (progress < 1) {
    yield progress;
    progress += Random().nextDouble() / 3;
    await Future.delayed(const Duration(milliseconds: 500));
  }
  yield 1;
}

// Our API (amplify_storage_s3)

// Just demonstrating the StreamSink usage, so the rest is commented out.
Future<void /* UploadFileResult */ > uploadFile({
  // required String key,
  // required File local,
  // UploadFileOptions? options,
  StreamSink<double>? progressSink,
}) async {
  // Event channel stuff. Get stream from platform.
  final eventChannelStream = nativeProgressEmitter.asBroadcastStream();
  progressSink?.addStream(eventChannelStream);

  await eventChannelStream.last;
  // return result
}

// User's App

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Progress Listener',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Stream<double>? _uploadProgress;
  bool _isFinished = false;

  @override
  void initState() {
    super.initState();
    _uploadFile();
  }

  Future<void> _uploadFile() async {
    final StreamController<double> _controller = StreamController(sync: true);
    setState(() {
      _uploadProgress = _controller.stream;
    });
    await uploadFile(progressSink: _controller.sink);
    setState(() {
      _isFinished = true;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Progress Listener'),
      ),
      body: Center(
        child: ConstrainedBox(
          constraints: const BoxConstraints(maxWidth: 300),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              const Text(
                'Uploading file:',
              ),
              const SizedBox(height: 10),
              if (_isFinished)
                const Icon(
                  Icons.check_circle,
                  color: Colors.green,
                  size: 40,
                )
              else
                StreamBuilder<double>(
                  stream: _uploadProgress,
                  builder: (context, snapshot) {
                    return LinearProgressIndicator(value: snapshot.data ?? 0);
                  },
                ),
            ],
          ),
        ),
      ),
    );
  }
}