How to Save Files from URL to Download Folder in Flutter

import 'dart:async';
import 'dart:io';

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(home: ModernFileDownloadDemo()));
}

/// Download progress state model.
class DownloadProgress {
  final int bytesReceived;
  final int totalBytes;
  final double progressRatio;
  final bool isCompleted;

  const DownloadProgress({
    required this.bytesReceived,
    required this.totalBytes,
    required this.progressRatio,
    this.isCompleted = false,
  });

  String get formattedReceived =>
      '${(bytesReceived / 1024).toStringAsFixed(1)} KB';
  String get formattedTotal => totalBytes > 0
      ? '${(totalBytes / 1024).toStringAsFixed(1)} KB'
      : 'Unknown';
}

/// Robust file download utility with chunked streaming and progress reporting.
class ChunkedFileDownloader {
  /// Simulates or executes chunked stream reading into a target [sink] with progress.
  static Future<void> downloadStream({
    required Stream<List<int>> byteStream,
    required IOSink targetSink,
    required int contentLength,
    required void Function(DownloadProgress) onProgress,
  }) async {
    int received = 0;

    await for (final chunk in byteStream) {
      targetSink.add(chunk);
      received += chunk.length;
      final ratio = contentLength > 0
          ? (received / contentLength).clamp(0.0, 1.0)
          : 0.0;
      onProgress(
        DownloadProgress(
          bytesReceived: received,
          totalBytes: contentLength,
          progressRatio: ratio,
        ),
      );
    }

    await targetSink.flush();
    await targetSink.close();

    onProgress(
      DownloadProgress(
        bytesReceived: received,
        totalBytes: contentLength,
        progressRatio: 1.0,
        isCompleted: true,
      ),
    );
  }
}

class ModernFileDownloadDemo extends StatefulWidget {
  const ModernFileDownloadDemo({super.key});

  @override
  State<ModernFileDownloadDemo> createState() => _ModernFileDownloadDemoState();
}

class _ModernFileDownloadDemoState extends State<ModernFileDownloadDemo> {
  DownloadProgress _progress = const DownloadProgress(
    bytesReceived: 0,
    totalBytes: 10240,
    progressRatio: 0.0,
  );
  bool _isDownloading = false;

  Future<void> _startMockDownload() async {
    setState(() {
      _isDownloading = true;
      _progress = const DownloadProgress(
        bytesReceived: 0,
        totalBytes: 10240,
        progressRatio: 0.0,
      );
    });

    for (int i = 2048; i <= 10240; i += 2048) {
      await Future.delayed(const Duration(milliseconds: 50));
      if (!mounted) return;
      setState(() {
        _progress = DownloadProgress(
          bytesReceived: i,
          totalBytes: 10240,
          progressRatio: i / 10240,
          isCompleted: i == 10240,
        );
      });
    }

    if (mounted) {
      setState(() {
        _isDownloading = false;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);

    return Scaffold(
      appBar: AppBar(title: const Text('Chunked Download Architecture')),
      body: SingleChildScrollView(
        padding: const EdgeInsets.all(24.0),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: [
            Card(
              elevation: 0,
              color: theme.colorScheme.surfaceContainerHighest,
              child: Padding(
                padding: const EdgeInsets.all(20.0),
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: [
                    Text(
                      'Download Progress',
                      style: theme.textTheme.titleMedium,
                    ),
                    const SizedBox(height: 12),
                    LinearProgressIndicator(
                      value: _progress.progressRatio,
                      minHeight: 10,
                      borderRadius: BorderRadius.circular(5),
                    ),
                    const SizedBox(height: 12),
                    Row(
                      mainAxisAlignment: MainAxisAlignment.spaceBetween,
                      children: [
                        Text('Received: ${_progress.formattedReceived}'),
                        Text('Total: ${_progress.formattedTotal}'),
                      ],
                    ),
                  ],
                ),
              ),
            ),
            const SizedBox(height: 24),
            FilledButton.icon(
              onPressed: _isDownloading ? null : _startMockDownload,
              icon: const Icon(Icons.download),
              label: Text(
                _isDownloading ? 'Downloading...' : 'Start Download Stream',
              ),
            ),
            const SizedBox(height: 24),
            Card(
              child: Padding(
                padding: const EdgeInsets.all(16.0),
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: [
                    const Text(
                      'Modern Android & iOS Storage Architecture:',
                      style: TextStyle(fontWeight: FontWeight.bold),
                    ),
                    const SizedBox(height: 8),
                    const Text(
                      '• Android 10+ (API 29+) Scoped Storage: No WRITE_EXTERNAL_STORAGE permission required for app documents.\n'
                      '• Public Downloads: Use path_provider getDownloadsDirectory() on desktop/supported platforms or MediaStore API on Android.\n'
                      '• HTTP Streams: Stream bytes chunk-by-chunk to disk to prevent out-of-memory crashes on large files.',
                    ),
                  ],
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}