Introduction

Downloading large files on mobile reliably requires more than a simple HTTP request. Users expect visible progress, automatic resume after interruptions, and guarantees that the file wasn't tampered with. In Flutter mobile development you can combine HTTP range requests, persistent partial files, and cryptographic integrity checks to create a secure, resilient downloader. This tutorial shows a practical architecture and sample Dart snippets to implement resumable downloads with progress and integrity verification.

Planning The Download Flow

Design the flow before coding. Key steps:

• Request the server for file metadata (size, MIME type, accepted ranges, and ideally a canonical hash like SHA256 or an HMAC-signed value).

• If the server supports Range requests (Accept-Ranges: bytes), perform chunked or resume requests.

• Persist partial downloads to a temporary file in app storage and keep a small state file (JSON) with current byte offset, expected total size, and server-provided hash.

• On completion, compute a cryptographic checksum and compare against the server value before promoting the file to the final location.

Server cooperation matters: for resume you need Accept-Ranges support and for integrity you need a trusted server-supplied checksum (or use HTTPS + signed metadata).

Implementing Resumable Downloads

Use a robust HTTP client that can set custom headers and stream bytes to disk. Dio and http packages both work; file operations use dart:io and path_provider to choose a safe directory. The approach: open file in append mode and request a Range starting from the current file length.

Example: issue a ranged GET and append bytes to file.

// Request with Range header, write bytes in append mode
final req = await dio.get<ResponseBody>(url,
  options: Options(responseType: ResponseType.stream, headers: {
    'Range': 'bytes=$existingLength-'
  }));
await req.data.stream.listen((chunk) async {
  await file.writeAsBytes(chunk, mode: FileMode.append);
  existingLength += chunk.length;
});

Keep a small JSON state file alongside the temporary file:

• url

• expectedSize

• downloadedBytes

• expectedHash

On relaunch, load state and resume from downloadedBytes. If the server indicates the file has changed (different Content-Length or ETag), discard the partial file and restart.

Verifying Integrity

Never trust the raw file without verification. Preferred options:

• Server-supplied SHA256 or SHA512 of the full file (over HTTPS). Compute the same hash locally and compare.

• Use HMAC or signed metadata if you control both client and server for higher assurance.

Compute a SHA256 checksum once the file is complete. The following snippet uses the crypto package to compute a hash incrementally (streaming to avoid memory spikes):

import 'dart:io';
import 'package:crypto/crypto.dart';

Future<String> sha256File(File file) async {
  final sink = sha256.startChunkedConversion(StringConversionSink.withCallback((_) {}));
  await for (final chunk in file.openRead()) sink.add(chunk);
  sink.close();
  return sha256.convert(await file.readAsBytes()).toString();
}

(Alternative: hash while streaming write to avoid double reads.)

If the checksum mismatches, delete the file, clear state, and retry with a clean download. Log mismatch events and surface an error to the user.

Handling Errors And Security

Handle the common failure modes explicitly:

• Network interruptions: implement exponential backoff and limited retry attempts. Persist partial file and state between attempts.

• Server changes: if Content-Length or ETag differs, restart the download.

• Insufficient storage: detect and fail early with a clear message.

Security best practices:

• Always use HTTPS. Validate server certificates by relying on platform TLS. For higher assurance, use certificate pinning only if you can manage key rotation.

• Obtain file integrity metadata from a trusted endpoint. Consider signing metadata with your server private key and verifying the signature on the client.

• Minimize permissions and store temporary files in app-specific storage provided by path_provider.

UI and UX considerations:

• Show a byte-level progress bar (downloadedBytes / totalBytes). Persist progress so the UI can show accurate state on relaunch.

• Allow users to pause/resume and cancel. On cancel, delete partial files unless the user wants to keep for resuming later.

Vibe Studio

Vibe Studio, powered by Steve’s advanced AI agents, is a revolutionary no-code, conversational platform that empowers users to quickly and efficiently create full-stack Flutter applications integrated seamlessly with Firebase backend services. Ideal for solo founders, startups, and agile engineering teams, Vibe Studio allows users to visually manage and deploy Flutter apps, greatly accelerating the development process. The intuitive conversational interface simplifies complex development tasks, making app creation accessible even for non-coders.

Conclusion

Implementing secure, resumable file downloads in Flutter combines server support (range and integrity metadata), careful local state management, and cryptographic checks. Use ranged requests and append to a temporary file, persist a tiny state file, and verify the file hash before promoting the asset. Handle network and storage errors gracefully and favor HTTPS and signed metadata for security. With these patterns you can deliver robust downloads with accurate progress and trustworthy content to mobile users.