transfer_manager

A protocol-aware transfer engine for Dart and Flutter. It provides one task model for uploads and downloads, a persistent queue, retries, progress and ETA, authentication renewal, integrity verification, and pluggable execution engines.

This repository contains the 1.0 transfer core and the first federated Android packages. Android background downloads, multipart uploads, and resumable TUS uploads are available as explicit WorkManager engines, including persistent pause/resume notification actions. iOS background URLSession, S3 multipart, and Flutter widgets remain future milestones.

What works

Streamed HTTP multipart uploads
TUS 1.0 uploads with persisted sessions and resumable chunks
HTTP downloads with .part files, Range and If-Range
Pause, resume, cancel, manual retry, and exponential automatic retry
Global, upload/download, and per-host concurrency limits
Priority ordering with FIFO inside one priority
Atomic download completion
SHA-256, SHA-512, and compatibility-only MD5 verification
Fresh auth headers and single-flight refresh after HTTP 401
In-memory storage and atomic JSON-file persistence
Authorization and cookie header redaction in persisted records
Atomic managed-source staging for uploads that must survive cache eviction
Throttled progress with exponentially smoothed speed and ETA
Android WorkManager downloads with foreground progress notifications
Android WorkManager multipart uploads with managed-source integration
Android WorkManager TUS uploads with persistent sessions and chunk resumption
Persistent pause/resume actions on active Android transfer notifications
Android low-storage preflight protection and device durability tests

Android background downloads

The Android implementation is split into transfer_manager_platform_interface and transfer_manager_android. Add the Android engine before the foreground HTTP fallback:

final manager = TransferManager(
  engines: [
    AndroidBackgroundDownloadEngine(),
    AndroidBackgroundTusUploadEngine(),
    AndroidBackgroundUploadEngine(),
    TusTransferEngine(),
    HttpTransferEngine(),
  ],
);

The current Android capability set includes persistent background downloads, unmetered-network constraints, process-restart task reconnection, progress notifications, notification cancellation, range resumption, and atomic completion. Persisted ETag/Last-Modified validators protect resumed files from remote-resource changes. Successful downloads post a completion notification—even while the app is visible—and tapping it opens the file using a temporary FileProvider permission. Multipart uploads can also run in WorkManager; retrying them restarts the request. Native TUS uploads persist the server-created URL and acknowledged offset, then reconcile and continue in bounded chunks after retries or process restarts. Active native transfers can be paused or resumed from their notification or through the platform API.

On Android 13+, call TransferManagerAndroid().requestNotificationPermission() from a visible screen before expecting progress or completion notifications.

Authorization and cookie headers are rejected by the Android worker because WorkManager persists its input. Authenticated background transfers require a future native credential-provider contract rather than storing access tokens.

Quick start

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

Future<void> main() async {
  final manager = TransferManager(
    storage: JsonFileTransferStorage(File('.transfers/tasks.json')),
    configuration: const TransferConfiguration(maxConcurrentTasks: 3),
  );
  await manager.initialize();

  final task = await manager.enqueue(
    DownloadRequest(
      source: Uri.parse('https://example.com/report.pdf'),
      destinationPath: 'downloads/report.pdf',
      checksum: Checksum.sha256,
      expectedChecksum: 'hex digest supplied by the server',
    ),
  );

  task.events.listen((event) {
    print('${event.state}: ${event.progress.fraction}');
  });
}

For a resumable upload, point TusUploadRequest at the server's TUS creation endpoint. The engine records the returned upload URL and reconciles Upload-Offset before resuming:

final task = await manager.enqueue(
  TusUploadRequest(
    sourcePath: 'videos/large.mp4',
    endpoint: Uri.parse('https://uploads.example.com/files'),
    chunkSize: 8 * 1024 * 1024,
    metadata: const {'contentType': 'video/mp4'},
    authScope: 'current-user',
  ),
);

Use authScope to persist an opaque credential lookup key. Do not put short-lived credentials in request headers. A TransferAuthProvider is asked for fresh headers immediately before execution and again after a 401.

Uploads selected from a cache or temporary picker location can be staged before enqueueing. The original remains untouched, while the managed copy is retained after failure and removed after completion or cancellation:

final manager = TransferManager(
  configuration: const TransferConfiguration(
    managedStoragePath: '/app-support/transfer_manager',
  ),
);

await manager.enqueue(
  UploadRequest(
    sourcePath: '/temporary-picker/video.mp4',
    destination: Uri.parse('https://example.com/upload'),
    sourcePolicy: UploadSourcePolicy.copyToManagedStorage,
  ),
);

Persistence and recovery

Every meaningful transition is saved before its event is emitted. On initialization, tasks interrupted in queued, preparing, running, retryWaiting, or verifying are returned to the queue. Downloads retain their .part file and continue with an HTTP range request when supported.

The bundled JSON store is intentionally simple and atomically replaces its database file. Production Flutter integrations can implement TransferStorage with SQLite without changing the manager API.

Platform boundary

The federated platform interface reports each native engine independently. Android currently advertises durable background downloads, multipart uploads, TUS uploads, notifications, and notification cancellation. Future platform packages should report only the capabilities they actually support.

See ROADMAP.md for the staged path to the full federated plugin.