background_downloader

Description:

background downloader

A background file downloader and uploader for iOS, Android, MacOS, Windows and Linux #
Create a DownloadTask to define where to get your file from, where to store it, and how you want to monitor the download, then call FileDownloader().download and wait for the result. Background_downloader uses URLSessions on iOS and DownloadWorker on Android, so tasks will complete also when your app is in the background. The download behavior is highly consistent across all supported platforms: iOS, Android, MacOS, Windows and Linux.
Monitor progress by passing an onProgress listener, and monitor detailed status updates by passing an onStatus listener to the download call. Alternatively, monitor tasks centrally using an event listener or callbacks and call enqueue to start the task.
Optionally, keep track of task status and progress in a persistent database, and show mobile notifications to keep the user informed and in control when your app is in the background.
To upload a file, create an UploadTask and call upload. To make a regular server request, create a Request and call request, or a enqueue a DataTask. To download in parallel from multiple servers, create a ParallelDownloadTask.
The plugin supports headers, retries, priority, requiring WiFi before starting the up/download, user-defined metadata and display name and GET, POST and other http(s) requests, and can be configured by platform. You can manage the tasks in the queue (e.g. cancel, pause and resume), and have different handlers for updates by group of tasks. Downloaded files can be moved to shared storage to make them available outside the app.
No setup is required for Android (except when using notifications), Windows and Linux, and only minimal setup for iOS and MacOS.
Usage examples #
Downloads example #
// Use .download to start a download and wait for it to complete

// define the download task (subset of parameters shown)
final task = DownloadTask(
url: 'https://google.com/search',
urlQueryParameters: {'q': 'pizza'},
filename: 'results.html',
headers: {'myHeader': 'value'},
directory: 'my_sub_directory',
updates: Updates.statusAndProgress, // request status and progress updates
requiresWiFi: true,
retries: 5,
allowPause: true,
metaData: 'data for me');

// Start download, and wait for result. Show progress and status changes
// while downloading
final result = await FileDownloader().download(task,
onProgress: (progress) => print('Progress: ${progress * 100}%'),
onStatus: (status) => print('Status: $status')
);

// Act on the result
switch (result.status) {
case TaskStatus.complete:
print('Success!');

case TaskStatus.canceled:
print('Download was canceled');

case TaskStatus.paused:
print('Download was paused');

default:
print('Download not successful');
}
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Enqueue example #
// Use .enqueue for true parallel downloads, i.e. you don't wait for completion of the tasks you
// enqueue, and can enqueue hundreds of tasks simultaneously.

// First define an event listener to process `TaskUpdate` events sent to you by the downloader,
// typically in your app's `initState()`:
FileDownloader().updates.listen((update) {
switch (update) {
case TaskStatusUpdate():
// process the TaskStatusUpdate, e.g.
switch (update.status) {
case TaskStatus.complete:
print('Task ${update.task.taskId} success!');

case TaskStatus.canceled:
print('Download was canceled');

case TaskStatus.paused:
print('Download was paused');

default:
print('Download not successful');
}

case TaskProgressUpdate():
// process the TaskProgressUpdate, e.g.
progressUpdateStream.add(update); // pass on to widget for indicator
}
});

// Next, enqueue tasks to kick off background downloads, e.g.
final successfullyEnqueued = await FileDownloader().enqueue(DownloadTask(
url: 'https://google.com',
filename: 'google.html',
updates: Updates.statusAndProgress));
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Uploads example #
/// define the multi-part upload task (subset of parameters shown)
final task = UploadTask(
url: 'https://myserver.com/uploads',
filename: 'myData.txt',
fields: {'datafield': 'value'},
fileField: 'myFile',
updates: Updates.statusAndProgress // request status and progress updates
);

// Start upload, and wait for result. Show progress and status changes
// while uploading
final result = await FileDownloader().upload(task,
onProgress: (progress) => print('Progress: ${progress * 100}%'),
onStatus: (status) => print('Status: $status')
);

// Act on result, similar to download
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Batch download example #
final tasks = [task1, task2, task3]; // a list of Download tasks

// download the batch
final result = await FileDownloader().downloadBatch(tasks,
batchProgressCallback: (succeeded, failed) =>
print('Completed ${succeeded + failed} out of ${tasks.length}, $failed failed')
);
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Task tracking database example #
// activate tracking at the start of your app
await FileDownloader().trackTasks();

// somewhere else: enqueue a download (does not complete immediately)
final task = DownloadTask(
url: 'https://google.com',
filename: 'testfile.txt');
final successfullyEnqueued = await FileDownloader().enqueue(task);

// query the tracking database, returning a record for each task
final records = await FileDownloader().database.allRecords();
for (record in records) {
print('Task ${record.tasksId} status is ${record.status}');
if (record.status == TaskStatus.running) {
print('-- progress ${record.progress * 100}%');
print('-- file size ${record.expectedFileSize} bytes');
}
};

// or get record for specific task
final record = await FileDownloader().database.recordForId(task.taskId);
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Notifications example #
// configure notification for all tasks
FileDownloader().configureNotification(
running: TaskNotification('Downloading', 'file: {filename}'),
complete: TaskNotification('Download finished', 'file: {filename}'),
progressBar: true
);

// all downloads will now show a notification while downloading, and when complete.
// {filename} will be replaced with the task's filename.
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Contents #

Basic use

Tasks and the FileDownloader
Monitoring the task
Specifying the location of the file to download or upload
A batch of files


Central monitoring and tracking in a persistent database

Using an event listener
Using callbacks
Using the database to track Tasks


Notifications
Shared and scoped storage
Permissions
Uploads
Parallel downloads
Managing tasks in the queue

Canceling, pausing and resuming tasks
Grouping tasks
Task queues and holding queues
Changing WiFi requirements


Server requests
Cookies
Optional parameters
Initial setup
Configuration
Limitations

Basic use #
Tasks and the FileDownloader #
A DownloadTask or UploadTask (both subclasses of Task) defines one download or upload. It contains the url, the file name and location, what updates you want to receive while the task is in progress, etc. The FileDownloader class is the entrypoint for all calls. To download a file:
final task = DownloadTask(
url: 'https://google.com',
filename: 'testfile.txt'); // define your task
final result = await FileDownloader().download(task); // do the download and wait for result
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The result will be a TaskStatusUpdate, which has a field status that indicates how the download ended: .complete, .failed, .canceled or .notFound.
It may also contain the responseHeaders (with lowercase header names), the responseStatusCode, and the mimeType and charSet if the server provided that information via the Content-Type header.
If the status is .failed, the result.exception field will contain a TaskException with information about what went wrong. For uploads and some unsuccessful downloads, the responseBody will contain the server response.
Monitoring the task #
Progress
If you want to monitor progress during the download itself (e.g. for a large file), then add a progress callback that takes a double as its argument:
final result = await FileDownloader().download(task,
onProgress: (progress) => print('Progress update: $progress'));
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Progress updates start with 0.0 when the actual download starts (which may be in the future, e.g. if waiting for a WiFi connection), and will be sent periodically, not more than twice per second per task. If a task completes successfully you will receive a final progress update with a progress value of 1.0 (progressComplete). Failed tasks generate progress of progressFailed (-1.0), canceled tasks progressCanceled (-2.0), notFound tasks progressNotFound (-3.0), waitingToRetry tasks progressWaitingToRetry (-4.0) and paused tasks progressPaused (-5.0).
Use await task.expectedFileSize() to query the server for the size of the file you are about
to download. The expected file size is also included in TaskProgressUpdates that are sent to
listeners and callbacks - see Using an event listener and Using callbacks
A DownloadProgressIndicator widget is included with the package, and the example app shows how to wire it up.
The widget can be configured to include pause and resume buttons, and to expand to show multiple
simultaneous downloads, or to collapse and show a file download counter.
To provide progress updates (as a percentage of total file size) the downloader needs to know the size of the file when starting the download. Most servers provide this in the "Content-Length" header of their response. If the server does not provide the file size, yet you know the file size (e.g. because you have stored the file on the server yourself), then you can let the downloader know by providing a {'Range': 'bytes=0-999'} or a {'Known-Content-Length': '1000'} header to the task's header field. Both examples are for a content length of 1000 bytes. The downloader will assume this content length when calculating progress.
Status
If you want to monitor status changes while the download is underway (i.e. not only the final state, which you will receive as the result of the download call) you can add a status change callback that takes the status as an argument:
final result = await FileDownloader().download(task,
onStatus: (status) => print('Status update: $status'));
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The status will follow a sequence of .enqueued (waiting to execute), .running (actively
downloading) and then one of the final states mentioned before, or .waitingToRetry if retries
are enabled and the task failed.
If a task fails with TaskStatus.failed then in some cases it is possible to resume the task without having to start from scratch. You can test whether this is possible by calling FileDownloader().taskCanResume(task) and if true, call resume instead of download or enqueue.
Elapsed time
If you want to keep an eye on how long the download is taking (e.g. to warn the user that there may be an issue with their network connection, or to cancel the task if it takes too long), pass an onElapsedTime callback to the download method. The callback takes a single argument of type Duration, representing the time elapsed since the call to download was made. It is called at regular intervals (defined by elapsedTimeInterval which defaults to 5 seconds), so you can react in different ways depending on the total time elapsed. For example:
final result = await FileDownloader().download(
task,
onElapsedTime: (elapsed) {
print('This is taking rather long: $elapsed');
},
elapsedTimeInterval: const Duration(seconds: 30));
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The elapsed time logic is only available for download, upload, downloadBatch and uploadBatch. It is not available for tasks started using enqueue, as there is no expectation that those complete imminently.
Specifying the location of the file to download or upload #
In the DownloadTask and UploadTask objects, the filename of the task refers to the filename without directory. To store the task in a specific directory, add the directory parameter to the task. That directory is relative to the base directory. By default, the base directory is the directory returned by the call to getApplicationDocumentsDirectory() of the path_provider package, but this can be changed by also passing a baseDirectory parameter (BaseDirectory.temporary for the directory returned by getTemporaryDirectory(), BaseDirectory.applicationSupport for the directory returned by getApplicationSupportDirectory() and BaseDirectory.applicationLibrary for the directory returned by getLibraryDirectory() on iOS and MacOS, or subdir 'Library' of the directory returned by getApplicationSupportDirectory() on other platforms).
So, to store a file named 'testfile.txt' in the documents directory, subdirectory 'my/subdir', define the task as follows:
final task = DownloadTask(
url: 'https://google.com',
filename: 'testfile.txt',
directory: 'my/subdir');
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To store that file in the temporary directory:
final task = DownloadTask(
url: 'https://google.com',
filename: 'testfile.txt',
directory: 'my/subdir',
baseDirectory: BaseDirectory.temporary);
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If you already have a path to a file or a File object, you can extract the values for baseDirectory, directory and filename to create the task by using Task.split:
final (baseDirectory, directory, filename) = await Task.split(filePath: yourPath);
final task = UploadTask(
url: 'https://google.com',
baseDirectory: baseDirectory,
directory: directory,
filename: filename);
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The downloader will only store the file upon success (so there will be no partial files saved), and if so, the destination is overwritten if it already exists, and all intermediate directories will be created if needed.
You can also pass an absolute path to the downloader by using BaseDirectory.root combined with the path in directory. This allows you to reach any file destination on your platform. However, be careful: the reason you should not normally do this (and use e.g. BaseDirectory.applicationDocuments instead) is that the location of the app's documents directory may change between application starts (on iOS, and on Android in some cases), and may therefore fail for downloads that complete while the app is suspended. You should therefore never store permanently, or hard-code, an absolute path, unless you are absolutely sure that that path is 'stable'.
Android has two storage modes: internal (default) and external storage. Read the configuration document for details on how to configure your app to use external storage instead of the default.
Server-suggested filename
If you want the filename to be provided by the server (instead of assigning a value to filename yourself), you have two options. The first is to create a DownloadTask that pings the server to determine the suggested filename:
final task = await DownloadTask(url: 'https://google.com')
.withSuggestedFilename(unique: true);
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The method withSuggestedFilename returns a copy of the task it is called on, with the filename field modified based on the filename suggested by the server, or the last path segment of the URL, or unchanged if neither is feasible (e.g. due to a lack of connection). If unique is true, the filename will be modified such that it does not conflict with an existing filename by adding a sequence. For example "file.txt" would become "file (1).txt". You can also supply a taskWithFilenameBuilder to suggest the filename yourself, based on response headers.
The second approach is to set the filename field of the DownloadTask to DownloadTask.suggestedFilename, to indicate that you would like the server to suggest the name. In this case, you will receive the name via the task's status and/or progress updates, so you have to be careful not to use the original task's filename, as that will still be DownloadTask.suggestedFilename. For example:
final task = await DownloadTask(url: 'https://google.com', filename: DownloadTask.suggestedFilename);
final result = await FileDownloader().download(task);
print('Suggested filename=${result.task.filename}'); // note we don't use 'task', but 'result.task'
print('Wrong use filename=${task.filename}'); // this will print '?' as 'task' hasn't changed
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A batch of files #
To download a batch of files and wait for completion of all, create a List of DownloadTask objects and call downloadBatch:
final result = await FileDownloader().downloadBatch(tasks);
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The result is a Batch object that contains the result for each task in .results. You can use .numSucceeded and .numFailed to check if all files in the batch downloaded successfully, and use .succeeded or .failed to iterate over successful or failed tasks within the batch. If you want to get progress updates for the batch (in terms of how many files have been downloaded) then add a callback:
final result = await FileDownloader().downloadBatch(tasks, batchProgressCallback: (succeeded, failed) {
print('$succeeded files succeeded, $failed have failed');
print('Progress is ${(succeeded + failed) / tasks.length} %');
});
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The callback will be called upon completion of each task (whether successful or not), and will start with (0, 0) before any downloads start, so you can use that to start a progress indicator.
To also monitor status and progress for each file in the batch, add a TaskStatusCallback and/or a TaskProgressCallback
To monitor based on elapsed time, see Elapsed time.
For uploads, create a List of UploadTask objects and call uploadBatch - everything else is the same.
Central monitoring and tracking in a persistent database #
Instead of monitoring in the download call, you may want to use a centralized task monitoring approach, and/or keep track of tasks in a database. This is helpful for instance if:

You start download in multiple locations in your app, but want to monitor those in one place, instead of defining onStatus and onProgress for every call to download
You have different groups of tasks, and each group needs a different monitor
You want to keep track of the status and progress of tasks in a persistent database that you query
Your downloads take long, and your user may switch away from your app for a long time, which causes your app to get suspended by the operating system. A download started with a call to download will continue in the background and will finish eventually, but when your app restarts from a suspended state, the result Future that you were awaiting when you called download may no longer be 'alive', and you will therefore miss the completion of the downloads that happened while suspended. This situation is uncommon, as the app will typically remain alive for several minutes even when moving to the background, but if you find this to be a problem for your use case, then you should process status and progress updates for long running background tasks centrally.

Central monitoring can be done by listening to an updates stream, or by registering callbacks. In both cases you now use enqueue instead of download or upload. enqueue returns almost immediately with a bool to indicate if the Task was successfully enqueued. Monitor status changes and act when a Task completes via the listener or callback.
To ensure your callbacks or listener capture events that may have happened when your app was suspended in the background, call resumeFromBackground right after registering your callbacks or listener.
In summary, to track your tasks persistently, follow these steps in order, immediately after app startup:

If using a non-default PersistentStorage backend, initialize with FileDownloader(persistentStorage: MyPersistentStorage()) and wait for the initialization to complete by calling await FileDownloader().ready (see using the database for details on PersistentStorage).
Register an event listener or callback(s) to process status and progress updates
call await FileDownloader().trackTasks() if you want to track the tasks in a persistent database
call await FileDownloader().resumeFromBackground() to ensure events that happened while your app was in the background are processed

The rest of this section details event listeners, callbacks and the database in detail.
Using an event listener #
Listen to updates from the downloader by listening to the updates stream, and process those updates centrally. For example, the following creates a listener to monitor status and progress updates for downloads, and then enqueues a task as an example:
final subscription = FileDownloader().updates.listen((update) {
switch(update) {
case TaskStatusUpdate():
print('Status update for ${update.task} with status ${update.status}');
case TaskProgressUpdate():
print('Progress update for ${update.task} with progress ${update.progress}');
}
});

// define the task
final task = DownloadTask(
url: 'https://google.com',
filename: 'google.html',
updates: Updates.statusAndProgress); // needed to also get progress updates

// enqueue the download
final successFullyEnqueued = await FileDownloader().enqueue(task);
// updates will be sent to your subscription listener
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A TaskProgressUpdate includes expectedFileSize, networkSpeed and timeRemaining. Check the associated hasExpectedFileSize, hasNetworkSpeed and hasTimeRemaining before using the values in these fields. Use networkSpeedAsString and timeRemainingAsString for human readable versions of these values.
Note that successFullyEnqueued only refers to the enqueueing of the download task, not its result, which must be monitored via the listener. Also note that in order to get progress updates the task must set its updates field to a value that includes progress updates. In the example, we are asking for both status and progress updates, but other combinations are possible. For example, if you set updates to Updates.status then the task will only generate status updates and no progress updates. You define what updates to receive on a task by task basis via the Task.updates field, which defaults to status updates only.
Best practice is to start your subscription in a singleton object that you initialize upon app startup, so that you only ever listen to the stream once, and use that singleton object to maintain state for your downloads. Note the stream can only be listened to once, though you can reset the stream controller by calling await FileDownloader().resetUpdates() to start listening again.
Using callbacks #
Instead of listening to the updates stream you can register a callback for status updates, and/or a callback for progress updates. This may be the easiest way if you want different callbacks for different groups.
The TaskStatusCallback receives a TaskStatusUpdate, so a simple callback function is:
void taskStatusCallback(TaskStatusUpdate update) {
print('taskStatusCallback for ${update.task) with status ${update.status} and exception ${update.exception}');
}
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The TaskProgressCallback receives a TaskProgressUpdate, so a simple callback function is:
void taskProgressCallback(TaskProgressUpdate update) {
print('taskProgressCallback for ${update.task} with progress ${update.progress} '
'and expected file size ${update.expectedFileSize}');
}
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A basic file download with just status monitoring (no progress) then requires registering the central callback, and a call to enqueue to start the download:
FileDownloader().registerCallbacks(taskStatusCallback: taskStatusCallback);
final successFullyEnqueued = await FileDownloader().enqueue(
DownloadTask(url: 'https://google.com', filename: 'google.html'));
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You define what updates to receive on a task by task basis via the Task.updates field, which defaults to status updates only. If you register a callback for a type of task, updates are provided only through that callback and will not be posted on the updates stream.
Note that all tasks will call the same callback, unless you register separate callbacks for different groups and set your Task.group field accordingly.
You can unregister callbacks using FileDownloader().unregisterCallbacks().
Using the database to track Tasks #
To keep track of the status and progress of all tasks, even after they have completed, activate tracking by calling trackTasks() and use the database field to query and retrieve the TaskRecord entries stored. For example:
// at app startup, after registering listener or callback, start tracking
await FileDownloader().trackTasks();

// somewhere else: enqueue a download
final task = DownloadTask(
url: 'https://google.com',
filename: 'testfile.txt');
final successfullyEnqueued = await FileDownloader().enqueue(task);

// somewhere else: query the task status by getting a `TaskRecord`
// from the database
final record = await FileDownloader().database.recordForId(task.taskId);
print('Taskid ${record.taskId} with task ${record.task} has '
'status ${record.status} and progress ${record.progress} '
'with an expected file size of ${record.expectedFileSize} bytes'
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You can interact with the database using allRecords, allRecordsOlderThan, recordForId,deleteAllRecords,
deleteRecordWithId etc. If you only want to track tasks in a specific group, call trackTasksInGroup instead.
By default, the downloader uses a modified version of the localstore package to store the TaskRecord and other objects. To use a different persistent storage solution, create a class that implements the PersistentStorage interface, and initialize the downloader by calling FileDownloader(persistentStorage: MyPersistentStorage()) as the first use of the FileDownloader.
As an alternative to LocalStore, use SqlitePersistentStorage, included in background_downloader_sql, which supports SQLite storage and migration from Flutter Downloader.
Notifications #
On iOS and Android, for downloads and uploads, the downloader can generate notifications to keep the user informed of progress also when the app is in the background, and allow pause/resume and cancellation of an ongoing download from those notifications.
Configure notifications by calling FileDownloader().configureNotification and supply a
TaskNotification object for different states. For example, the following configures
notifications to show only when actively running (i.e. download in progress), disappearing when
the download completes or ends with an error. It will also show a progress bar and a 'cancel'
button, and will substitute {filename} with the actual filename of the file being downloaded.
FileDownloader().configureNotification(
running: TaskNotification('Downloading', 'file: {filename}'),
progressBar: true);
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To also show a notifications for other states, add a TaskNotification for complete, error
and/or paused. If paused is configured and the task can be paused, a 'Pause' button will
show for the running notification, next to the 'Cancel' button. To open the downloaded file
when the user taps the complete notification, add tapOpensFile: true to your call to
configureNotification
There are four possible substitutions of the text in the title or body of a TaskNotification:

{filename} is replaced with the filename field of the Task
{displayName} is replaced with the displayName field of the Task
{progress} is substituted by a progress percentage, or '--%' if progress is unknown
{metadata} is substituted by the metaData field of the Task

Notifications on iOS follow Apple's guidelines, notably:

No progress bar is shown, and the {progress} substitution always substitutes to an empty string. In other words: only a single running notification is shown and it is not updated until the download/upload state changes
When the app is in the foreground, on iOS 14 and above the notification will not be shown but will appear in the NotificationCenter. On older iOS versions the notification will be shown also in the foreground. Apple suggests showing progress and download/upload controls within the app when it is in the foreground

No notifications will be generated:

On desktop platforms, as there is no true background mode, and progress updates and indicators can be shown within the app
For a DataTask, as those are meant for short data exchanges

The configureNotification call configures notification behavior for all tasks. You can specify a separate configuration for a group of tasks by calling configureNotificationForGroup and for a single task by calling configureNotificationForTask. A Task configuration overrides a group configuration, which overrides the default configuration.
Make sure to check for, and if necessary request, permission to display notifications - see permissions. For Android, starting with API 33, you need to add <uses-permission android:name="android.permission.POST_NOTIFICATIONS" /> to your app's AndroidManifest.xml. Also on Android you can localize the button text by overriding string resources bg_downloader_cancel, bg_downloader_pause, bg_downloader_resume and descriptions bg_downloader_notification_channel_name, bg_downloader_notification_channel_description. Localization on iOS can be done through configuration.
Grouping notifications #
If you download or upload multiple files simultaneously, you may not want a notification for every task, but one notification representing the group of tasks. To do this, set the groupNotificationId field in a notificationConfig and use that configuration for all tasks in this group. It is easiest to combine this with the group field of the task, e.g.:
FileDownloader.configureNotificationForGroup('bunchOfFiles', // refers to the Task.group field
running: const TaskNotification(
'{numFinished} out of {numTotal}', 'Progress = {progress}'),
complete:
const TaskNotification('Done!', 'Loaded {numTotal} files'),
error: const TaskNotification(
'Error', '{numFailed}/{numTotal} failed'),
progressBar: true,
groupNotificationId: 'myGroupNotification'); // unique ID for notification group

// start every task like this
await FileDownloader().enqueue(DownloadTask(
url: 'https://your_url.com',
filename: 'your_filename',
group: 'bunchOfFiles'));
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All tasks in group bunchOfFiles will now use the notification group configuration with ID myNotificationGroup. Any other task that uses a configuration with groupNotificationId set to 'myGroupNotification' will also be added to that group notification.
On iOS: If your running group notification contains a dynamic item (such as {numFinished} in the example above) then a new notification will be issued every time the notification message changes (different from Android, where the existing notification is updated so does not trigger a new one).
Tapping a notification #
To respond to the user tapping a notification, register a callback that takes Task and NotificationType as parameters:
FileDownloader().registerCallbacks(
taskNotificationTapCallback: myNotificationTapCallback);

void myNotificationTapCallback(Task task, NotificationType notificationType) {
print('Tapped notification $notificationType for taskId ${task.taskId}');
}
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Opening a downloaded file #
To open a file (e.g. in response to the user tapping a notification), call FileDownloader().openFile and supply either a Task or a full filePath (but not both) and optionally a mimeType to assist the Platform in choosing the right application to use to open the file.
The file opening behavior is platform dependent, and while you should check the return value of the call to openFile, error checking is not fully consistent.
Note that on Android, files stored in the BaseDirectory.applicationDocuments cannot be opened. You need to download to a different base directory (e.g. .applicationSupport) or move the file to shared storage before attempting to open it.
If all you want to do on notification tap is to open the file, you can simplify the process by
adding tapOpensFile: true to your call to configureNotifications, and you don't need to
register a taskNotificationTapCallback.
Setup for notifications #
On iOS: Add the following to your AppDelegate.swift:
UNUserNotificationCenter.current().delegate = self as UNUserNotificationCenterDelegate
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or if using Objective C, add to AppDelegate.m:
[UNUserNotificationCenter currentNotificationCenter].delegate = (id<UNUserNotificationCenterDelegate>) self;
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On Android: Optionally, supply your own notification icons by creating a version of the icons defined in android/src/main/res/drawable, e.g. outline_download_done_24.xml, and add those to your own app's android/src/main/res/drawable under the same name.
Shared and scoped storage #
The download directories specified in the BaseDirectory enum are all local to the app. To make downloaded files available to the user outside of the app, or to other apps, they need to be moved to shared or scoped storage, and this is platform dependent behavior. For example, to move the downloaded file associated with a DownloadTask to a shared 'Downloads' storage destination, execute the following after the download has completed:
final newFilepath = await FileDownloader().moveToSharedStorage(task, SharedStorage.downloads);
if (newFilePath == null) {
// handle error
} else {
// do something with the newFilePath
}
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Because the behavior is very platform-specific, not all SharedStorage destinations have the same result. The options are:

.downloads - implemented on all platforms, but 'faked' on iOS: files in this directory are not accessible to other users
.images - implemented on Android and iOS only. On iOS, this moves the image to the Photos Library and returns an identifier instead of a filePath - see below
.video - implemented on Android and iOS only. On iOS, this moves the video to the Photos Library and returns an identifier instead of a filePath - see below
.audio - implemented on Android and iOS only, and 'faked' on iOS: files in this directory are not accessible to other users
.files - implemented on Android only
.external - implemented on Android only

The 'fake' on iOS is that we create an appropriately named subdirectory in the application's Documents directory where the file is moved to. iOS apps do not have access to the system wide directories.
Methods moveToSharedStorage and the similar moveFileToSharedStorage also take an optional
directory argument for a subdirectory in the SharedStorage destination. They also take an
optional mimeType parameter that overrides the mimeType derived from the filePath extension.
If the file already exists in shared storage, then on iOS and desktop it will be overwritten,
whereas on Android API 29+ a new file will be created with an indexed name (e.g. 'myFile (1).txt').
On MacOS: For the .downloads to work you need to enable App Sandbox entitlements and set the key com.apple.security.files.downloads.read-write to true.
On Android: Depending on what SharedStorage destination you move a file to, and depending on the OS version your app runs on, you may require extra permissions WRITE_EXTERNAL_STORAGE and/or READ_EXTERNAL_STORAGE . See here for details on the new scoped storage rules starting with Android API version 30, which is what the plugin is using.
On iOS: For .images and .video SharedStorage destinations, you need user permission to add to the Photos Library, which requires you to set the NSPhotoLibraryAddUsageDescription key in Info.plist. The returned String is not a filePath, but a unique identifier. If you only want to add the file to the Photos Library you can ignore this identifier. If you want to actually get access to the file (and filePath) in the Photos Library, then the user needs to grant an additional 'modify' permission, which requires you to set the NSPhotoLibraryUsageDescription in Info.plist. To get the actual filePath, call pathInSharedStorage and pass the identifier obtained via the call to moveToSharedStorage as the filePath parameter:
final identifier = await FileDownloader().moveToSharedStorage(task, SharedStorage.images);
if (identifier != null) {
final path = await FileDownloader().pathInSharedStorage(identifier, SharedStorage.images);
debugPrint('iOS path to dog picture in Photos Library = ${path ?? "permission denied"}');
} else {
debugPrint('Could not add file to Photos Library, likely because permission denied');
}
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The reason for this two-step approach is that typically you only want to add to the library (requires PermissionType.iosAddToPhotoLibrary), which does not require the user to give read/write access to their entire photos library (PermissionType.iosChangePhotoLibrary, required to get the filePath).
Path to file in shared storage #
To check if a file exists in shared storage, obtain the path to the file by calling
pathInSharedStorage and, if not null, check if that file exists.
On Android 29+: If you
have generated a version with an indexed name (e.g. 'myFile (1).txt'), then only the most recently stored version is available this way, even if an earlier version actually does exist. Also, only files stored by your app will be returned via this call, as you don't have access to files stored by other apps.
On iOS: To make files visible in the Files browser, do not move them to shared storage. Instead, download the file to the BaseDirectory.applicationDocuments and add the following to your Info.plist:
<key>LSSupportsOpeningDocumentsInPlace</key>
<true/>
<key>UIFileSharingEnabled</key>
<true/>
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This will make all files in your app's Documents directory visible to the Files browser.
See moveToSharedStorage above for the special handling of .video and .images destinations on iOS.
Permissions #
User permissions may be needed to display notifications, to move files to shared storage (on Android) and to add images or video to the iOS Photo Library. These permissions should be checked and if needed requested before executing those operations.
You can use a package like permission_handler, or use the FileDownloader().permissions object, which has three methods:

status: returns a PermissionsStatus. On Android this is either granted or denied. If you have not asked for permission yet, then Android returns denied and iOS returns .undetermined. iOS can also return .partial
request: to request the actual permission. Only do this if you have confirmed that the permission is not already granted
shouldShowRationale: for Android only, if true you should show a UI element (e.g. a dialog) to explain to the user why this permission is necessary

All three methods take one PermissionType parameter:

notifications, to display notifications
androidSharedStorage, to move files to external storage on Android, before API 29
iosAddToPhotoLibrary, to move files to SharedStorage.images or SharedStorage.video on iOS, as this adds those files to the Photo Library
iosChangePhotoLibrary, to access the path to files moved to the Photos Library

For example, to request permissions for notifications:
final permissionType = PermissionType.notifications;
var status = await FileDownloader().permissions.status(permissionType);
if (status != PermissionStatus.granted) {
if (await FileDownloader().permissions.shouldShowRationale(permissionType)) {
await showRationaleDialog(permissionType); // Show a dialog with rationale
}
status = await FileDownloader().permissions.request(permissionType);
debugPrint('Permission for $permissionType was $status');
}
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The downloader will check permission status before each action, e.g. will not show notifications unless permissions for notifications have been granted.
Note that permissions are very platform and version dependent, e.g. notification permissions on Android are only required as of API 33, and iOS 14 introduced new Photo Library permissions. If you want to get into details, you can determine the platform version you're running by calling await FileDownloader().platformVersion().
Bypassing permissions on iOS #
By default, the downloader allows any of the permissions to be requested, but that also means that Apple requires you to add things like Photo Library Usage Description to your Info.plist, even if you never move files to the Photo Library.
On iOS, to bypass the permission code altogether at compile time (and therefore remove the need to provide the Info.plist entry) modify your app's Podfile as follows:
post_install do |installer|
installer.pods_project.targets.each do |target|
flutter_additional_ios_build_settings(target)

# The following loop has been added to bypass compilation of specific
# permissions.
# If you want to bypass one or more permissions (so that you don't
# have to include things like a Photo Library Usage Description
# if you don't add files to the Photo Library) then add this loop
# and uncomment the permissions you want to bypass.
# If you bypass (by including the line below) then the
# check will not happen, and the permission is aways denied. If you
# bypass you do not need to include the associated entry in your
# Info.plist file
target.build_configurations.each do |config|
config.build_settings['OTHER_SWIFT_FLAGS'] ||= ['$(inherited)']
#config.build_settings['OTHER_SWIFT_FLAGS'] << '-D BYPASS_PERMISSION_NOTIFICATIONS'
#config.build_settings['OTHER_SWIFT_FLAGS'] << '-D BYPASS_PERMISSION_IOSADDTOPHOTOLIBRARY'
#config.build_settings['OTHER_SWIFT_FLAGS'] << '-D BYPASS_PERMISSION_IOSCHANGEPHOTOLIBRARY'
end
end
end
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and uncomment the line items that you want to bypass by deleting the # mark at the start of the line.
Uploads #
Uploads are very similar to downloads, except:

define an UploadTask object instead of a DownloadTask
the file location now refers to the file you want to upload
call upload instead of download, or uploadBatch instead of downloadBatch

There are two ways to upload a file to a server: binary upload (where the file is included in the POST body) and form/multi-part upload. Which type of upload is appropriate depends on the server you are uploading to. The upload will be done using the binary upload method only if you have set the post field of the UploadTask to 'binary'.
If you already have a File object, you can create your UploadTask using UploadTask.fromFile, though note that this will create a task with an absolute path reference and BaseDirectory.root, which can cause problems on mobile platforms (see here). Preferably, use Task.split to break your File or filePath into appropriate baseDirectory, directory and filename and use that to create your UploadTask.
For multi-part uploads you can specify name/value pairs in the fields property of the UploadTask as a Map<String, String>. These will be uploaded as form fields along with the file. To specify multiple values for a single name, format the value as '"value1", "value2", "value3"' (note the double quotes and the comma to separate the values).
You can also set the field name used for the file itself by setting fileField (default is "file") and override the mimeType by setting mimeType (default is derived from filename extension).
If you need to upload multiple files in a single request, create a MultiUploadTask instead of an UploadTask. It has similar parameters as the UploadTask, except you specify a list of files to upload as the files argument of the constructor, and do not use fileName, fileField and mimeType. Each element in the files list is either:

a filename (e.g. "file1.txt"). The fileField for that file will be set to the base name (i.e. "file1" for "file1.txt") and the mime type will be derived from the extension (i.e. "text/plain" for "file1.txt")
a record containing (fileField, filename), e.g. ("document", "file1.txt"). The fileField for that file will be set to "document" and the mime type derived from the file extension (i.e. "text/plain" for "file1.txt")
a record containing (filefield, filename, mimeType), e.g. ("document", "file1.txt", "text/plain")

The baseDirectory and directory fields of the MultiUploadTask determine the expected location of the file referenced, unless the filename used in any of the 3 formats above is an absolute path (e.g. "/data/user/0/com.my_app/file1.txt"). In that case, the absolute path is used and the baseDirectory and directory fields are ignored for that element of the list.
Once the MultiUpoadTask is created, the fields fileFields, filenames and mimeTypes will contain the parsed items, and the fields fileField, filename and mimeType contain those lists encoded as a JSON string.
Use the MultiTaskUpload object in the upload and enqueue methods as you would a regular UploadTask.
Parallel downloads #
Some servers may offer an option to download part of the same file from multiple URLs or have multiple parallel downloads of part of a large file using a single URL. This can speed up the download of large files. To do this, create a ParallelDownloadTask instead of a regular DownloadTask and specify chunks (the number of pieces you want to break the file into, i.e. the number of downloads that will happen in parallel) and urls (as a list of URLs, or just one). For example, if you specify 4 chunks and 2 URLs, then the download will be broken into 8 pieces, four each for each URL.
Note that the implementation of this feature creates a regular DownloadTask for each chunk, with the group name 'chunk' which is now a reserved group. You will not get updates for this group, but you will get normal updates (status and/or progress) for the ParallelDownloadTask.
Managing tasks and the queue #
Canceling, pausing and resuming tasks #
To enable pausing, set the allowPause field of the Task to true. This may also cause the task to pause un-commanded. For example, the OS may choose to pause the task if someone walks out of WiFi coverage.
To cancel, pause or resume a task, call:

cancelTaskWithId to cancel the tasks with that taskId
cancelTasksWithIds to cancel all tasks with a taskId in the provided list of taskIds
pause to attempt to pause a task. Pausing is only possible for download GET requests, only if the Task.allowPause field is true, and only if the server supports pause/resume. Soon after the task is running (TaskStatus.running) you can call taskCanResume which will return a Future that resolves to true if the server appears capable of pause & resume. If it is not, then pause will have no effect and return false
resume to resume a previously paused task (or certain failed tasks), which returns true if resume appears feasible. The task status will follow the same sequence as a newly enqueued task. If resuming turns out to be not feasible (e.g. the operating system deleted the temp file with the partial download) then the task will either restart as a normal download, or fail.

To manage or query the queue of waiting or running tasks, call:

reset to reset the downloader, which cancels all ongoing download tasks (may not yield proper status updates)
allTaskIds to get a list of taskId values of all tasks currently active (i.e. not in a final state). You can exclude tasks waiting for retries by setting includeTasksWaitingToRetry to false. Note that paused tasks are not included in this list
allTasks to get a list of all tasks currently active (i.e. not in a final state). You can exclude tasks waiting for retries by setting includeTasksWaitingToRetry to false. Note that paused tasks are not included in this list
taskForId to get the Task for the given taskId, or null if not found.
tasksFinished to check if all tasks have finished (successfully or otherwise)

Each of these methods accept a group parameter that targets the method to a specific group. If tasks are enqueued with a group other than default, calling any of these methods without a group parameter will not affect/include those tasks - only the default tasks.
NOTE: Only tasks that are active (ie. not in a final state) are guaranteed to be returned or counted, but returning a task does not guarantee that it is active.
This means that if you check tasksFinished when processing a task update, the task you received an update for may still show as 'active', even though it just finished, and result in false being returned. To fix this, pass that task's taskId as ignoreTaskId to the tasksFinished call, and it will be ignored for the purpose of testing if all tasks are finished:
void downloadStatusCallback(TaskStatusUpdate update) async {
// process your status update, then check if all tasks are finished
final bool allTasksFinished = update.status.isFinalState &&
await FileDownloader().tasksFinished(ignoreTaskId: update.task.taskId) ;
print('All tasks finished: $allTasksFinished');
}
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Grouping tasks #
Because an app may require different types of downloads, and handle those differently, you can specify a group with your task, and register callbacks specific to each group. If no group is specified the default group FileDownloader.defaultGroup is used. For example, to create and handle downloads for group 'bigFiles':
FileDownloader().registerCallbacks(
group: 'bigFiles'
taskStatusCallback: bigFilesDownloadStatusCallback,
taskProgressCallback: bigFilesDownloadProgressCallback);
final task = DownloadTask(
group: 'bigFiles',
url: 'https://google.com',
filename: 'google.html',
updates: Updates.statusAndProgress);
final successFullyEnqueued = await FileDownloader().enqueue(task);
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The methods registerCallBacks, unregisterCallBacks, reset, allTaskIds, allTasks and tasksFinished all take an optional group parameter to target tasks in a specific group. Note that if tasks are enqueued with a group other than default, calling any of these methods without a group parameter will not affect/include those tasks - only the default tasks.
If you listen to the updates stream instead of using callbacks, you can test for the task's group field in your listener, and process the update differently for different groups.
Task queues and Holding queues #
Once you enqueue a task with the FileDownloader it is added to an internal queue that is managed by the native platform you're running on (e.g. Android). Once enqueued, you have limited control over the execution order, the number of tasks running in parallel, etc, because all that is managed by the platform. If you want more control over the queue, you need to use a TaskQueue or a HoldingQueue:

A TaskQueue is a Dart object that you can add to the FileDownloader. You can create this object yourself (implementing the TaskQueue interface) or use the bundled MemoryTaskQueue implementation. This queue sits "in front of" the FileDownloader and instead of using the enqueue and download methods directly, you now simply add your tasks to the TaskQueue. Because this is a Dart object, the queue will suspend when the OS suspends your application, and if the app gets killed, tasks held in the TaskQueue will be lost (unless you have implemented persistence)
A HoldingQueue is native to the OS and can be configured using FileDownloader().configure to limit the number of concurrent tasks that are executed (in total, by host or by group). When using this queue you do not change how you interact with the FileDownloader, but you cannot implement your own holding queue. Because this queue is native, it will continue to run when your app is suspended by the OS, but if the app is killed then tasks held in the holding queue will be lost (unlike tasks already enqueued natively, which persist)

TaskQueue
The MemoryTaskQueue bundled with the background_downloader allows:

pacing the rate of enqueueing tasks, based on minInterval, to avoid 'choking' the FileDownloader when adding a large number of tasks
managing task priorities while waiting in the queue, such that higher priority tasks are enqueued before lower priority ones, even if they are added later
managing the total number of tasks running concurrently, by setting maxConcurrent
managing the number of tasks that talk to the same host concurrently, by setting maxConcurrentByHost
managing the number of tasks running that are in the same Task.group, by setting maxConcurrentByGroup

A TaskQueue conceptually sits 'in front of' the FileDownloader queue, and the TaskQueue makes the call to FileDownloader().enqueue. To use it, add it to the FileDownloader and instead of enqueuing tasks with the FileDownloader, you now add tasks to the queue:
final tq = MemoryTaskQueue();
tq.maxConcurrent = 5; // no more than 5 tasks active at any one time
tq.maxConcurrentByHost = 2; // no more than two tasks talking to the same host at the same time
tq.maxConcurrentByGroup = 3; // no more than three tasks from the same group active at the same time
FileDownloader().addTaskQueue(tq); // 'connects' the TaskQueue to the FileDownloader
FileDownloader().updates.listen((update) { // listen to updates as per usual
print('Received update for ${update.task.taskId}: $update')
});
for (var n = 0; n < 100; n++) {
task = DownloadTask(url: workingUrl, metData: 'task #$n'); // define task
tq.add(task); // add to queue. The queue makes the FileDownloader().enqueue call
}
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Because it is possible that an error occurs when the taskQueue eventually actually enqueues the task with the FileDownloader, you can listen to the enqueueErrors stream for tasks that failed to enqueue.
A common use for the MemoryTaskQueue is enqueueing a large number of tasks. This can 'choke' the downloader if done in a loop, but is easy to do when adding all tasks to a queue. The minInterval field of the MemoryTaskQueue ensures that the tasks are fed to the FileDownloader at a rate that does not grind your app to a halt.
The default TaskQueue is the MemoryTaskQueue which, as the name suggests, keeps everything in memory. This is fine for most situations, but be aware that the queue may get dropped if the OS aggressively moves the app to the background. Tasks still waiting in the queue will not be enqueued, and will therefore be lost. If you want a TaskQueue with more persistence, or add different prioritization and concurrency roles, then subclass the MemoryTaskQueue and add your own persistence or logic.
In addition, if your app is suspended by the OS due to resource constraints, tasks waiting in the queue will not be enqueued to the native platform and will not run in the background. TaskQueues are therefore best for situations where you expect the queue to be emptied while the app is still in the foreground.
Holding queue
Use a holding queue to limit the number of tasks running concurrently. Calling await FileDownloader().configure(globalConfig: (Config.holdingQueue, (3, 2, 1))) activates the holding queue and sets the constraints maxConcurrent to 3, maxConcurrentByHost to 2, and maxConcurrentByGroup to 1. Pass null for no constraint for that parameter.
Using the holding queue adds a queue on the native side where tasks may have to wait before being enqueued with the Android WorkManager or iOS URLSessions. Because the holding queue lives on the native side (not Dart) tasks will continue to get pulled from the holding queue even when the app is suspended by the OS. This is different from the TaskQueue, which lives on the Dart side and suspends when the app is suspended by the OS
When using a holding queue:

Tasks will be taken out of the queue based on their priority and time of creation, provided they pass the constraints imposed by the maxConcurrent values
Status messages will differ slightly. You will get the TaskStatus.enqueued update immediately upon enqueuing. Once the task gets enqueued with the Android WorkManager or iOS URLSessions you will not get another "enqueue" update, but if that enqueue fails the task will fail. Once the task starts running you will get TaskStatus.running as usual
The holding queue and the native queues managed by the Android WorkManager or iOS URLSessions are treated as a single queue for queries like taskForId and cancelTasksWithIds. There is no way to determine whether a task is in the holding queue or already enqueued with the Android WorkManager or iOS URLSessions

Changing WiFi requirements #
By default, whether a task requires WiFi or not is determined by its requireWiFi property (iOS and Android only). To override this globally, call FileDownloader().requireWifi and pass one of the RequireWiFi enums:

asSetByTask (default) lets the task's requireWiFi property determine if WiFi is required
forAllTasks requires WiFi for all tasks
forNoTasks does not require WiFi for any tasks

When calling FileDownloader().requireWifi, all enqueued tasks will be canceled and rescheduled with the appropriate WiFi requirement setting, and if the rescheduleRunningTasks parameter is true, all running tasks will be paused (if possible, independent of the task's allowPause property) or canceled and resumed/restarted with the new WiFi requirement. All newly enqueued tasks will follow this setting as well.
The global setting persists across application restarts. Check the current setting by calling FileDownloader().getRequireWiFiSetting.
Server requests #
To make a regular server request (e.g. to obtain a response from an API end point that you process directly in your app) use:

A Request object, for requests that are executed immediately, expecting an immediate return
A DataTask object, for requests that are scheduled on the background queue, similar to DownloadTask

Request: immediate execution #
A regular foreground request works similar to the download method, except you pass a Request object that has fewer fields than the DownloadTask, but is similar in structure. You await the response, which will be a Response object as defined in the dart http package, and includes getters for the response body (as a String or as UInt8List), statusCode and reasonPhrase.
Because requests are meant to be immediate, they are not enqueued like a Task is, and do not allow for status/progress monitoring.
DataTask: scheduled execution #
To make a similar request using the background mechanism (e.g. if you want to wait for WiFi to be available), create and enqueue a DataTask.
A DataTask is similar to a DownloadTask except it:

Does not accept file information, as there is no file involved
Does not allow progress updates
Accepts post data as a String, or
Accepts json data, which will be converted to a String and posted as content type application/json
Accepts contentType which will set the Content-Type header value
Returns the server responseBody, responseHeaders and possible taskException in the final TaskStatusUpdate fields

Typically you would use enqueue to enqueue a DataTask and monitor the result using a listener or callback, but you can also use transmit to enqueue and wait for the final result of the DataTask.
Cookies #
Servers may ask you to set a cookie (via the 'Set-Cookie' header in the response), to be passed along to the next request (in the 'Cookie' header).
This may be needed for authentication, or for session state.
The method Request.cookieHeader makes it easy to insert cookies in a request. The first argument cookies is either a http.Response object (as returned by the FileDownloader().request method), a List<Cookie>, or a String value from a 'Set-Cookie' header. It returns a {'Cookie': '...'} header that can be added to the next request.
The second argument is the url you intend to use the cookies with. This is needed to filter the appropriate cookies based on domain and path.
For example:
final loginResponse = await FileDownloader()
.request(Request(url: 'https://server.com/login', headers: {'Auth': 'Token'}));
const downloadUrl = 'https://server.com/download';
// add the cookies from the response to the task
final task = DownloadTask(url: downloadUrl, headers: {
'Auth': 'Token',
...Request.cookieHeader(loginResponse, downloadUrl) // inserts the 'Cookie' header
});
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Optional parameters #
The DownloadTask, UploadTask and Request objects all take several optional parameters that define how the task will be executed. Note that a Task is a subclass of Request, and both DownloadTask and UploadTask are subclasses of Task, so what applies to a Request or Task will also apply to a DownloadTask and UploadTask.
Request, DownloadTask & UploadTask #
urlQueryParameters
If provided, these parameters (presented as a Map<String, String>) will be appended to the url as query parameters. Note that both the url and urlQueryParameters must be urlEncoded (e.g. a space must be encoded as %20).
Headers
Optionally, headers can be added to a Request or Task, which will be added to the HTTP request. This may be needed for authentication or session cookies.
HTTP request method
If provided, this request method will be used to make the request. By default, the request method is GET unless post is not null, or the Task is a DownloadTask, in which case it will be POST. Valid HTTP request methods are those listed in Request.validHttpMethods.
POST requests
For downloads, if the required server request is a HTTP POST request (instead of the default GET request) then set the post field of a DownloadTask to a String or UInt8List representing the data to be posted (for example, a JSON representation of an object). To make a POST request with no data, set post to an empty String.
For an UploadTask the POST field is used to request a binary upload, by setting it to 'binary'. By default, uploads are done using the form/multi-part format.
Retries
To schedule automatic retries of failed requests/tasks (with exponential backoff), set the retries field to an
integer between 1 and 10. A normal Task (without the need for retries) will follow status
updates from enqueued -> running -> complete (or notFound). If retries has been set and
the task fails, the sequence will be enqueued -> running ->
waitingToRetry -> enqueued -> running -> complete (if the second try succeeds, or more
retries if needed). A Request will behave similarly, except it does not provide intermediate status updates.
Note that certain failures can be resumed, and retries will therefore attempt to resume from a failure instead of retrying the task from scratch.
DownloadTask & UploadTask #
Requiring WiFi
On Android and iOS only: If the requiresWiFi field of a Task is set to true, the task won't start unless a WiFi network is available. By default requiresWiFi is false, and downloads/uploads will use the cellular (or metered) network if WiFi is not available, which may incur cost. Note that every task requires a working internet connection: local server connections that do not reach the internet may not work.
Priority
The priority field must be 0 <= priority <= 10 with 0 being the highest priority, and defaults to 5. On Desktop and iOS all priority levels are supported. On Android, priority levels <5 are handled as 'expedited', and >=5 is handled as a normal task.
Metadata and displayName
metaData and displayName can be added to a Task. They are ignored by the downloader but may be helpful when receiving an update about the task, and can be shown in notifications using {metaData} or {displayName}.
UploadTask #
File field
Set fileField to the field name the server expects for the file portion of a multi-part upload. Defaults to "file".
Mime type
Set mimeType to the MIME type of the file to be uploaded. By default the MIME type is derived from the filename extension, e.g. a .txt file has MIME type text/plain.
Form fields
Set fields to a Map<String, String> of name/value pairs to upload as "form fields" along with the file.
Initial setup #
No setup is required for Windows or Linux.
Android #
This package needs Kotlin 1.9.20 or above to compile.
For modern Flutter projects this should be added to the /android/settings.gradle file.
plugins {
// ...
id "org.jetbrains.kotlin.android" version "1.9.20" apply false
// ...
}
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For older flutter projects, the kotlin version is set in the android/build.gradle file as follows.
buildScript {
ext.kotlin_version = '1.9.20'
}
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If using notifications you need to:

Starting with API 33, you need to add <uses-permission android:name="android.permission.POST_NOTIFICATIONS" /> to your app's AndroidManifest.xml
If needed, localize the button text by overriding string resources bg_downloader_cancel, bg_downloader_pause, bg_downloader_resume and descriptions bg_downloader_notification_channel_name, bg_downloader_notification_channel_description.

iOS #
On iOS, ensure that you have the Background Fetch capability enabled:

Select the Runner target in XCode
Select the Signing & Capabilities tab
Click the + icon to add capabilities
Select 'Background Modes'
Tick the 'Background Fetch' mode

Note that iOS by default requires all URLs to be https (and not http). See here for more details and how to address issues.
If using notifications, add the following to your AppDelegate.swift:
UNUserNotificationCenter.current().delegate = self as UNUserNotificationCenterDelegate
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or if using Objective C, add to AppDelegate.m:
[UNUserNotificationCenter currentNotificationCenter].delegate = (id<UNUserNotificationCenterDelegate>) self;
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MacOS #
MacOS needs you to request a specific entitlement in order to access the network. To do that open macos/Runner/DebugProfile.entitlements and add the following key-value pair.
<key>com.apple.security.network.client</key>
<true/>
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Then do the same thing in macos/Runner/Release.entitlements.
Configuration #
Several aspects of the downloader can be configured on startup:

Setting the request timeout value and, for iOS only, the 'resourceTimeout'
Checking available space before attempting a download
Activating a holding queue to manage how many tasks are executed concurrently
On Android, when to use the cacheDir for temporary files
Setting a proxy
Bypassing TLS Certificate validation (for debug mode only, Android and Desktop only)
On Android, running tasks in 'foreground mode' to allow longer runs
On Android, whether or not to use external storage
On iOS, localizing the notification button texts

Please read the configuration document for details on how to configure.
Limitations #

iOS 13.0 or greater; Android API 21 or greater
On Android, downloads are by default limited to 9 minutes, after which the download will end with TaskStatus.failed. To allow for longer downloads, set the DownloadTask.allowPause field to true: if the task times out, it will pause and automatically resume, eventually downloading the entire file. Alternatively, configure the downloader to allow tasks to run in the foreground
On iOS, once enqueued (i.e. TaskStatus.enqueued), a background download must complete within 4 hours. Configure 'resourceTimeout' to adjust.
Redirects will be followed
Background downloads and uploads are aggressively controlled by the native platform. You should therefore always assume that a task that was started may not complete, and may disappear without providing any status or progress update to indicate why. For example, if a user swipes your app up from the iOS App Switcher, all scheduled background downloads are terminated without notification