background_downloader 3.0.1

A background file downloader and uploader for iOS and Android. Define the task, enqueue and monitor progress

A background file downloader and uploader for iOS and Android

Define where to get your file from, where to store it, and how you want to monitor the download in a DownloadTask, then call 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.

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.

To upload a file, create an UploadTask and call upload. To make a regular server request, create a Request and call request.

The plugin supports headers, retries, requiring WiFi before starting the up/download, user-defined metadata and GET and POST http(s) requests. You can manage and monitor the tasks in the queue, and have different handlers for updates by group.

No setup is required for Android, and only minimal setup for iOS.

Contents

• Basic use
  • Tasks and the FileDownloader
  • Monitoring the task
  • Specifying the location of the file to download or upload
  • A batch of files
• Advanced use
  • Using an event listener
  • Using callbacks
• Uploads
• Managing and monitoring tasks in the queue
  • Grouping tasks
• Server requests
• Optional parameters

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 static class is the entrypoint for all plugin calls. You must initialize it once (when you start your app), then call FileDownloader.download while passing the DownloadTask, then wait for the result:

    FileDownloader.initialize(); // just once when you start your app
    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

The result will be a TaskStatus that represents how the download ended: .complete, .failed, .canceled or .notFound.

Monitoring the task

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'));

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) and waitingToRetry tasks progressWaitingToRetry (-4.0).

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'));

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.

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, so cannot start with a /. By default, the base directory is the directory returned by the call to getApplicationDocumentsDirectory(), but this can be changed by also passing a baseDirectory parameter (BaseDirectory.temporary for the directory returned by getTemporaryDirectory() and BaseDirectory.applicationSupport for the directory returned by getApplicationSupportDirectory()).

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');

To store that file in the temporary directory:

final task = DownloadTask(
        url: 'https://google.com',
        filename: 'testfile.txt',
        directory: 'my/subdir',
        baseDirectory: BaseDirectory.temporary);

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.

Note: the reason you cannot simply pass a full absolute directory path to the downloader is that the location of the app's documents directory may change between application starts (on iOS), 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.

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);

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, (succeeded, failed) {
      print('$succeeded files succeeded, $failed have failed');
      print('Progress is ${(succeeded + failed) / tasks.length} %');
   });

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. Note that it is not possible to monitor download progress of individual files within the batch.

For uploads, create a List of UploadTask objects and call uploadBatch - everything else is the same.

Advanced use

The download method works well for most cases, e.g. when you have a few downloads associated with a widget. If you have a large number of downloads, or very long running downloads, the user may move your app to the background and the operating system may suspended it. The downloads 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. You do this by listening to an updates stream, or by registering callbacks, and using enqueue instead of download or upload. As long as you start listening to the updates (or register your callbacks) as soon as your app starts, you will get notified of status and progress update changes that happened while your app was suspended, immediately after the app awakes.

In this scenario, to start a download or upload, call enqueue, which returns a Future<bool> to indicate if the Task was successfully enqueued. You are then responsible for monitoring status changes and acting when a Task completes via the listener or callback.

Status updates may still get lost in unusual situations (e.g. if the user kills your app when it is suspended). It may therefore be helpful to check upon startup if a file you were expecting exists, even if you may not have been notified of a completed download. In addition, you can check which downloads are still active by querying the task queue.

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:

  FileDownloader.initialize();  // initialize before starting to listen
  final subscription = FileDownloader.updates.listen((update) {
      if (update is TaskStatusUpdate) {
        print('Status update for ${update.task} with status ${update.status}');
      } else if (update is 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

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.

You can start your subscription in a convenient place, like a widget's initState, and don't forget to cancel your subscription to the stream using subscription.cancel(). Note the stream can only be listened to once: to listen again, first call FileDownloader.initialize().

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 - see below.

The TaskStatusCallback receives the Task and the updated TaskStatus, so a simple callback function is:

void taskStatusCallback(
    Task task, TaskStatus status) {
  print('taskStatusCallback for $task with status $status');
}

The TaskProgressCallback receives the Task and progess as a double, so a simple callback function is:

void taskProgressCallback(Task task, double progress) {
  print('taskProgressCallback for $task with progress $progress');
}

A basic file download with just status monitoring (no progress) then requires initialization to register the callback, and a call to enqueue to start the download:

  FileDownloader.initialize(taskStatusCallback: taskStatusCallback);
  final successFullyEnqueued = await FileDownloader.enqueue(
      DownloadTask(url: 'https://google.com', filename: 'google.html'));

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.

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'.

Managing and monitoring tasks in the queue

To manage or monitor tasks, use the following methods:

• reset to reset the downloader by cancelling all ongoing download tasks
• 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
• 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
• cancelTasksWithIds to cancel all tasks with a taskId in the provided list of taskIds
• taskForId to get the DownloadTask for the given taskId, or null if not found. Only tasks that are running (ie. not in a final state) are guaranteed to be returned, but returning a task does not guarantee that it is running

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 named default 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);

The methods initialize, registerCallBacks, reset, allTaskIds and allTasks 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.

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 the request method. It 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 Resonse 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, do not allow for status/progress monitoring, and will not execute in the background.

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 the Task, which will be added to the HTTP request. This may be useful for authentication, for example.

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.

DownloadTask & UploadTask

Requiring WiFi

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.

Metadata

metaData can be added to a Task. It is ignored by the downloader but may be helpful when receiving an update about the task.

Initial setup for 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.

No setup is required for Android.

Limitations

• On Android, once started (i.e. TaskStatus.running), a task must complete within 8 minutes
• On iOS, once enqueued (i.e. TaskStatus.enqueued), a background download must complete within 4 hours
• 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