flutter_workmanager

Introduction: A Flutter plugin which allows you to execute code in the background on Android and iOS.
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Flutter WorkManager is a wrapper around Android's WorkManager, iOS' performFetchWithCompletionHandler and iOS BGAppRefreshTask, effectively enabling headless execution of Dart code in the background.

This is especially useful to run periodic tasks, such as fetching remote data on a regular basis.

This plugin was featured in this Medium blogpost

Platform Setup

In order for background work to be scheduled correctly you should follow the Android and iOS setup first.

How to use the package?

See sample folder for a complete working example.
Before registering any task, the WorkManager plugin must be initialized.

void callbackDispatcher() {
  Workmanager().executeTask((task, inputData) {
    print("Native called background task: $backgroundTask"); //simpleTask will be emitted here.
    return Future.value(true);
  });
}

void main() {
  Workmanager().initialize(
    callbackDispatcher, // The top level function, aka callbackDispatcher
    isInDebugMode: true // If enabled it will post a notification whenever the task is running. Handy for debugging tasks
  );
  Workmanager().registerOneOffTask("task-identifier", "simpleTask");
  runApp(MyApp());
}

The callbackDispatcher needs to be either a static function or a top level function to be accessible as a Flutter entry point.

The workmanager runs on a separate isolate from the main flutter isolate. Ensure to initialize all dependencies inside the Workmanager().executeTask.

Debugging tips

Wrap the code inside your Workmanager().executeTask in a try and catch in order to catch any exceptions thrown.

void callbackDispatcher() {
  Workmanager().executeTask((task, inputData) {

    int? totalExecutions;
    final _sharedPreference = await SharedPreferences.getInstance(); //Initialize dependency

    try { //add code execution
      totalExecutions = _sharedPreference.getInt("totalExecutions");
      _sharedPreference.setInt("totalExecutions", totalExecutions == null ? 1 : totalExecutions+1);
    } catch(err) {
      Logger().e(err.toString()); // Logger flutter package, prints error on the debug console
      throw Exception(err);
    }

    return Future.value(true);
  });
}

Android tasks are identified using their taskName. iOS tasks are identitied using their taskIdentifier.

However, there is an exception for iOS background fetch: Workmanager.iOSBackgroundTask, a constant for iOS background fetch task.


Work Result

The Workmanager().executeTask(... block supports 3 possible outcomes:

  1. Future.value(true): The task is successful.
  2. Future.value(false): The task did not complete successfully and needs to be retried. On Android, the retry is done automatically. On iOS (when using BGTaskScheduler), the retry needs to be scheduled manually.
  3. Future.error(...): The task failed.

On Android, the BackoffPolicy will configure how WorkManager is going to retry the task.

Refer to the example app for a successful, retrying and a failed task.

iOS specific setup and note

iOS supports One off tasks with a few basic constraints:

Workmanager().registerOneOffTask(
  "task-identifier",
  simpleTaskKey, // Ignored on iOS
  initialDelay: Duration(minutes: 30),
  constraints: Constraints(
    // connected or metered mark the task as requiring internet
    networkType: NetworkType.connected,
    // require external power
    requiresCharging: true,
  ),
  inputData: ... // fully supported
);

For more information see the BGTaskScheduler documentation.

Customisation (Android)

Not every Android WorkManager feature is ported.

Two kinds of background tasks can be registered :

  • One off task : runs only once
  • Periodic tasks : runs indefinitely on a regular basis
// One off task registration
Workmanager().registerOneOffTask(
    "oneoff-task-identifier", 
    "simpleTask"
);

// Periodic task registration
Workmanager().registerPeriodicTask(
    "periodic-task-identifier", 
    "simplePeriodicTask", 
    // When no frequency is provided the default 15 minutes is set.
    // Minimum frequency is 15 min. Android will automatically change your frequency to 15 min if you have configured a lower frequency.
    frequency: Duration(hours: 1),
)

Each task must have an unique name;
This allows cancellation of a started task.
The second parameter is the String that will be sent to your callbackDispatcher function, indicating the task's type.

Tagging

You can set the optional tag property.
Handy for cancellation by tag.
This is different from the unique name in that you can group multiple tasks under one tag.

Workmanager().registerOneOffTask("1", "simpleTask", tag: "tag");

Existing Work Policy

Indicates the desired behaviour when the same task is scheduled more than once.
The default is KEEP

Workmanager().registerOneOffTask("1", "simpleTask", existingWorkPolicy: ExistingWorkPolicy.append);

Initial Delay

Indicates how along a task should waitbefore its first run.

Workmanager().registerOneOffTask("1", "simpleTask", initialDelay: Duration(seconds: 10));

Constraints

Constraints are mapped at best effort to each platform. Android's WorkManager supports most of the specific constraints, whereas iOS tasks are limited.

  • NetworkType Constrains the type of network required for your work to run. For example, Connected. The NetworkType lists various network conditions. .connected & .metered will be mapped to requiresNetworkConnectivity on iOS.
  • RequiresBatteryNotLow (Android only) When set to true, your work will not run if the device is in low battery mode. Enabling the battery saving mode on the android device prevents the job from running
  • RequiresCharging When set to true, your work will only run when the device is charging.
  • RequiresDeviceIdle (Android only) When set to true, this requires the user’s device to be idle before the work will run. This can be useful for running batched operations that might otherwise have a - negative performance impact on other apps running actively on the user’s device.
  • RequiresStorageNotLow (Android only) When set to true, your work will not run if the user’s storage space on the device is too low.
Workmanager().registerOneOffTask(
    "1",
    "simpleTask",
    constraints: Constraints(
        networkType: NetworkType.connected,
        requiresBatteryNotLow: true,
        requiresCharging: true,
        requiresDeviceIdle: true,
        requiresStorageNotLow: true
    )
);

InputData

Add some input data for your task. Valid value types are: int, bool, double, String and their list

 Workmanager().registerOneOffTask(
    "1",
    "simpleTask",
    inputData: {
    'int': 1,
    'bool': true,
    'double': 1.0,
    'string': 'string',
    'array': [1, 2, 3],
    },
);

BackoffPolicy

Indicates the waiting strategy upon task failure.
The default is BackoffPolicy.exponential.
You can also specify the delay.

Workmanager().registerOneOffTask("1", "simpleTask", backoffPolicy: BackoffPolicy.exponential, backoffPolicyDelay: Duration(seconds: 10));

Cancellation

A task can be cancelled in different ways :

By Tag

Cancels the task that was previously registered using this Tag, if any.

Workmanager().cancelByTag("tag");

By Unique Name

Workmanager().cancelByUniqueName("<MyTask>");

All

Workmanager().cancelAll();
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