Registration of tasks

[Kymer]

We want to be able to register a single and periodic task:

registerSingleTask

• String id
• String taskName
• String tag
• Enum existingWorkPolicy:
  • keep
  • replace
• TimeInterval initialDelay
• Enum constraints:
  • requiresNetworkConnectivity
  • requiresExternalPower
• Enum backoffPolicy
  • exponential
  • linear
• TimeInterval backoffPolicyDelay
• String inputData

registerPeriodicTask

• TimeInterval frequency
• String id
• String taskName
• String tag
• Enum existingWorkPolicy:
  • keep
  • replace
• TimeInterval initialDelay
• Enum constraints:
  • requiresNetworkConnectivity
  • requiresExternalPower
• Enum backoffPolicy
  • exponential
  • linear
• TimeInterval backoffPolicyDelay
• String inputData

We'll start with supporting only BGProcessingTaskRequest for now. Later we can look at BGAppRefreshTaskRequest.

Internally we'll need to keep a complete state of the different tasks.

We need to keep a reference to the tasks in order to know how to behave when a new registration comes in with the same id. Depending on the existingWorkPolicy, the previously registered task needs to be left alone or resubmitted. We also want to be able to cancel all requests with a certain tag. If a task fails it needs to be resubmitted based on the backoffPolicy and backoffPolicyDelay.

For periodic tasks there is a frequency property. Meaning that after each succesful 'run' of the task a new task needs to be submitted with an earliestBeginDate calculated using the frequency that was passed along.

General notes from BGTaskScheduler class docs:

The system runs only tasks registered with identifiers on a whitelist of task identifiers. To add the whitelist, add the identifiers to the Info.plist file.

This is something we'll need to clearly mention in our documentation.

From BGTaskScheduler submit(_:) docs:

Submitting a task request for an unexecuted task that’s already in the queue replaces the previous task request.

There can be a total of 1 refresh task and 10 processing tasks scheduled at any time. Trying to schedule more tasks returns BGTaskScheduler.Error.Code.tooManyPendingTaskRequests.

Useful links:

• Starting and Terminating Tasks During Development
• WWDC 2019: Advances in App Background Execution
• Apple sample code
• Android Workmanager API

[phranck]

@Kymer @olivierscalais JFYI: I'm actively working on our (Open-Xchange) fork of this manager for our Open Source Flutter project OX COI Messenger. I made lots of changes and already added registration for App Refresh Tasks. Please see here.

[AAverin]

Hi guys. Your project seems to be the only one so far trying to leverage iOS BackgroundTask API.
Is there any progress on this already or was the project frozen so far?

[petermnt]

Hi @AAverin , we are currently not actively developing this project.
We are open to pull requests though.