Background Tasks in iOS

Overview

This lecture explains how to use Continuous Background Tasks in iOS and iPadOS to perform long-running operations even when the app is backgrounded, including setup, execution, reporting progress, and handling completion or cancellation.

Background Task Concepts

• Continuous Background Tasks let apps finish critical work if the user backgrounds the app before the job ends.
• BGContinuedProcessingTask starts in the foreground and can continue in the background if needed.
• Suitable for tasks like exporting video, processing images, or analyzing sensor data.
• Tasks can use GPU and CPU resources, as well as network access, during background execution.
• The system may queue tasks or reject them based on available resources and priority.

Creating and Configuring a Background Task

• Create a BGContinuedProcessingTaskRequest with a unique identifier (using your app's bundle ID).
• Set a title and subtitle for task identification in the system UI.
• To enable GPU use, set requiredResources to .gpu and ensure the app has Background GPU Access entitlement.
• Check BGTaskScheduler.supportedResources for .gpu before enabling GPU processing.

Submission Strategy

• The default strategy queues tasks if immediate resources aren't available.
• Set request.strategy = .fail to have submission fail if the system can't execute the task right away.

Running and Registering a Task

• Register the task with BGTaskScheduler using the taskIdentifier and a launch handler.
• The launch handler receives a BGContinuedProcessingTask instance to manage and execute the long-running job.

Submitting a Task Request

• Use BGTaskScheduler.shared.submit(request) to submit the properly configured task request for execution.

Reporting Progress

• Report accurate progress using the Progress class (conforms to ProgressReporting protocol).
• Update task.progress.completedUnitCount regularly to reflect real job progress.
• Users can monitor and cancel tasks via the Live Activity system UI if stuck.

Handling Task Completion and Expiry

• Use the task.expirationHandler to handle cancellation or early termination.
• On normal completion, update progress and return success; if expired or canceled, handle as failure.
• Tasks can fail due to app errors, user cancellation, or system-imposed expiration.

Key Terms & Definitions

• Continuous Background Task — A background task that continues running after the app moves to the background.
• BGContinuedProcessingTask — The API class representing a continuous background processing task.
• BGContinuedProcessingTaskRequest — The request object used to configure and submit a continuous background task.
• BGTaskScheduler — The system scheduler for managing background tasks.
• ProgressReporting — A protocol for reporting task progress to the system.
• Expiration Handler — Code run if a task is canceled or expires before completion.

Action Items / Next Steps

• Implement task registration, request creation, and submission in your app as needed.
• Enable Background GPU Access in Xcode if your tasks require GPU resources.
• Regularly report progress and handle task completion, expiration, or failure in your code.