How to use background in iOS
When your app moves to the background, the system calls your app delegate’s applicationDidEnterBackground(_:) method. That method has five seconds to perform any tasks and return. Shortly after that method returns, the system puts your app into the suspended state. For most apps, five seconds is enough to perform any crucial tasks, but if you need more time, you can ask UIKit to extend your app’s runtime.
You extend your app’s runtime by calling the beginBackgroundTask(withName:expirationHandler:) method. Calling this method gives you extra time to perform important tasks.
Use the BackgroundTasks framework to keep your app content up to date and run tasks requiring minutes to complete while your app is in the background. Longer tasks can optionally require a powered device and network connectivity.
Register launch handlers for tasks when the app launches and schedule them as required. The system will launch your app in the background and execute the tasks.
The main API for using this framework is the BGTaskScheduler . This API constantly monitors the system state such as battery level, background usage, and more, so it chooses the optimal time to run your tasks.
To use this API, you begin working when your app is on the foreground. You need to create Background task request. The framework provides an abstract class BGTask, you never use this task directly. Instead, the framework provides two concrete subclasses you can interact with: BGProcessingTask, for long running and maintenance tasks such backup and cleanup, and BGAppRefreshTask to keep your app up-to-date throughout the day.
When you create your background download or upload tasks with URLSession, you’re actually scheduling a download (or upload) with the ‘nsurlsessiond’ which is a daemon service that runs as a separate process.