AdvancesInAppBackgroundExecution.md

Advances in App Background Execution

목차

• Overview of Background Execution
• Best practices
• New BackgroundTasks framework

Apple Document: BackgroundTasks 참고

Overview of Background Execution

• background execution이란 앱이 foreground 상태가 아닌 상황에서 동작하고 실행되는 것을 말한다.

Why Do We Enter This State?

• App request
• Event trigger

Important Considerations for Background Execution

• Power
• Performance
• Privacy

Best Practices

Send Messages

• User expects immediate completion
• Protect completion
• Protect by using Background Task Completion
• Start based on user action

Background Task Completion

Gives app additional time to run in the background before being suspended

• UIApplication.beginBackgroundTask(expirationHandler:)
• ProcessInfo.performExpiringActivity(withReason:using:)

Complete work started in the foreground

• Saving files to disk, completing user-initiated requests

// Guarding Important Tasks while App is Still in the Foreground
func send(_ message: Message) {
    var sendOperation = SendOperation(message: message)
    var identifier: UIBackgroundTaskIdentifier!
    identifier = UIApplication.shared.beginBackgroundTask(expirationHandler: {
        sendOperation.cancel()
        postUserNotification("Message not sent, please resend")
        // Background task will be ended in the operation's completion block below
    })
    sendOperation.completionBlock = {
        UIApplication.shared.endBackgroundTask(identifier)
    }
    operationQueue.addOperation(sendOperation)
}

Phone Calls

• VoIP push notificatons
• Special type of push that launches your app

func registerForVoIPPushes() {
    self.voipRegistry = PKPushRegistry(queue: nil)
    self.voipRegistry.delegate = self
    self.voipRegistry.desiredPushTypes = [.voIP]
}

VoIP Pushes

• Must report incoming call with CallKit in didReceiveIncomingPush callback
• If not, system may stop launching your app for VoIP pushes

let provider = CXProvider(configuration: providerConfiguration)

func pushRegistry(_ registry: PKPushRegistry, didReceiveIncomingPushWith payload: PKPushPayload, for type: PKPushType, completion: @escaping () -> Void) {
    if type == .voIP {
        if let handle = payload.dictionaryPayload["handle"] as? String {
            let callUpdate = CXCallUpdate()
            callUpdate.remoteHandle = CXHandle(type: .phoneNumber, value: handle)
            let callUUID = UUID()
            provider.reportNewIncomingCall(with: callUUID, update: callUpdate) { _ in
                completion()
            }
            establishConnection(for: callUUID)
        }
    }
}

• Other tips
  • Caller info in push payload
  • Set apns-expiration on push to 0 or something small
  • Prefer a banner? Use standard pushes
    • Notification Service Extension to modify content

Muted Threads

• Many different threads

• User may not want alerts for specific thread

• Message content may be relevant

• Alert device, not user

• Use background push as best effort way to download content

• Can always download when app re-enters foreground

Background Pushes

• Mechanism to tell device new data is available without alerting user

• Send push with "content-available: 1" without "alert", "sound", or "badge"

• System intelligently decides when to launch app to download content

• Must set apns-priority = 5 or app will not launch

• Should set apns-push-type = background

Download Past Attachments

• User signs into account
• Download conversation list, recent messages immediately
• Defer download of older content
• Defer work if possible to minimize user-visible impact
• Same principles to any deferrable download or upload
  • Batching analytics uploads
  • Photos

Discretionary Background URL Session

• Allows the system to defer the download until a better time

• Provide information to system for smarter scheduling

  // Set up background URL session
  let config = URLSessionConfiguration.background(withIdentifier: "com.app.attachments")
  let session = URLSession(configuration: config, delegate: ..., delegateQueue: ...)
  
  // Set discretionary
  config.discretionary = true
  
  // Set timeout intervals
  config.timeoutIntervalForResource = 24 * 60 * 60
  config.timeOutIntervalForRequest = 60
  
  // Create request and task
  var request = URLRequest(url: url)
  request.addValue("...", forHTTPHeaderField: "...")
  let task = session.downloadTask(with: request)
  
  // Set time window
  task.earliestBeginDate = Date(timeIntervalSinceNow: 2 * 60 * 60)
  
  // Set workload size
  task.countOfBytesClientExpectsToSend = 160
  task.countOfBytesClientExpectsToReceive = 4096
  task.resume()

New BackgroundTasks Framework

BackgroundTasks가 실행되는 시점 조절

BackgroundTasks

• A new framework for scheduling background work
• Available on iOS, iPadOS, tvOS, and iPad Apps on Mac

Background Processing Tasks

• Several minutes of runtime at system-friendly times
  • Deferrable maintenance work
  • Core ML training/inference
• Can turn off CPU Monitoring for intensive work
• Eligible if requested in foreground or if app has been recently used

Background App Refresh Task

• New API, same policies

  • 30 seconds of runtime
  • Keep app up-to-date throughout the day

• Eligibility based on usage patterns

• Old UIApplication fetch API is deprecated, and not supported on Mac

  UIApplication.setMinimumBackgroundFetchInterval(_:)
  UIApplicationDelegate.application(_:performFetchWithCompletionHandler:)

Additional Considerations

• Don't set earliestBeginDate too far into the future
• Ensure files are accessible while device is locked
  • FileProtectionType.completeUntilFirstUserAuthentication
• UIScene apps should call UIApplication.requestSceneSessionRefresh(_:)
• Consider calling BGTaskScheduler.submit(_:) on a background queue if submitting at launch

Summary

• Be conscientious when running in the background
• Use the right background mode for the job
• Schedule deferrable work with BackgroundTasks

BackgroundTasks에 대한 경험이 거의 전무하다 보니 이해에 좀 부족함이 있는 것 같다. 문서랑 예제 코드를 다시 살펴볼 필요가 있다.