Apps in the Background

When building an app, a lot of focus and effort is put on crafting the best foreground experience for users. However, our apps also have a hidden life when we move them the background. And the system watchdog is here to make sure they all behave! In this talk, you will learn what the common pitfalls of background execution are, how you can make sure you don’t exceed system resources by applying best practices, and finally, how to optimize network requests when your app is not in the foreground.

Transcript

1. Apps in the Background
   The secret life of apps when you minimize them.
   Alexis Aubry — App Builders 2019
   
   @_alexaubry
   

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2. Delivering a good UX means
   More than a foreground experience
   

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3. Our Plan
   How do we provide the best experience to our users even when they
   don’t interact with the app?
   1. What does it mean to be in the background?
   
   2. What are the common problems and how to detect them?
   
   3. How can we fix these issues and improve the UX?
   

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4. What does it mean to
   be in the background?
   

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5. Possible States
   Minimized App Push Notification Background Refresh
   

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6. Moving to the Background
   applicationWillResignActive(_:)
   UI Snapshot Suspended
   applicationDidEnterBackground(_:)
   

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7. Handling Background Events
   UI Snapshot
   Process the events
   applicationDidEnterBackground(_:)
   Launch if needed
   Suspended
   

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8. How to identify issues?
   

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9. Common Exception Codes
   • 0x8badf00d = took too long to respond to a system event
   • 0xdead10cc = forgot to release a file lock after suspension
   • 0xc51bad01 = CPU usage too high in the background
   
   • 0xc51bad02 = took too long to move to the background
   • 0xc51bad03 = system resources too low to continue
   • 0xbada5e47 = exceeded the limit of background tasks
   

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10. Other Symptoms
    • Missing data or expected event not happening
    • Example: taking too long to process a push payload.
    
    • Routinely check your crash reports for the aforementioned codes.
    

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11. Other Symptoms
    • Unexpectedly high battery usage
    
    • If you take a lot of time to process events, but stay under the limit, the
    system will probably not terminate your app.
    
    • Example: you can spend 20 seconds for each push notification, under
    the limit, but will have an impact on user’s battery.
    
    • At Wire: reduced background activity by 5 hours a week.
    

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12. What are the techniques for the
    best background experience?
    

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13. Use background tasks
    1
    

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14. ✅ Get more time to complete
    your background work
    

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15. How it Works
    UIApplication
    Your Code
    

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16. Starting a Task
    beginBackgroundTask
    Your Code
    Expiration Handler
    

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17. Starting a Task
    beginBackgroundTask
    Your Code
    Expiration Handler
    

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18. Starting a Task
    beginBackgroundTask
    Your Code
    Expiration Handler
    Task Identifier
    

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19. Check for Expiration
    beginBackgroundTask
    Your Code
    Expiration Handler
    .invalid
    

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20. Performing your work
    UIApplication
    Task Identifier Expiration Handler
    Async Queue
    Your Background Work
    

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21. Finishing your work
    endBackgroundTask
    Your Code
    Expiration Handler
    Task Identifier
    

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22. Expiring Properly
    endBackgroundTask
    Your Code
    Expiration Handler
    Task Identifier
    

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23. For App Extensions
    ProcessInfo.processInfo.performExpiringActivity(withReason: "Request") {
    [weak self] expiring in
    guard let self = self else { return }
    if expiring {
    // Clean up your code before suspension
    self.currentRequest?.cancel()
    } else {
    // Otherwise perform the task
    let request = self.networkingLayer.createRequest()
    self.currentRequest = request
    request.resume()
    }
    }
    

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24. When to Use It
    • Every time interrupting the work would be detrimental to the UX
    • Example: when sending a message or saving a file.
    
    • When operating in the foreground
    • Example: asking for extra time to send a message before going in the
    background, so you don’t need to care about the state of the app.
    
    • Caveat: having too many active background tasks can cause a crash
    

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25. How to Optimize It
    • Give names to tasks when creating them.
    
    • Create all your tasks from a single thread.
    
    • Centralize the tasks creation inside one object.
    

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26. Benefits of a Task Manager
    • Manages a single background task to keep the app awake.
    
    • Avoids repeated overhead of creating background tasks.
    
    • Avoids issues with gaps between tasks.
    
    • Avoids issues with needing to create tasks from multiple threads.
    

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27. Building a Task Manager
    BackgroundTasksManager UIKit
    Your
    Code
    

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28. Building a Task Manager
    beginBackgroundTask UIKit
    Your
    Code
    

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29. Building a Task Manager
    BackgroundTasksManager
    Begin task
    with UIKit
    Your
    Code
    No Internal Background Task
    

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30. Building a Task Manager
    BackgroundTasksManager
    Save the
    token of the
    custom task
    When we have an Internal Background Task
    UIKit
    

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31. Building a Task Manager
    endBackgroundTask UIKit
    Your
    Code
    After ending your work
    

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32. Building a Task Manager
    When there are no more active tasks
    BackgroundTasksManager
    End the
    internal task
    Your
    Code
    

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33. Building a Task Manager
    BackgroundTasksManager
    Expire all
    active tasks
    When the extended lifetime expires
    UIKit
    expiration
    callback
    

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34. Building a Task Manager
    BackgroundTasksManager
    End the
    internal task
    Your
    Code
    When the extended lifetime expires
    

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35. Building a Task Manager
    BackgroundTasksManager
    Expire
    immediately
    When creating tasks while the app is expiring
    UIKit cannot
    start a real
    task
    

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36. Implementation on GitHub
    github.com/alexaubry/background-tasks-manager
    

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37. Keep your memory usage
    under control
    2
    

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38. Reducing Memory Usage
    • Makes the system more likely to keep your app suspended for a longer
    period of time.
    
    • Example: Use a purgeable cache with a disk counterpart.
    
    • Example: Reduce your use of images.
    
    • Recommended Reads
    • https://developer.apple.com/library/archive/technotes/tn2434/_index.html
    
    • http://www.thomashanning.com/building-memory-efficient-apps/
    

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39. Use the right API for the
    right task
    3
    

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40. Using the Right API
    • Processing notifications in the background?
    
    • Use a notification service extension.
    • Downloading files in the background?
    
    • Use a background URLSession with sessionSendsLaunchEvents.
    • https://developer.apple.com/documentation/foundation/
    url_loading_system/downloading_files_in_the_background
    

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41. Design for immediately
    cancellable tasks and restoration
    4
    

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42. “Apps must be prepared for

    termination to happen at any time.”
    

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43. Architecture Tips
    • Don’t wait for the user to close the app to save data.
    
    • Make every task cancellable, so that you can stop them during expiration.
    
    • Design background behavior with UX in mind.
    
    • Example: if we cancel sending a message, how do we communicate
    that to the user?
    

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44. What should happen if we cancel a
    running task? Is it recoverable? Does it
    need user interaction to recover?
    

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45. Wrapping Up
    • Different scenarios, different rules.
    
    • Comply to stay frozen in the background as long as possible.
    
    • Look for the signs to detect potential bottlenecks.
    
    • Use background tasks as often and efficiently as possible.
    
    • Use the latest APIs and tools to optimize background work.
    
    • Design for immediately cancellable tasks, think about the UX.
    

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46. Thank you!
    Questions? Tweet me @_alexaubry
    

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