The complicated life of a backgrounded iOS app (360|iDev '18)

Background transfer is an invaluable feature if your app is relying on any critical network operation that could potentially drive your user’s attention away when running long, or being interrupted.

There are non-trivial challanges to solve before being able to support it in your apps though. Including limitations around authentication methods, response/error handling, and keeping the mysterious resume rate limiter happy.

After solving this puzzle with some pieces under-documented, almost impossible to debug and nerve-wracking to test, I’m here to share all our findings to help you understand and work comfortably with background sessions.

Transcript

1. THE COMPLICATED LIFE OF A BACKGROUNDED iOS APP Agnes Vasarhelyi @vasarhelyia
2. WHY DO ANYTHING IN THE BACKGROUND?
4. SUMMARY OF THIS TALK Handling background transfers ⏰ Silent (content) push notifications Authentication Resume rate limiter % Testing Debugging
5. REMINDER App states Source: developer.apple.com
6. REMINDER "Background apps that consume large amounts of memory are
 terminated before those with small memory footprints." Source: developer.apple.com
7. HANDLING BACKGROUND TRANSFER Completion handler-based Delegate-based URLSession API
8. HANDLING BACKGROUND TRANSFER download task Papp Pdaemon ACTIVE BACKGROUND SUSPENDED temporary file URL BACKGROUND
9. HANDLING BACKGROUND TRANSFER Surprises
 • Storing session IDs • Storing completion handler • Tasks are not failing • Users can directly turn off background transfer capability • isDiscretionary might be treated as true
10. HANDLING BACKGROUND TRANSFER isDiscretionary "For time-insensitive tasks, enable the isDiscretionary property, so the system can wait for optimal conditions..." Source: developer.apple.com
11. HANDLING BACKGROUND TRANSFER isDiscretionary "...if the transfer is initiated while the app is in the background,
 ... isDiscretionary property is treated as being true." Source: developer.apple.com
13. ⏰ SILENT (CONTENT) PUSH NOTIFICATIONS Silent push • Silent push with content-available flag set • App gets resumed in the background • Limited time window to execute (30 s) • No user permission necessary • Very similar to background fetch
14. ⏰ SILENT (CONTENT) PUSH NOTIFICATIONS Surprises • User can directly switch it off (Settings / Background Refresh) • Rate limited by APNS • No more than 2, or 3 per hour • Won't receive it when app was terminated
15. ⏰ SILENT (CONTENT) PUSH NOTIFICATIONS "The system tracks the elapsed time, power usage, and data costs..." Source: developer.apple.com
16. ⏰ SILENT (CONTENT) PUSH NOTIFICATIONS "Apps that use significant amounts of ... may not always be woken up..." Source: developer.apple.com
17. ⏰ SILENT (CONTENT) PUSH NOTIFICATIONS What counts? • Number of notifications received • Time executing in the background • Resources used (power, memory)
18. ⏰
19. AUTHENTICATION Client trust validation Server trust validation
20. AUTHENTICATION 1. Certificate pinning ⚠ • Can be simple, and secure • No self-signed certificates in the background • Only system-trusted CA-signed certs (ATS req) • Rolling certs: public key has to be static, backup keys • Long-living certs: $$$
21. AUTHENTICATION 2. Authentication challenges ⚠ • HTTP Basic, HTTP Digest • Easier to implement on both client / server • Too many resumes
22. AUTHENTICATION 3. Custom authentication scheme • Ask for token in a foreground session • Authenticate that request, no matter how • Use that token with the background session requests • Server evaluates that • Expire the token
24. RESUME RATE LIMITER download task Papp Pdaemon ACTIVE BACKGROUND SUSPENDED temporary file URL BACKGROUND
25. RESUME RATE LIMITER nsurlsessiond • delay that doubles with each resume • delay resets to 0 when user activates app • delay resets to 0 when delay elapsed without new task • applies to your app as a whole, not per session
26. RESUME RATE LIMITER download task Papp Pnsurlsessiond ACTIVE BACKGROUND SUSPENDED temporary file URL BACKGROUND BACKGROUND SUSPENDED authentication challenge ⏳
27. RESUME RATE LIMITER 1 download 1 resume No custom authentication challenge 2 resumes 1 download 2n resumes n downloads Custom authentication challenge = delay * 1 = delay * 2 = delay * 22n n resumes n downloads = delay * 2n
28. RESUME RATE LIMITER download task A Pnsurlsessiond Papp ACTIVE temporary file URL A BACKGROUND ⏳ BACKGROUND auth challenge A SUSPENDED isDiscretionary = true download task B ⏳ * 2 SUSPENDED SUSPENDED ⏳ * 4 BACKGROUND auth challenge B temporary file URL B BACKGROUND BACKGROUND SUSPENDED
29. RESUME RATE LIMITER How to make the rate limiter happy? • Batch the transfers • Authentication that doesn't require additional resumes
30. RESUME RATE LIMITER No custom authentication challenge 1 resume [n downloads] = delay * 1 Custom authentication challenge n + 1 resumes [n downloads] = delay * 2n + 1
32. % TESTING Surprises • Free pass for Xcode builds (RRL delays, isDiscretionary) • APNS device token only valid in specific environment • Call registerForRemoteNotifications on every launch
33. %
34. DEBUGGING Console.app 1. Look at nsurlsessiond logs for networking 2. Look at dasd logs for scheduling 3. Search for your bundle ID 4. Put the pieces together
49. Why Must Not Proceed? • An active app downloading? • Not the time of the day? • Got enough background downloading for the hour?
51. Tips for implementing background transfer • Only use it for time-insensitive work • Design your networking in a way that pleases the RRL • Same goes for authentication • Master the usage of Console.app
52. Thank you! 
 
 
 
 
 
 
 
 @vasarhelyia
53. Some useful links
 
 https://developer.apple.com/documentation/uikit/core_app/managing_your_app_s_life_cycle/ preparing_your_app_to_run_in_the_background https://www.nowsecure.com/blog/2017/06/15/certificate-pinning-for-android-and-ios-mobile- man-in-the-middle-attack-prevention/
 https://forums.developer.apple.com/message/42352#42352 https://forums.developer.apple.com/thread/28713
 https://developer.apple.com/documentation/foundation/url_loading_system/ downloading_files_in_the_background https://developer.apple.com/documentation/uikit/uiapplicationdelegate/1623013-application