Apps in the Background - Speaker Deck

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

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Delivering a good UX means More than a foreground experience

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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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What does it mean to be in the background?

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Possible States Minimized App Push Notiﬁcation Background Refresh

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

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

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How to identify issues?

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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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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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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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What are the techniques for the best background experience?

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Use background tasks 1

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✅ Get more time to complete your background work

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How it Works UIApplication Your Code

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Starting a Task beginBackgroundTask Your Code Expiration Handler

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Starting a Task beginBackgroundTask Your Code Expiration Handler

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Starting a Task beginBackgroundTask Your Code Expiration Handler Task Identiﬁer

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Check for Expiration beginBackgroundTask Your Code Expiration Handler .invalid

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

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Finishing your work endBackgroundTask Your Code Expiration Handler Task Identiﬁer

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Expiring Properly endBackgroundTask Your Code Expiration Handler Task Identiﬁer

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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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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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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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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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Building a Task Manager BackgroundTasksManager UIKit Your Code

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Building a Task Manager beginBackgroundTask UIKit Your Code

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

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

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

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

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

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

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

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Keep your memory usage under control 2

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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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Use the right API for the right task 3

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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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Design for immediately cancellable tasks and restoration 4

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“Apps must be prepared for  termination to happen at any time.”

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

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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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Thank you! Questions? Tweet me @_alexaubry