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Mobile apps

A mobile engineer interview on building apps that survive background limits, flaky networks, old versions in the wild, and devices you cannot reach.

These prompts are original practice. They are not questions from a named employer or a live hiring loop.

  1. 1.A field-inspection app for a water utility uploads photos when the inspector taps Submit. Uploads stall when inspectors switch to the camera or lock the phone. What is the most likely cause?

  2. 2.On both iOS and Android, what should an app assume about its process once it moves to the background?

  3. 3.A recipe app returns users to its home screen after a phone call, losing a half-typed note. Testers cannot reproduce it on a new flagship phone. What is the likely explanation?

  4. 4.A habit-tracker team wants a background sync every 5 minutes on all phones. What should they plan for?

  5. 5.A carpool app keeps tracking location in the background after a trip ends, just in case it is useful later. What is the main risk?

  6. 6.Why should an app save a user's draft when it moves to the background rather than when it is about to terminate?

  7. 7.A parking-meter app shows time remaining. After an hour in the background, the counter shows the old value and then slowly catches up. What is the better design?

  8. 8.What is the main purpose of the OS work scheduler (a job or task scheduling API) for background work?

  9. 9.A meditation app sends an analytics event when a session ends. Many sessions end with the user locking the phone, and those events are missing. What is the best fix?

  10. 10.Why must network calls and disk reads stay off the main (UI) thread?

  11. 11.A transit timetable app stutters while scrolling a long list of stops. Profiling shows each row parses and formats a date string during layout. What should you do first?

  12. 12.At 60 frames per second, roughly how much time does an app have to produce each frame?

  13. 13.A photo-journal app freezes for half a second when opening its gallery. The code decodes full-resolution photos to show 100-pixel thumbnails. What is the best fix?

  14. 14.A grocery-list app takes 4 seconds to show its first screen. Traces show it sets up analytics, ads, a crash reporter, and a database migration before drawing. What is the best first step?

  15. 15.What is the difference between a cold start and a warm start?

  16. 16.A budgeting app measures startup on the team's new test phones and sees 600 ms. Users on older devices complain about slow launches. What should the team do?

  17. 17.A hiking app's map drops frames whenever the user pans. A trace shows garbage collection pauses during each pan gesture. What is the likely cause?

  18. 18.A news reader parses a large JSON feed on a background thread, then posts 2,000 small UI updates to the main thread one at a time. Scrolling hitches after each refresh. What is the best change?

  19. 19.What does an application not responding (ANR) error or a watchdog termination usually indicate?

  20. 20.A regional theater's ticketing app runs a database migration on first launch after each update. Some users stare at the launch screen for 20 seconds, then the app is killed. What is the best fix?

  21. 21.What does offline-first mean for a mobile app?

  22. 22.A grain co-op app lets field workers log harvest weights in barns with no signal. Entries are lost when the app is closed before workers reach coverage. What design prevents this?

  23. 23.Why is a local relational database such as SQLite often chosen over one big JSON file for app data?

  24. 24.A dental clinic scheduler caches appointments in a local database. After a new version changes the schema, some users crash on launch. What was most likely missed?

  25. 25.A county library catalog app shows cached search results when the phone is offline. What should the UI do?

  26. 26.A ferry-ticket app stores purchased tickets in the cache directory. Some users find their tickets missing at the dock when the phone is low on storage. What is the cause?

  27. 27.What is the outbox (pending operations queue) pattern in an offline-first app?

  28. 28.A plant-care app uses optimistic UI. When a user marks a plant as watered, the screen updates at once and the request runs in the background. The request later fails permanently. What should the app do?

  29. 29.A field survey app stores 50,000 records locally, and its list screen is slow even offline. The query filters by county and sorts by date. What is the most likely fix?

  30. 30.What is a last-write-wins conflict strategy, and what is its main risk?

  31. 31.A shared shopping list app lets two family members edit the same list offline. Both add items and then sync, and one person's items vanish. What change fixes this best?

  32. 32.Why are device clocks a poor basis for ordering edits made on many phones?

  33. 33.A volunteer shift app downloads every shift each time it opens. There are now 80,000 shifts and sync takes a minute. What is the better approach?

  34. 34.A choir rehearsal app syncs by asking for songs updated since its last sync time. It never learns that some songs were deleted on the server. What is missing?

  35. 35.In a home inspection firm's app, two inspectors edit different fields of the same report while offline. How should sync handle this?

  36. 36.Why should each queued write from a phone carry a unique ID generated on the client?

  37. 37.A food bank inventory app syncs counts of canned goods. Two volunteers each record plus 10 cans while offline, and after sync the count went up by only 10. What went wrong?

  38. 38.Why should client retries use exponential backoff with random jitter?

  39. 39.A bike-share app retries a failed end-ride request. Some riders are charged twice for one ride. What is the best fix?

  40. 40.A delivery-driver app uses a 60-second timeout on every request. On a weak cell signal, the app looks frozen for a full minute. What should change?

  41. 41.For a non-idempotent POST with no idempotency key, which failures are generally safe to retry automatically?

  42. 42.A weather app polls its API every 30 seconds in the foreground and every 2 minutes in the background. Users report heavy battery drain. What is the main reason?

  43. 43.A podcast app downloads new episodes automatically. What default best respects users' data plans?

  44. 44.A pet-sitting app shows a spinner forever when a request fails in an elevator. Code review shows errors are logged but never shown. What is the core problem?

  45. 45.Why does batching several small analytics events into one request usually save battery?

  46. 46.A sports-score app's API goes down every time a big game ends. Every client retries failed requests on a fixed 2-second interval. What is happening?

  47. 47.A hotel check-in app decides it is online by checking whether a network interface is connected. Guests on Wi-Fi with a login page get endless errors. What is the better approach?

  48. 48.What does HTTP caching with ETag and If-None-Match let a mobile client do?

  49. 49.A wildlife survey app uploads 20 MB videos. On spotty rural networks, each upload starts over from zero after every drop. What is the best fix?

  50. 50.Why should an app not rely on push notifications alone to deliver data that must arrive?

  51. 51.A pharmacy refill app sends a notification reading that a named drug is ready for pickup. What is the main concern?

  52. 52.A coworking-space app's pushes stop reaching some users after they reinstall or move to a new phone. The server keeps sending to the old token. What is the fix?

  53. 53.When should a gardening app ask for notification permission?

  54. 54.What is a silent (data-only) push typically used for, and what is its limit?

  55. 55.A sports league scheduling app sends a push saying a game was moved. Users tap it and land on the home screen instead of the game. What is most likely missing?

  56. 56.Why are verified web links (universal links or app links) usually preferred over custom URL schemes for deep links?

  57. 57.A real-estate app's deep link opens a listing by ID. A link to a removed listing crashes the app. What is the right behavior?

  58. 58.A payroll service's password-reset link opens the app, which resets the password using a token from the URL with no other checks. What is the concern?

  59. 59.A concert app adds a new deep link path in version 5.0. Users still on 4.x tap one of the new links. What should the team plan for?

  60. 60.A recipe-sharing app's deep links work when the app is already running but do nothing after a cold start. What is the most likely cause?

  61. 61.What is deferred deep linking?

  62. 62.Why ask for a runtime permission in context rather than at launch?

  63. 63.A trail-running app asks for always-on location access at first launch, though it only records runs the user starts. What should it do instead?

  64. 64.A coupon app's users deny camera permission, and its barcode scanner then shows a black view. What is the right handling?

  65. 65.What is data minimization in a mobile app?

  66. 66.A clinic check-in app's crash reports include full request bodies with patient names. What should the team do first?

  67. 67.A notes app wants to read the clipboard at launch to suggest pasting a copied link. What is the main concern?

  68. 68.A kids' spelling app includes a third-party software development kit (SDK) that collects device identifiers. What should the team check?

  69. 69.Where should an app store a long-lived refresh token on the device?

  70. 70.A gym membership app embeds its backend API secret in the app binary to sign requests. What is the problem?

  71. 71.Why do mobile OAuth flows use PKCE (Proof Key for Code Exchange)?

  72. 72.A credit union's savings app runs on shared branch tablets. Logout deletes the local token, but the server still accepts that token afterward. What is missing?

  73. 73.Why should access tokens on a phone be short-lived, with a refresh token used to get new ones?

  74. 74.A tutoring marketplace app logs users in through an embedded web view and reads the password field with injected JavaScript. What is the better approach?

  75. 75.A home security camera app refreshes tokens from three screens in parallel, and users get logged out at random. The server rotates the refresh token on each use. What is the cause?

  76. 76.Why do staged rollouts matter more for mobile apps than for web apps?

  77. 77.A ski resort's lift-ticket app releases to 5 percent of users. Crash-free sessions drop from 99.8 to 97 percent on the new version. What should the team do first?

  78. 78.A bad mobile release reached 20 percent of users. The team can halt the rollout but cannot roll it back. What is the realistic path to a fix?

  79. 79.A city parking app's release is stuck in app review for a week, delaying a server change that went live the same day. What practice avoids this?

  80. 80.What is the main purpose of a beta channel, such as a store test track, before a production release?

  81. 81.A tide-tracker app's release build crashes on launch, while debug builds work. What is a common cause?

  82. 82.What should gate expanding a staged rollout from 10 to 50 percent?

  83. 83.Why must a mobile backend keep supporting older app versions for a long time?

  84. 84.A taxi dispatch API renames a JSON field from eta to etaSeconds. Older app versions now show blank arrival times. What should the team have done?

  85. 85.A kayak rental app's old version crashes when the server returns a new booking status value, paused. What client practice would have prevented this?

  86. 86.What is the main use of a forced upgrade (minimum version) gate?

  87. 87.A charity donations app is adding a forced upgrade mechanism. Where should the minimum supported version be defined?

  88. 88.A laundromat app forces an upgrade for every version below 3.2. Some users' phones cannot run 3.2 because their OS is too old. What should the team consider?

  89. 89.How do remote feature flags help with mobile releases?

  90. 90.A blood donor app reads feature flags at launch. With no network, a new appointment booking flow fails because its flag value is undefined. What is the better design?

  91. 91.A two-year-old fitness app has 140 feature flags, and many are fully rolled out. What should the team do?

  92. 92.A grain co-op pricing app's API adds a new required field to a POST body. Old app versions start getting 400 errors. What should the API do?

  93. 93.Why should a mobile app send its version and build number with API requests?

  94. 94.Why must symbol files, such as dSYM or mapping files, be uploaded for release builds?

  95. 95.A tutoring app crashes on only one phone model with one OS version, and no one on the team owns that phone. What should you do first?

  96. 96.A birdwatching app's crash rate looks healthy, but users say it just closes during long sessions. Crash reports show nothing. What is the likely cause?

  97. 97.What are breadcrumbs in crash reporting?

  98. 98.A food truck finder app wants to log user actions for debugging. What should those logs avoid?

  99. 99.What does a crash-free users rate tell you that a raw crash count does not?

  100. 100.A fishing license app's support team gets vague reports saying it does not work. What is the best low-cost addition?

  101. 101.What does a screen reader need from a custom icon-only button?

  102. 102.A bus pass app's layout breaks when users choose a large system text size. Text is cut off in fixed-height rows. What is the fix?

  103. 103.A volunteer fire department paging app shows alert status only by color, red for urgent and green for routine. What should change?

  104. 104.Roughly what minimum touch target size do mobile accessibility guidelines recommend?

  105. 105.A driving school quiz app shows each question with a countdown timer. Screen reader users fail most questions. What should the team do?

  106. 106.Users with vestibular disorders report nausea from a parallax effect in a travel app. What should the app do?

  107. 107.What does device fragmentation mean for a mobile team?

  108. 108.A vaccine record app's background reminders work on most Android phones but never fire on one manufacturer's devices. What is a likely cause?

  109. 109.How should a mobile team choose which devices to test on?

  110. 110.An art supply store app looks fine on phones, but on tablets and foldables its buttons stretch across the whole screen. What is the better approach?