App size & energy — thinning, ODR, what drains the battery

App size — how it works

• Slicing — a variant per device: arch + asset-catalog traits (scale, idiom, Metal GPU family, memory class). Only asset catalogs slice.
• Bitcode — deprecated in Xcode 14; the App Store no longer takes it.
• Three sizes — upload (universal) · download (compressed variant; the cellular prompt checks it) · install (unpacked). Over cellular iOS asks above 200 MB (iOS 13+; user-adjustable in Settings).
• ODR — tagged resources hosted by the App Store: initial install, prefetched (after install), on demand. NSBundleResourceRequest(tags:) + beginAccessingResources; the live request pins them — after endAccessingResources/deinit they are purgeable. Own CDN: Background Assets (iOS 16).
• Measure — Organizer export, App Thinning: all variants → App Thinning Size Report.txt (per variant, compressed + not); App Store Connect file sizes; assetutil on Assets.car.
• Shrink — catalog assets (HEIC, vectors), drop unused images/localizations; -Osize, dead-code stripping, symbols into the dSYM, fewer dynamic frameworks.

Energy — what drains

• Radio tail (diagram) — every wake of cellular/Wi-Fi pays seconds of high power. Polling every 10 s = the radio never sleeps; push (APNs rides one shared system connection) = the OS pays.
• CPU wakeups — timers, polling loops, chatty background work stop deep idle. timer.tolerance ≥ 10% lets iOS coalesce. Off-main ≠ cheaper: same joules.
• GPS — continuous kCLLocationAccuracyBest is among the worst. Least accuracy you need, distanceFilter, one-shot requestLocation(), significant-change (∼500 m, ≥ 5 min) or region monitoring, stop when the screen closes. “When In Use” can still drain: mode + accuracy decide, not the permission.
• Display + GPU — isIdleTimerDisabled left on, needless 120 Hz animation, offscreen passes.

Energy — what to do

• Batch — buffer analytics, prefetch in one burst, flush on Wi-Fi/charging. isDiscretionary = true: iOS picks the moment (automatic for transfers started in the background). Respect Low Data Mode (allowsConstrainedNetworkAccess).
• Low Power Mode — pause prefetch, autoplay, decoration; never break what the user just asked for. Read ProcessInfo.processInfo.isLowPowerModeEnabled, observe NSProcessInfoPowerStateDidChange.
• Thermal — thermalState .nominal/.fair/.serious/.critical: at .serious cut frame rate, quality, effects.
• Measure — Xcode Energy Impact gauge; Instruments Energy Log (on device); field: MetricKit (iOS 13) MXMetricPayload ∼daily — cpuMetrics, locationActivityMetrics, networkTransferMetrics, cellularConditionMetrics, MXCPUExceptionDiagnostic; Organizer → Energy.

Example

let req = NSBundleResourceRequest(tags: ["level-3"])
req.beginAccessingResources { error in // keep `req` alive
  if let error { return showRetry(error) }
  loadLevel3() }            // later: req.endAccessingResources()
let cfg = URLSessionConfiguration
  .background(withIdentifier: "sync")
cfg.isDiscretionary = true             // iOS picks the moment
cfg.allowsExpensiveNetworkAccess = false // no cellular
let pi = ProcessInfo.processInfo
if pi.isLowPowerModeEnabled
   || [.serious, .critical].contains(pi.thermalState) {
  prefetcher.pause() }                 // shed optional work

Interview traps

• “Enable bitcode for thinning” — removed; slicing + ODR remain.
• ODR read by bundle path without a live request — maybe never downloaded, or purged. ODR moves when bytes arrive: loading UI, retry, offline.
• “One small request can’t matter” — tail energy. Batch it.

Remember

Slice, tag, strip. Batch, defer, coarsen, stop.

Likely questions

1. Download vs install size? — compressed variant vs unpacked.
2. Keep ODR from purging? — hold the request object.
3. Real-user energy data? — MetricKit, Organizer.