# Audit code that branches on user interface idiom

> Apple's guidance is to avoid making display assumptions based on the user interface idiom. iPhone Duo reports the phone idiom with a regular-width display.

Source: https://iphoneduosupport.com/checklist/idiom-based-assumptions/

Severity: major
Category: layout
Applies to: swiftui, uikit, react-native, flutter, unity, capacitor
Symptom: iPad-quality layouts exist in the codebase but never appear on iPhone Duo, because the branch is gated on idiom.
Sources: https://developer.apple.com/videos/play/tech-talks/111461/
Last verified: 2026-09-09

---

This is the mirror image of adopting size classes: even after you branch on size class in new code, older idiom checks scattered through the app quietly keep forcing the phone layout.

## Find them

```bash
grep -rn "userInterfaceIdiom" --include="*.swift" .
grep -rn "UIDevice.current.model" --include="*.swift" .
grep -rn "horizontalSizeClass == .compact" --include="*.swift" .
```

The third pattern is worth checking carefully. `== .compact` is often used to mean "is a phone", which is now false on the inner display — and that is usually the correct new behaviour, so confirm each site rather than mass-replacing.

## What idiom is still good for

Idiom remains legitimate for genuinely platform-level questions: whether a hardware keyboard is typical, whether Catalyst-specific menus apply, whether a feature exists on the platform at all.

It is not a proxy for available space. That question now belongs to size classes and to the actual bounds you are given.

## The device-detection trap

Do not replace `idiom == .pad` with a check for iPhone Duo specifically. Hard-coding a device model recreates the same fragility one form factor later, and Apple offers no supported way to ask "am I on a Duo". Branch on the properties your layout actually depends on — width, size class, active reserved regions.

## Cross-platform equivalents

The same pattern appears outside Swift. In React Native, `Platform.isPad` and any device-model library used for layout decisions. In Flutter, `defaultTargetPlatform` combined with a hard-coded breakpoint. Both should be replaced with live window dimensions.

## How to verify

After removing idiom-based layout branches, run on the inner display and confirm you get the wide layout. Many apps discover a good iPad layout already in the codebase that was simply unreachable.

---

Published by iPhone Duo Support (https://iphoneduosupport.com). Not affiliated with Apple Inc.