# How to prepare your iOS app for iPhone Duo: a 12-step checklist

Source: https://foldready.dev/guides/prepare-your-app-for-iphone-duo

Published: 2026-09-12. Updated: 2026-09-12. Author: FoldReady (vity5.diduk@gmail.com).

Summary: To prepare an iOS app for iPhone Duo, replace idiom and orientation checks with size classes, adopt the iOS 27.1 ReservedRegion API for custom bars, remove hardcoded frame sizes, and test every screen in all six poses (closed, open and half folded, each in portrait and landscape) in the Xcode 27.1 Device Hub simulator.

Apple announced iPhone Duo, its first foldable iPhone, on September 9, 2026. It ships October 23, 2026. Existing apps launch on it without changes, but apps built against an SDK older than iOS 27 run letterboxed with limited screen usage, and most were designed for one compact portrait screen. This checklist covers what to change so an app feels native on both displays and in every pose.

## Before you start

- Install Xcode 27.1. The iPhone Duo simulator lives in Device Hub and has on-screen buttons to open, close, rotate and fold the device.
- Build with the iOS 27.1 SDK. The iOS 27 SDK extends content left of the status bar on the inner display; the 27.1 SDK reaches the screen edge and lays standard navigation and toolbar buttons out vertically.
- Watch Apple's two tech talks: "Prepare your app for iPhone Duo" and "Strike a pose with adaptive layouts on iPhone Duo".

## The checklist

1. Replace every UIDevice idiom check and interface orientation check with size classes. The outer display is compact width and regular height in portrait, like any iPhone. The inner display is regular width and regular height, like an iPad, which is why iPhone-only branches break.
2. Remove UIScreen.main.bounds and other fixed frame sizes. Window size changes live when the device opens, closes or is placed side by side with another app.
3. Adopt NavigationSplitView in SwiftUI or UISplitViewController in UIKit so the inner display gets a sidebar or two-column layout instead of a stretched single column.
4. Handle asymmetric safe areas and layout margins. On the inner display the system UI is not symmetric left to right.
5. Use the iOS 27.1 ReservedRegion API (SwiftUI) or UIViewReservedRegion (UIKit) for custom bars, toolbars and edge-to-edge canvases so they claim space without colliding with system UI.
6. Query reserved regions by kind. The division kind marks the fold itself; the occlusion kind marks the camera. Keep controls and important content out of both.
7. Design a half-folded pose. When the device is partly folded the inner display behaves like two halves. Media, camera, reading and video-call views should place content on one half and controls on the other rather than straddling the crease.
8. Preserve state across the outer-to-inner handoff. Opening the device while a screen is on the outer display must not reload data or lose scroll and edit state.
9. Support side-by-side multitasking. Users can run two apps at once on the inner display, so the app can be resized to a compact slice at any moment.
10. Adopt the Concentricity APIs for corner shapes so custom rounded containers match the display corners.
11. Test text scaling and Dynamic Type at both display sizes; the inner display is 7.6 inches at 430 ppi and exposes long lines that were previously wrapped.
12. Run every screen through all six poses (three fold states in two orientations) plus side-by-side multitasking in the simulator and capture screenshots for review. Do this again before each release.

## What to prioritize if you have one week

Fix size-class branching first because it affects every screen. Then remove hardcoded widths, which cause the visible clipping that reviewers screenshot. Then handle the half-folded pose for any media or camera screen. Everything else can follow in a point release.

> Need this done for you? FoldReady runs a fixed-price iPhone Duo readiness audit that covers every item on this list and returns a prioritized fix list in five business days.

## FAQ

**Do I need to rewrite my app for iPhone Duo?**

No. Apple states that legacy iOS apps do not require a full refactor. Most apps need targeted changes to size-class handling, safe areas, hardcoded dimensions and custom bars.

**Which Xcode version supports iPhone Duo?**

Xcode 27.1 includes the iPhone Duo simulator in Device Hub and the iOS 27.1 SDK with the ReservedRegion API.

## Sources

- [Apple Tech Talk: Prepare your app for iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111461/)
- [Apple Tech Talk: Strike a pose with adaptive layouts on iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111463/)
- [Apple Newsroom: Apple unveils iPhone Duo](https://www.apple.com/newsroom/2026/09/apple-unveils-iphone-duo/)