How to build an app for iPhone Duo, Apple's foldable iPhone

What changes on iPhone Duo

• Two screens, one app. The outer display (1398 by 2034 pixels at 3x) acts like other iPhones. The inner display (2007 by 2853 pixels at 3x) is regular width and regular height, so there is room for a sidebar or two panes side by side.
• Folding is a live resize. When someone opens or closes the phone, your app gets a new size and new size classes while it is running. State must survive that change.
• The fold and the cameras take space. iOS 27.1 lets you ask where the fold and camera areas are, so content does not sit under the hinge.

Layout patterns that work

• Use NavigationSplitView for list and detail apps. It shows one column folded and two columns unfolded, with no extra code.
• Use ViewThatFits to offer a compact and a wide arrangement and let SwiftUI pick.
• Read the horizontal size class from the environment instead of checking the device model.
• Never hard code a screen width, and avoid UIScreen.main.bounds.
• Give toolbar items both a title and an icon, because iOS 27.1 can move the toolbar to a vertical edge on the inner screen.

New iOS 27.1 APIs for the fold

• ArrangementView manages a primary and a secondary region, shown split side by side or as an overlay.
• Reserved regions: proxy.reservedRegions() in a GeometryReader reports the fold and camera areas.
• onHingeChange reports the hinge angle, for optional effects such as a half-open tabletop layout.
• Vertical toolbar placement through axisBehavior and the toolbarVerticalEdge environment value.

These need the iOS 27.1 SDK. Wrap them in if #available(iOS 27.1, *) so the same app still runs on every other iPhone. Start with adaptive layouts and add fold-specific touches only where they help.

Testing folded and unfolded

Install Xcode 27.1 with the iOS 27.1 simulator. The iPhone Duo simulator has toolbar buttons to fold and unfold the device, and holding Option reveals a slider to set the exact hinge angle. Test every screen in both states, and fold in the middle of a task to check that nothing resets. Xcode 27.1 is in beta as of September 2026, and Apple does not accept App Store builds made with a beta Xcode, so plan to ship any 27.1-only APIs once Xcode 27.1 is released.

How appflight builds Duo-ready apps

appflight tells its AI to build every app with adaptive layouts: NavigationSplitView for list and detail screens, ViewThatFits for flexible rows, size classes instead of device checks, and no fixed screen sizes. That means an app you describe today already fits the Duo's outer and inner screens, and iPad. When Xcode 27.1 is installed on your Mac, appflight can preview your app on its own iPhone Duo simulator, which it creates and manages without touching your other simulators. Fold and unfold with the Simulator app's toolbar, then describe any change in plain English. When the app is ready, appflight signs it and uploads it to TestFlight so you can try it on a real device.