DuoReady

Getting Started with iPhone Duo Development

What iPhone Duo is, whether your existing app already works, and the shortest path to running and testing it in Xcode.

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Quick answer

Existing iPhone apps run on Duo without any changes — Apple built Duo support on the same size-class and window-scene systems that already power iPad and Split View. Install Xcode 27.1 beta, run your app on the Duo Simulator, and check whether your layout reflows correctly before working through the full compatibility checklist.

What is iPhone Duo?

iPhone Duo is a foldable iPhone that gives apps significantly more available layout space when opened. Rather than introducing a new device category to design for individually, Apple built Duo support on top of the size class and window scene systems that already power iPad and Split View — meaning most of the work to support Duo well is the same work that makes an app resize correctly anywhere.

Do existing apps work?

Apple recommendation

Yes. Apple states that apps run on iPhone Duo even if they weren't built with the iOS 27 SDK. Rebuilding against the iOS 27 or 27.1 SDK unlocks the ability to use the full available screen space, adaptive layout behavior, and new APIs like ReservedRegion.

In practice this means you can ship on Duo today with zero changes, but you'll want to update your layouts to take advantage of the extra space rather than leaving it letterboxed or blank.

Which SDK should I use?

Build against the iOS 27.1 SDK with Xcode 27.1 beta for the latest Duo-specific APIs and simulator support.

  • Use Xcode 27.1 / iOS 27.1 when you need the Duo Simulator, ReservedRegion, or ArrangementView.
  • Stay on iOS 27 if you must keep a stable Xcode — core adaptive layout is already there.
  • Ship without a rebuild if you only need the app to launch: existing apps run on Duo with any SDK.

Install Xcode 27.1

Download Xcode 27.1 beta from the Apple Developer site — that is the release that includes Duo Simulator support. See the full Xcode testing guide for simulator setup details.

Open Device Hub

Device Hub is where you add the Duo Simulator and drive pose changes — fold, unfold, and rotate — while your app is running.

Run your existing app

Select the Duo Simulator as your run destination and launch — no code changes are required for an existing app to start.

Check layout

The first layout bugs to hunt are UIScreen.main and hardcoded widths — they freeze the layout to one screen size, so folding, the outer display, and Split View all clip or letterbox.

With the app running, resize the simulator window and change poses in Device Hub. Watch for content that doesn't reflow, controls that land under the fold, or text that clips — each is a sign your layout is assuming a fixed screen rather than reading the available space.

Common mistake

Don't size views to UIScreen.main or magic-number screen widths. Search the project for those reads, then use the window scene instead. Walkthrough: Replace UIScreen.main.

Follow the compatibility checklist

Once your app runs and resizes reasonably, work through the full compatibility checklist to catch the less obvious issues — safe area handling, Dynamic Type, and testing every pose.

Next APIs

After the first simulator pass, pick the next API from the layout you actually have — not from the device name.

  • Fix screen-size reads first: Replace UIScreen.main.
  • Adopt ReservedRegion after the app resizes, if custom chrome — floating buttons, overlays, extra toolbars — would still need a hardcoded inset to miss the hinge. System bars already avoid reserved areas.
  • Reach for ArrangementView when two related regions should sit across the fold (list and detail, canvas and inspector, split or overlay). Skip it for a single column that already follows size classes.

Frequently asked questions

Will my existing app run on iPhone Duo without changes?

Yes. Apple states that apps run on iPhone Duo even if they weren't built with the iOS 27 SDK. You can ship on Duo today with zero changes, though the extra screen space will be letterboxed or unused until you update your layouts.

Which SDK should I target for iPhone Duo?

Build against the iOS 27.1 SDK with Xcode 27.1 beta for the latest Duo-specific APIs and simulator support. iOS 27 already includes the core adaptive layout behavior if you need to stay on a stable Xcode release.

What is the first thing to check when running my app on Duo?

Run it on the Duo Simulator, then resize the window and change poses in Device Hub. Watch for content that doesn't reflow, controls landing under the fold, or clipped text — each signals a layout assuming a fixed screen instead of reading available space.

What is the first layout bug I should hunt?

UIScreen.main and hardcoded widths. They freeze the layout to one screen size, so folding, the outer display, and Split View all clip or letterbox. Search the project for UIScreen.main and magic numbers used as screen width, then read the window scene instead. Walkthrough: /uiscreen-main.

When should I adopt ReservedRegion?

After the app resizes, if you still have custom chrome — floating buttons, overlays, extra toolbars — that would need a hardcoded inset to miss the hinge. System bars already avoid reserved areas; ReservedRegion is for your chrome. Start with /reservedregion. UIKit equivalent: UIViewReservedRegion.

When should I reach for ArrangementView?

When you have two related regions that should sit across the fold (list and detail, canvas and inspector, split or overlay). Skip it for a single column that already follows size classes. Placement rules: /arrangementview.