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Is your iPhone app ready for iPhone Duo?

· 5 min read

#apple-dev

iPhone Duo reaches customers on October 23, 2026, and Apple's guidance for developers comes down to one idea: on this device, nothing about the screen tells you the shape of your app. Orientation, idiom and screen bounds can all mislead you, so layout has to follow the space your view is actually given.

What happens if you do nothing

Apple says every app will run on iPhone Duo, but what it looks like depends on the SDK you build with.

That last point matters. Apple's own advice is to check that your content adapts to the bars becoming vertical before you ship a build with the 27.1 SDK. Recompiling is not a free upgrade; it changes your layout.

One more thing from the same page: the app's size changes whenever the device folds or unfolds, and it does not always match the screen size. A fold can resize your app in the middle of a session.

The checklist, in plain terms

Apple's Prepare page groups the fixes into three steps: run an assessment, fix patterns, test resizing. The patterns are the useful part.

Stop sizing from the main screen. Search for UIScreen.main. Replace UIScreen.main.bounds with view.bounds or window.bounds. In SwiftUI, use GeometryReader or onGeometryChange. Reading the size once at launch, or in viewIsAppearing, is not enough. Apple points to layoutSubviews, viewDidLayoutSubviews and viewWillTransition(to:with:) for size-dependent work. Scale comes from traitCollection.displayScale or @Environment(\.displayScale), windows from UIWindow(windowScene:), and you should not store a UIScreen.

Stop using orientation to decide layout. Apple lists UIDevice.current.orientation, statusBarOrientation and interfaceOrientation (including windowScene.effectiveGeometry.interfaceOrientation) as things to remove from layout logic. If you need to know whether you are wider than tall, compare width and height of the space you have.

Stop assuming idiom or size class identifies the device. With iPhone Duo and resizable iPhone Mirroring, an iPhone app can have any combination of size classes. So:

A small SwiftUI version

This is my own sketch of the idea, not Apple's sample. The layout depends only on the space the view receives.

import SwiftUI

struct PhotoGallery: View {
    let names: [String]

    var body: some View {
        GeometryReader { proxy in
            let isWide = proxy.size.width > proxy.size.height
            let columns = Array(repeating: GridItem(.flexible()), count: isWide ? 4 : 2)
            ScrollView {
                LazyVGrid(columns: columns) {
                    ForEach(names, id: \.self) { name in
                        Image(systemName: name)
                            .resizable()
                            .scaledToFit()
                    }
                }
            }
        }
    }
}

GeometryReader re-evaluates when the size changes, so a fold or unfold produces a new column count without any orientation code. Apple names GeometryReader and onGeometryChange as the SwiftUI tools for this. A fixed column count would be a better choice for some screens; the point is only that the input is the size, not the device.

How to test

Apple lists three ways to test resizing: the iOS resizable simulator in Device Hub, iPhone Mirroring on macOS 27 (resize the window to extreme sizes in both directions), and the iPhone Duo simulator in Xcode 27.1. Apple calls the Duo simulator the best way to test the full native experience.

For the first pass, Apple says to ask the coding assistant in Xcode to "get my app ready for iPhone Duo". It runs a resizability skill that scans for these patterns and suggests fixes. Apple adds that it finds most issues, not all, so the manual search for the patterns above is still worth doing. For other coding agents, Apple documents xcrun agent skills export.

App Store dates

Apple's news post says you can submit iPhone Duo optimized apps in App Store Connect today. Starting April 2027, any app or game submitted will need to include screenshots for iPhone Duo. Updated screenshot and preview specs are published, and App Store Connect has a new preview tool for how your assets look on Duo. There is also a featuring nomination option where you can say the app supports all device poses.

Where I would start

I would grep for UIScreen.main, .orientation and userInterfaceIdiom first. Those three searches cover most of what Apple's page lists, and each hit is a place where the app assumes something the device no longer guarantees.

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