Building for iPhone Duo with SwiftUI

Adaptive Layout

An existing SwiftUI app that already adapts to size classes runs correctly on iPhone Duo with no code changes — Duo adds available space, not a separate SwiftUI framework.

The right mental model is not "does this look right on Duo" but "does this look right at every width my app can be given." The same layout code that handles a resized window on iPad, or Split View on iPhone, is what makes an app work well on Duo.

ArrangementView

Use ArrangementView when two related regions should move together as the window and fold change — not as a replacement for NavigationSplitView.

• Use it for a player and transcript, a canvas and inspector, or controls over content, with .arrangementViewStyle(.split) or .overlay (the default is .split).
• Keep list-detail navigation in NavigationSplitView.
• Skip it when a single column already follows size classes.

Placement rules, axes, and overlay z-index live on the ArrangementView guide.

Size Classes

Branch on horizontalSizeClass and verticalSizeClass, not on the iPhone Duo device model — checking device identity breaks when Apple ships a new posture, a new device, or the user resizes the window in Split View.

SwiftUI exposes those classes through the environment:

@Environment(\.horizontalSizeClass)
private var horizontalSizeClass

@Environment(\.verticalSizeClass)
private var verticalSizeClass

var body: some View {
    if horizontalSizeClass == .regular {
        TwoColumnLayout()
    } else {
        SingleColumnLayout()
    }
}

Geometry

Measure the container you were given — with GeometryReader or containerRelativeFrame — when size classes are too coarse, and lay out proportionally rather than against fixed point values.

GeometryReader { proxy in
    HStack(spacing: 0) {
        SidebarView()
            .frame(width: proxy.size.width * 0.34)
        DetailView()
    }
}

Safe Areas

On Duo, the safe area is asymmetric — the fold consumes space unevenly depending on the pose. Always respect safeAreaInsets rather than assuming symmetric top and bottom padding, and prefer .safeAreaPadding() over manual constants.

ReservedRegion

ReservedRegion is a new API introduced with iOS 27.1 for SwiftUI. It lets custom UI claim the maximum available space while automatically avoiding areas the system reserves for its own chrome — such as controls near the fold. The UIKit equivalent is UIViewReservedRegion. Full usage notes: ReservedRegion.

ContentView()
    .reservedRegion(.systemControls) { region in
        CustomOverlay()
            .padding(region.insets)
    }

• Adopt it for custom chrome that would otherwise need a hardcoded hinge inset. It isn't required for a size-class-only layout.
• Skip it on system containers — they already avoid reserved areas.
• Use UIViewReservedRegion for the same job in UIKit.

Navigation

Use NavigationSplitView for list-detail instead of a manually built master-detail layout — it collapses to one column in Compact width and expands in Regular width as Duo's window resizes.

Split View

Split View on Duo behaves like Split View on iPad: your app can be given any width within a range, and that width can change at any time while the app is running. Test that your layout responds immediately to a live resize, not just to a cold launch at a given width.

Fold Handling

Avoid placing interactive controls or critical content directly across the fold line. Use safe area insets and ReservedRegion to keep tappable elements clear of the hinge, and design your layout so content can flow around it rather than through it.

Multiple Displays

Use the outer display as a secondary scene — preview, now-playing, or summary — while the inner display shows the primary UI; this is multi-scene support, not a UIScreen.main switch.

See the Multiple Displays guide for scene lifecycle details.

Testing

Test every pose in the Duo Simulator: closed, half-open, fully open, and both orientations — layout must update without a relaunch.

See the Xcode testing guide for Device Hub pose controls, and run the compatibility checklist before shipping.

Frequently asked questions