SwiftUI on iPhone Duo

SwiftUI is the best-supported way to build for iPhone Duo. Declarative layout already recomputes when the available size changes, so a SwiftUI app that avoids fixed sizes gets much of the adaptation for free.

What you get without changes

Build against the iOS 27.1 SDK and the following already adapt: NavigationSplitView, TabView, sheets, popovers, context menus, and alerts. Columns collapse when the device is closed and tile or overlay when it is open. Standard navigation and toolbar buttons move into a vertical bar on the inner display in landscape.

What you need to change

The work concentrates in four places:

1. Fixed sizes. Any .frame(width:) tuned for a 402-point iPhone is wrong on a 626-point display. See hard-coded screen dimensions.
2. Orientation logic. The inner display does not honor supported interface orientations. Move to @Environment(\.horizontalSizeClass). See size classes.
3. Safe area math. Insets are asymmetric now. See asymmetric safe areas.
4. Toolbar content. A vertical bar holds fewer items. See overflow priorities.

The new APIs worth adopting

ArrangementView is the highest-value addition — it places a primary and secondary view across every pose, taking size classes, aspect ratio, and active division regions as input:

ArrangementView {
    PlayerView()
} secondary: {
    UpNextView()
}
.arrangementViewStyle(.split)

ReservedRegion reports where the hinge divides the display and where the under-display camera occludes it:

GeometryReader { proxy in
    let regions = proxy.reservedRegions(kind: .division)
}

And one modifier that is close to free:

TabView { … }.defaultTabBarPlacement(.sidebar)

Read the full walkthrough in SwiftUI on iPhone Duo.