Three steps to make your app shine on iPhone Duo

Before you begin

While all apps will run on iPhone Duo, building with the latest SDKs let your app use more of the display and adapt gracefully across layouts. The following examples show what your app will look like on iPhone Duo depending on which SDK you build with.

Step 1: Run the app resizability skill

In Xcode, ask the coding assistant to “get my app ready for iPhone Duo.” The skill scans your project for the patterns described in Step 2. For each issue it finds, it explains the problem and suggests a fix.

If you use a different coding agent, export the skill by running this in Terminal:

xcrun agent skills export

The skill finds most issues, but not all of them. Use the list below to review what it flags and catch anything it misses.

Step 2: Fix patterns

Stop sizing from the main screen

On iPhone Duo, your app’s size changes whenever the device folds or unfolds, and it doesn’t always match the size of the screen. Search your project for UIScreen.main and replace each use:

• Size from the view or window. Replace UIScreen.main.bounds with view.bounds or window.bounds. In SwiftUI, use GeometryReader or onGeometryChange.
• Re-read the size whenever it changes. Reading the size once at launch or in viewIsAppearing isn’t enough, because a fold can resize your app in the middle of a session. Do size-dependent layout in layoutSubviews or viewDidLayoutSubviews, and use viewWillTransition(to:with:) for work that needs to run when the size changes.
• Get scale from the trait collection. Use traitCollection.displayScale in UIKit or @Environment(\.displayScale) in SwiftUI.
• Create windows from a scene. Replace UIWindow(frame: UIScreen.main.bounds) with UIWindow(windowScene:).
• Don’t store a UIScreen. When screen information is needed, access it dynamically with view.window?.windowScene?.screen.

Avoid using orientation to make layout decisions

On iPhone Duo, the device’s orientation doesn’t tell you what shape your app is. Search for these properties and remove any layout logic that depends on them:

• UIDevice.current.orientation
• statusBarOrientation
• interfaceOrientation, including windowScene.effectiveGeometry.interfaceOrientation.

To find out whether your app is wider than it is tall, compare the width and height of the space you have: view.bounds in a view controller, superview.bounds in a view, or the size from GeometryReader in SwiftUI.

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. On the inner display, your app should look like a natural extension of its iPad layout. For more information, see Prepare your app for iPhone Duo.

• Don’t choose a layout by idiom. Search for userInterfaceIdiom == .phone and == .pad. Base layout choices on size classes or available space instead.
• Don’t treat “regular width” as “iPad.” Code that shows an iPad-only layout when horizontalSizeClass == .regular, or that assumes iPhone is always compact, will now run that layout on iPhone too.
• Remove checks for specific device sizes. Comparisons such as bounds.height == 844 or a list of known iPhone sizes won’t match iPhone Duo.
• Check full-bleed media. Hero images and video set to .scaleAspectFill or .aspectRatio(contentMode: .fill) can lose important content on wider displays. Use the current size classes or aspect ratio to choose between fill and fit, or set a focal point.

Step 3: Test resizing

Test resizing using these tools to make sure your app runs as expected.

• The iOS resizable simulator in Device Hub.
• iPhone Mirroring on macOS 27. Resize the window to extreme sizes in both directions.
• The iPhone Duo simulator in Xcode 27.1.
  • Note: The iPhone Duo simulator is the best way to test the full native experience.