# UIKit on iPhone Duo

> UIKit has full first-party iPhone Duo support, with UIArrangementViewController, UIViewReservedRegion and vertical bar APIs in the iOS 27.1 SDK.

Source: https://iphoneduosupport.com/frameworks/uikit/

Support status: first-party
Every SwiftUI capability has a UIKit counterpart. Apps built on standard containers adapt with little work.
Last verified: 2026-09-09

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UIKit is fully supported on iPhone Duo, and every SwiftUI adaptive API has a UIKit counterpart. Apps built on `UINavigationController`, `UITabBarController`, and `UISplitViewController` get most of the adaptation automatically.

## What adapts automatically

Standard containers are fully adaptive across all poses. Custom `UIToolbar` content is **not** considered for vertical bar placement, so an app that replaced the system bars with its own has more work than one that did not.

## The main migrations

`UIScreen.main` is being deprecated on two-display devices. Read scale from traits and the screen from the window scene:

```swift
let scale  = traitCollection.displayScale
let screen = view.window?.windowScene?.screen
```

Safe area insets are asymmetric, so inset the rectangle rather than doubling one side:

```swift
let width = view.bounds.inset(by: view.safeAreaInsets).width
```

Audit `userInterfaceIdiom` checks. iPhone Duo reports the phone idiom with a regular-width display, so idiom-gated iPad layouts never appear. See [idiom assumptions](/checklist/idiom-based-assumptions/).

## The new UIKit APIs

```swift
let arrangementVC = UIArrangementViewController()
arrangementVC.setViewController(playerVC, for: .primary)
arrangementVC.setViewController(upNextVC, for: .secondary)
```

```swift
let regions = view.reservedRegions(kind: .division)
```

For vertical bars, `item.axisBehavior`, `traitCollection.verticalBarEdge`, `navigationItem.verticalBarCompressionBehavior`, and `preferredVerticalBarBehavior` control placement, adaptation, compression, and opt-out respectively.

Start with the **App Resizability** modernization skill in Xcode 27.1, which now covers this migration. See [UIKit on iPhone Duo](/blog/uikit-iphone-duo-vertical-bars/).

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Published by iPhone Duo Support (https://iphoneduosupport.com). Not affiliated with Apple Inc.