iPhone Duo: First Developer Good-to-Knows Apple has released six new developer videos covering app development for its upcoming foldable iPhone Duo, which runs iOS 27.1 and requires Xcode 27.1. The videos emphasize using size classes over device assumptions, keeping the same app hierarchy across poses, and adapting to asymmetric safe areas and vertical bars. Astra, an AI agent, generated a summary of the videos, noting that existing apps run but rebuilding with the iOS 27.1 SDK enables edge-to-edge content and vertical bars. It’s time for Apple to get into the… fold… with foldables. Anyways, on the day of the announcement, Cupertino and Friends™️ dropped six new videos over developing, designing, and crafting great apps for the nascent device: And there’s some ground to cover, because it seems like this thing can contort your app’s layout about 7,000 different ways: I’ve only watched one, and the other five are next for me to marathon, but in the interim I had Astra check out each one because I’m excited and impatient. Here’s what it thinks the critical things to know are, with the obvious caveat that this is at the discretion of an agent. let concatenatedThoughts = """ These takeaways come from Apple's English video captions. Some new iOS 27.1 API reference pages weren't available at writing; documentation links below point to available references, with timestamps for the new APIs. """ AI-generated summary The following section was generated by Astra from Apple's video captions. Follow the linked videos and documentation for the original guidance. Build With iOS 27.1 Existing apps run, but rebuilding with the iOS 27.1 SDK enables edge-to-edge content and vertical bars. Use Xcode 27.1’s Device Hub to test opening, closing, rotating, and partially folding; its updated App Resizability skill also covers SwiftUI and Duo. - Video: Prepare your app — 0:30 https://developer.apple.com/videos/play/tech-talks/111461/?time=30 - Video: App Resizability skill — 9:12 https://developer.apple.com/videos/play/tech-talks/111461/?time=552 - Docs: Device Hub https://developer.apple.com/documentation/xcode/device-hub Size Classes Beat Device Assumptions Duo remains an iPhone even when its inner display has regular width and height size classes. Use size classes and local view or scene geometry; avoid layout branches based on idiom, interface orientation, or UIScreen.main . The inner display doesn’t honor supported interface orientations for normally resizable apps. - Video: Prepare your app — 2:46 https://developer.apple.com/videos/play/tech-talks/111461/?time=166 - Docs: horizontalSizeClass https://developer.apple.com/documentation/swiftui/environmentvalues/horizontalsizeclass Keep the Same App Inside and Out Use the extra space for columns or richer navigation while preserving the same hierarchy and functionality across poses. NavigationSplitView and UISplitViewController adapt automatically. A tabletop layout can put media above controls, but opening or folding the phone shouldn’t be required to access a feature. - Video: Design for iPhone Duo — 3:42 https://developer.apple.com/videos/play/tech-talks/111466/?time=222 - Video: Inner display layouts — 7:34 https://developer.apple.com/videos/play/tech-talks/111466/?time=454 - Docs: NavigationSplitView https://developer.apple.com/documentation/swiftui/navigationsplitview Safe Areas Are Asymmetric Vertical controls can occupy either edge, especially in Split View. Handle each safe-area inset and layout margin independently: keep interactive content inside, let backgrounds extend behind bars, and test your app on both sides of the inner display. Let Containers Move Your Bars Use bars managed by navigation and tab containers; standalone UIKit bar instances don’t participate in the new vertical layout. Supply both titles and symbols so the system can choose a representation. Custom controls need an explicit vertical opt-in and must fit the bar’s fixed width; meaningful text can stay horizontal. - Video: Raise the bar — 2:00 https://developer.apple.com/videos/play/tech-talks/111462/?time=120 - Video: Toolbar content and axis behavior — 5:56 https://developer.apple.com/videos/play/tech-talks/111462/?time=356 - Docs: ToolbarItem https://developer.apple.com/documentation/swiftui/toolbaritem Decide What Overflows First Toolbars compress before tabs by default; task-focused screens can change that preference. Give frequent actions and status-bearing items higher visibility priority, and consolidate custom overflow actions into ToolbarOverflowMenu or UIKit’s additionalOverflowItems so users get one system overflow menu. - Video: Raise the bar — 11:40 https://developer.apple.com/videos/play/tech-talks/111462/?time=700 - Docs: ToolbarOverflowMenu https://developer.apple.com/documentation/swiftui/toolbaroverflowmenu - Docs: visibilityPriority