# Iphone Duo Displays > Use iPhone Duo's hinge, two displays, two front cameras and multiple windows in SwiftUI and UIKit apps. Use when a developer asks about the hinge angle or fold state, onHingeChange or UIHingeInteraction, interactions driven by folding, Split View multitasking or the stacked video layout on iPhone Duo, multiple scenes or windows on iPhone, requestSceneSessionActivation errors on the outer display, UIWindowSceneActivationAction, scene accessories, sceneAccessory, CameraCaptureAccessory, a teleprompter or subject preview on the outer display, the virtual front camera, AVCaptureDeviceDirectionCoordinator and cameras that change direction when the device opens or closes, preview mirroring, or a widget so the app appears in StandBy on the outer display. Based on Apple's "Leverage multiple displays and scenes" and "Build a great camera experience for iPhone Duo". Not for column widths, safe areas or content around the fold (layout) or toolbar items (bars). - Skill: `alexey1312/iphone-duo-displays` (Agent Skill, multi-file: 12 files) - Install (CLI): `npx skillmds@latest add alexey1312/iphone-duo-displays` - Raw SKILL.md: https://api.skillmd.com/api/skills/alexey1312/iphone-duo-displays/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: alexey1312 (https://skillmd.com/u/alexey1312) - Updated: 2026-09-22 - Page: https://skillmd.com/skills/alexey1312/iphone-duo-displays --- # iPhone Duo displays, hinge, scenes and cameras Code: `references/displays-code.md`. Availability: `references/api-availability.md` — the hinge APIs, `CameraCaptureAccessory`, the direction coordinator and the physical front camera types need the **iOS 27.1 SDK**, which ships with Xcode 27.1; `sceneAccessory`, `onAvailabilityChange`, `UISceneAccessory` and `UIWindowScene.ActivationAction` exist earlier. The hinge reaches you only through `UIHingeInteraction` / `onHingeChange`: there is no trait, no notification and no `UIDevice` property. StandBy cannot be verified in the iPhone Duo simulator (Xcode 27.1 known issues 187708663, 187708767). Which half carries the outer display and the inner camera, how each pose faces, and what each camera can do: `references/device-geometry.md`. ## Hinge (Tech Talk 111464, 0:49–2:35) - SwiftUI `onHingeChange { previous, context in … }`; UIKit `UIHingeInteraction { _, update in … }` added to a view with `addInteraction`. - Reports a status — closed, partially open, fully open (UIKit also `unknown`) — and a continuous angle (`UIHinge.angle` is in radians; the SwiftUI sample uses `Angle`). - `context.hinge == nil` (UIKit: `update.hinge == nil`) means the device has no hinge or the view left a hierarchy that provides hinge updates: every hinge feature must degrade to nothing on other devices. `UIHingeInteraction.isEnabled` pauses updates. - Filter for `.partiallyOpen` when reading the angle, and **reset in the else branch** so the effect does not stick when the device opens flat or closes. - **Hinge data drives interactions and effects, not layout.** Layout uses arrangements and reserved regions (`iphone-duo-layout`). - Good fits: instruments, games, camera and media controls, playful physical interactions. Poor fits: anything essential — it must be reachable without folding. ## Split View multitasking (2:59) - **All apps participate**: two apps side by side on iPhone Duo, plus a new layout that stacks video and apps. There is no opt-out to lean on. - Apps that already resize on iPad or in iPhone Mirroring start in good shape. Handle it with size classes and scene geometry, and the asymmetric safe areas it produces. - Verify: pose P8 in `references/pose-test-matrix.md`. ## Multiple scenes (3:38) - iPhone Duo is the first iPhone that shows multiple instances of an app's UI; apps that support multiple scenes on iPad get this too. - **New windows cannot be created on the outer display.** Every scene request must handle failure (scanner rule `DUO010` flags `errorHandler: nil`). - Prefer `UIWindowSceneActivationAction` (Swift: `UIWindowScene.ActivationAction`) for "Open in New Window" controls: it hides itself when new windows are not available. - Apps that enable `UIApplicationSupportsMultipleScenes` must not rely on global window state (`iphone-duo-adaptivity-audit`, rule `DUO009`). - An iPhone app that set the key to `false` to avoid coordinating state across windows now leaves Split View pairs and "open in new window" on the table. Treat it as a Tier 3 item: enable it only if the app tolerates several windows, test two windows on an iPad first, then on iPhone Duo. The scanner reports the key's value under `project.supports_multiple_scenes`. ## Scene accessories (4:22–6:25) - Show content on more than one display at once, paired with the main UI — like a phone acting as a controller for a game on an external display. - **The system controls availability** and it can change at any time; accessories are enabled by default and can be toggled. Observe availability and update any control that turns the accessory on. - **Camera capture accessory** (camera apps): extra UI on the outer display while the main UI stays on the inner display — show the subject a preview, a countdown, a teleprompter. Available only when the app is **full screen on the inner display with an active camera session**. Register it on the **same view as the camera UI**, so it appears only while that view is visible. - Pattern: `.sceneAccessory { CameraCaptureAccessory(isEnabled: $model.isEnabled) { … } .onAvailabilityChange { model.isAvailable = $0 } }` and a toolbar toggle disabled when unavailable (for example when the device is closed). - UIKit: `UISceneAccessory.cameraCapture(sceneConfiguration:userInfo:)` passed to `registerSceneAccessory(_:)` on the capture view controller; keep the returned `UISceneAccessoryRegistration` (`isAvailable` is observable — read it in `updateProperties()`; set `isEnabled` from the toggle) and call `unregisterSceneAccessory(_:)` when the app stops offering the content. The accessory scene's session role is `windowCameraCaptureAccessory`; the shared model arrives as `connectionOptions.sceneAccessoryUserInfo`. No scene-manifest entry is needed or honored. (Apple documentation › Registering a camera capture accessory) - The system presents the top-most registration of a kind, withdraws content when capture stops, the app leaves the foreground or the device closes, and lets different accessory kinds coexist (a connected external display and the outer display at once). Share one model between the capture UI and the accessory instead of passing messages; the outer display accepts touch, so a single control (pause the script, tap to focus) is fine, a second interface is not. - Simulator has no camera: previews and Simulator check the accessory's layout, only a device shows it. ### Before proposing a camera accessory An accessory is only available with an active camera session, so check the session really runs: a camera usage description (`NSCameraUsageDescription`), an authorization request, inputs added without swallowed errors, and outputs for what the app records. Missing prerequisites come first in the plan — otherwise the accessory silently never appears and looks like an iPhone Duo bug. Recording itself must never wait on the accessory being available. ## Cameras (Tech Talk 111465; Apple documentation › Choosing a camera by the direction it faces) iPhone Duo has two front cameras, both square ultrawide sensors: the outer camera and the under-display inner camera. Which way a camera faces depends on the display the app is on, and opening or closing the device moves the app to the other display. - **Existing front-camera code keeps working.** A `DiscoverySession` with position `.front` and the wide or ultra wide type returns the **Virtual Front Camera**, one `AVCaptureDevice` that streams from the inner camera when open and the outer camera when closed (`isVirtualDevice == true`; `activePrimaryConstituent` names the streaming camera). It offers only what both cameras share — 1080p, 60 fps, no depth. (1:01, 2:28) - **Capture-first apps use the physical cameras.** `builtInOuterUltraWideCamera` (up to 4K at 120 fps) and `builtInInnerUltraWideCamera` (1080p up to 60 fps) expose each camera fully, and switching on open/close becomes the app's job. (1:53) - **`position` is where a camera sits, not where it points.** Both front cameras report `.front`; a front camera can face away from the person looking at the app, and the rear cameras face the person when the open device is turned around. Create an `AVCaptureDeviceDirectionCoordinator(view:deviceTypes:)` on the preview view, listing every built-in camera the app captures from — rear types included, the two physical front types instead of the virtual one — and keep it alive while the view is on screen. Its handler reports an `AVCaptureDeviceDirectionMap`; pick from `forwardFacingDeviceDescriptors` when the active camera is no longer in it. (2:53–4:49) - **The handler runs on the main actor and never touches AVFoundation.** It hands an `AVCaptureDeviceDescriptor` (Sendable) to the actor that owns the session, which resolves it with `AVCaptureDevice(uniqueID:)`, handles a `nil` result, and swaps the single video input inside `beginConfiguration` / `commitConfiguration`. Two displays showing a preview (a capture accessory) mean two coordinators, one per view. (5:37–6:16) - **Mirror by direction, not position.** A forward-facing rear camera is a selfie camera and needs a mirrored preview; a backward-facing front camera does not. Set `automaticallyAdjustsVideoMirroring = false` before assigning `isVideoMirrored` (assigning while it adjusts raises), only when position and direction disagree, and reapply after reconnecting an input. Mask the preview while the new camera starts. - **Rotation.** Adopt `AVCaptureDevice.RotationCoordinator`; it updates when the app moves displays, and it is created per device, so make a new one on every camera switch. Then disable camera-sensor-orientation compensation, which is on for every front camera on iPhone Duo, to save work. (8:08) - **Preview polish.** A full-field-of-view rear preview on the inner display leaves room around it: offset the preview and put controls in the remaining space, or fill with `videoGravity`. `dynamicAspectRatio` lets the square front sensors deliver a landscape frame on the inner display. (7:03) - Verify on a device — Simulator has no camera (pose P13). ## StandBy on the outer display (Apple newsroom; HIG › Live Activities) StandBy runs on either display "even when it's not charging": a folded or tented iPhone Duo on a desk is a screen facing the room, filled with widgets and Live Activities. An app with neither is absent from it. For apps that have a natural glanceable state (next event, timer, score, order status), a widget or Live Activity is a Tier 3 item; the scan inventory reports `WidgetKit` and `Live Activities` adoption. The Dynamic Island on the side stack expands vertically for Live Activities (HIG › Reserved regions), so nothing extra is needed there. ## Workflow 1. From the scan inventory: `AVCaptureSession` → camera accessory candidate, and with `front camera discovery` or `video mirroring` → camera direction review; `multiple scene requests` or `UIApplicationSupportsMultipleScenes` → scene handling review; custom gesture or sensor-driven features → hinge candidates; no `WidgetKit` / `Live Activities` → StandBy presence item. 2. Correctness first: scene request errors, Split View sizes, cameras that face the wrong way after opening or closing. Then opportunities: accessory, hinge, widget. 3. Recommend with `references/recommendation-format.md`; mark 27.1 APIs *blocked* when the SDK lacks them; apply after approval; build. 4. Verify poses P7, P8, P9, P10 and P13 in `references/pose-test-matrix.md`. ## Guardrails - Never gate core functionality on the hinge or an accessory. - Don't read hinge state once and cache it; it is live. - Don't present a new-window affordance you have not checked is available. - Don't infer which way a camera faces from its type or `position`; ask a direction coordinator, and write one code path that also runs on single-display iPhones (there the coordinator reports once and never changes).