DuoAppsOpen, close, lose nothing: keeping your app’s state on iPhone Duo
People open and close iPhone Duo while they use your app. Each time, the app moves to the other display and resizes, and nothing should be lost: the same screen, the same selection and scroll position, what people typed, what’s playing. Keep the same features and the same hierarchy on both displays, with one more level on the inner display when it helps, like Mail’s list beside the open email. Tie no feature to a pose.
The problem
People open and close iPhone Duo while they use an app, sometimes often. Each time, the app moves from one display to the other and changes size: compact width on the outer display, regular width on the inner display. Apple treats it as a resize, like resizing an iPhone app in iPhone Mirroring on the Mac: the app may cross a size class boundary, but it’s still an iPhone app.
What gets lost is whatever the app rebuilds on the way: a list back at the top, a selection gone, a reply half written, a video starting over. Or a feature that exists only open, or only closed. Each time they change displays, people then have to find their place again.
The rules
- 1
Treat opening and closing as a resize. Closed, your app has the outer display: compact width, like on other iPhone models. Open, it has the inner display: regular width and height, with room for more. Build one app that resizes, with size classes, layout margins, and safe areas, not a layout for each display or each pose: don’t reinvent your app when it resizes, let the existing layout expand into the space. Even an app that asks for full screen still resizes when the device opens or closes.
- 2
Keep the state of elements the same between displays: when the device opens or closes, people find what they left. In a list, the selection and the scroll position; in a form, what they typed; in an article, the reading position; in a carousel, the item showing; in a video, playback in progress. People may open and close the device often while using your app: it has to stay predictable and consistent, inside and out, a single experience that adapts to each display.
- 3
Keep the same hierarchy on both displays. The inner display can show one more level at once, when it makes sense for your content: closed, Mail shows either the list of emails or one email; open, it shows both, side by side. The levels and their order stay the same: the hierarchy doesn’t change between the outer and inner displays.
- 4
Use the system’s containers: they handle the resize. A split view (list and detail) expands on the inner display and collapses to a single stack of screens on the outer display, as it does between regular and compact widths on other iPhone models. Tab bars, navigation stacks, and arrangement views handle the resize too.
- 5
When the device closes on a list and an open item, keep the item on screen, with Back to the list; when it opens on an item, show the list beside it, with that item selected. Apple shows Mail, closed, with either the list or one email; keeping the one people were looking at is our recommendation, since it’s the state they left.
- 6
Keep controls where people expect them, from one pose to the next, so they don’t have to learn again where actions live. That’s why the system keeps the bars on the side when the device opens in landscape, at the same height as on the outer display (Where the bars go).
- 7
Tie no feature to a pose. In every pose, people reach the same controls and the same content: controls may move into the overflow menu and content may move or change size, but nothing exists only open or only closed. A layout made for the device on a table is optional; if you make one, give it all the same controls and the same hierarchy as the other poses.
- 8
Only what the hardware ties to one display may depend on it, and the system tells the app. A new window of your app opens only on the inner display: with the device closed, the system’s action for it hides on its own, and the app handles a request that fails. Showing a view on the outer display while a camera app runs on the inner one works only with the device open: Apple’s example turns its button off when that isn’t available, like when the device is closed.
- 9
A camera that’s streaming keeps streaming. The Virtual Front Camera switches between the inner and outer front cameras on its own as the device opens or closes. An app that picks its cameras itself handles the switch: when the device opens or closes, the system tells it which camera now faces the person, and the app keeps streaming from that one.
- 10
Test by opening and closing, screen by screen. In Xcode, Device Hub simulates iPhone Duo, with buttons to open, close, rotate, or fold it. Go through each view, sheet, and popover, closed, open, and partially folded, and look for any that position awkwardly when you fold or open the device (Don’t stretch).
Demo
Static version, no JavaScript
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Closed. On the outer display: the fourth item of the list, a reply started at the bottom. Back, at the top of the column, leads to the list.
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Open. On the inner display, one more level at once: the list beside the item (rule 3). The fourth item is still selected, the list is at the same place, and the reply is still there, half written (rule 2).
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Back to the list. The device closes on the item people were reading, not on the list (rule 5). Back then shows the list at the same place, the item they read still marked.
FAQ
What happens to my app when someone opens or closes iPhone Duo?
It moves to the other display and resizes: compact width closed, regular width open. For your app, it’s a resize, like in iPhone Mirroring on the Mac, and Apple asks you to keep the state of elements the same on both displays.
How do I keep the scroll position and the selection when iPhone Duo opens?
Let the same layout expand instead of rebuilding the screen: use size classes and the system’s containers, like the split view, which expands open and collapses closed. When it opens on an item, show the list beside it, that item selected.
Can a feature exist only when iPhone Duo is open?
Not by design: Apple asks for the same features in every pose. The exceptions come from the hardware, like a new window, which opens only on the inner display: the system’s action for it hides on its own when the device is closed.
Should my app show the list or the item when iPhone Duo closes?
Closed, Mail shows either the list or one email. Our recommendation: the one people were looking at. If an item was open, keep it on screen, with Back to the list.
Can my app refuse to resize on iPhone Duo?
No. iPhone Duo still honors the setting that asks for full screen, but the app resizes anyway when someone opens or closes the device. Build it to resize, with size classes.
Sources
- Prepare your app for iPhone Duo, Apple Developer, Tech Talk
- Designing for iPhone Duo, Apple Human Interface Guidelines, September 9, 2026
- Design for iPhone Duo, Apple Developer, Tech Talk
- Preparing your app for iPhone Duo, Apple Developer Documentation
- Raise the bar with iPhone Duo, Apple Developer, Tech Talk
- Strike a pose with adaptive layouts on iPhone Duo, Apple Developer, Tech Talk
- Leverage multiple displays and scenes on iPhone Duo, Apple Developer, Tech Talk
- Build a great camera experience for iPhone Duo, Apple Developer, Tech Talk