iPhone Duo: What Apple's 'Boring' Keynote Actually Means for iOS Developers

Apple's September keynote felt like "nothing interesting": a slightly faster Pro, a slightly pricier Pro, new watches, new earbuds, and a foldable that looked almost exactly like the leaks. For buyers, that's a fair summary. For iOS developers, it's the opposite — the evening quietly delivered the biggest layout change since the iPhone X notch, and the clock on it runs out on October 23.

The day before the event I published 13 predictions with probabilities. Here's how they did, what was actually shown, and — the part I really care about — what the iPhone Duo's inner display means for your code.

Scorecard: 11 out of 13

Probabilities were set on the morning of September 9, before the stream, and haven't been touched since. Anything below 50% means I was betting against the event.

Brier score: 0.118 (answering 50/50 on everything scores 0.25). I missed exactly where I warned I would: specs leak for months because thousands of factory workers know them, while the name is known by about twenty people who stay quiet. And I got cocky betting against a surprise.

What was announced

The good: a foldable whose two displays share (almost) the same proportions, so content scales instead of reflowing; Split View on an iPhone for the first time; a real variable aperture; 45 hours of video on the Pro Max; ANC in the $129 AirPods; and Apple Reference Image, which signs sensor data so a photo can later be proven to be a photo — arguably the most underrated announcement of the night.

The not-so-good: +$100 on the Pro and +$300 on the 1 TB tier, price bumps on older models; Siri AI ships in beta, English-only, with daily usage limits and "expanded access for a fee in the future" (and not at all on iPhone in the EU or mainland China); the iPhone 18 Pro display is unchanged apart from a narrower Dynamic Island; the Duo is eSIM-only and doesn't support Apple Pencil Pro.

Not shown: the base iPhone 18, the home hub, HomePod mini, Apple TV, any Mac — and, for developers, the iOS 27.1 SDK.

The new screen in numbers

For 19 years the iPhone has been a tall, narrow strip: 1.5 on the original, 2.17 on today's Pro Max. Every mobile layout we write grew up inside that strip. The Duo's inner display breaks it.

Three takeaways:

1. 1.42 is roughly √2 — the A4 paper ratio. Fold such a sheet in half and each half keeps the same shape. In Split View the inner display splits into two panes of roughly 445 × 626 points, each almost the shape of the outer display. Two apps side by side each get an "outer screen". That's hardware designed with software in mind.
2. The open Duo is wider than tall. It's not a phone turned sideways — it's the new default posture. The Dynamic Island sits vertically on the side, so safe-area insets are asymmetric, and with the 27.1 SDK vertical bars move to the left edge.
3. 16:9 video loses. At 890 points wide a video is 890 × 501, leaving 125 points of black — a fifth of the screen.

How the Duo shows your app if you do nothing

• Built without the iOS 27 SDK: a familiar phone-sized window in the middle of the inner display, surrounded by black. A postcard in a giant frame.
• Built with the iOS 27 SDK: wider, but part of the screen is still empty.
• Built with the iOS 27.1 SDK: edge to edge, toolbars and tab bars move to the left. This is the native look — and the SDK for it isn't even in beta yet.

What to grep for this week

The good news: almost all of this is stuff Apple has asked us to do for years. The bad news: iPhones were always narrow, and plenty of codebases quietly relied on it. The Duo is still an iPhone, but inside it's regular × regular, like an iPad.

Biometrics is the easy one:

import LocalAuthentication

func biometryLabel() -> String {
    let context = LAContext()
    _ = context.canEvaluatePolicy(.deviceOwnerAuthentication, error: nil)
    return switch context.biometryType {
    case .faceID:  "Sign in with Face ID"
    case .touchID: "Sign in with Touch ID"
    case .opticID: "Sign in with Optic ID"
    default:       "Sign in with passcode"
    }
}

Layout isn't rocket science either if you're on SwiftUI — NavigationSplitView collapses to one column in compact width and expands on the inner display:

struct RootView: View {
    @Environment(\.horizontalSizeClass) private var hSize

    var body: some View {
        NavigationSplitView {
            SidebarList()
        } detail: {
            DetailView()
                .padding(hSize == .regular ? 32 : 16)
        }
    }
}

Apple's "Preparing your app for iPhone Duo" material also names foldable-specific APIs: onHingeChange / UIHingeInteraction for hinge states and angle, ArrangementView / UIArrangementViewController for two-pane layouts around the fold, reservedRegion on GeometryProxy / UIView to keep controls off the fold, CameraCaptureAccessory for a preview or teleprompter on the outer display, and AVCaptureDeviceDirectionCoordinator — because camera position is no longer camera direction.

Two easy-to-miss extras: StandBy on the Duo runs on either display even when not charging, so apps without widgets or Live Activities simply don't exist there. And health apps should expect a lot more data — Series 12 and Ultra 4 sample heart rate every five seconds all day.

The next six weeks

• Sep 14 — iOS 27 ships. Its SDK (already in RC) decides whether your app resizes.
• Sep 16–17 — Apple group labs on iPhone Duo; Q&As on Sep 23.
• Xcode 27.1 beta — iOS 27.1 SDK and a Duo simulator you can open, close, rotate and partially fold.
• Oct 23 — iPhone Duo ships on iOS 27.1, and people who paid $1,999 start opening your app.

Bottom line

If you own an iPhone 16 Pro or 17 Pro, you could have skipped this keynote. If you write iOS apps, this is the busiest autumn since 2017, when the iPhone X taught everyone the words "safe area". This time the lesson is that an iPhone can be wider than it is tall, that size classes on a phone aren't a formality, and that one window per app is no longer a given. The keynote was boring precisely because everything important in it was for Xcode, not for the store window.

How many UIScreen.main references are in your project?

Originally published at klukyanov.ru.