Fit a UIKit app to every iPhone Duo window in five steps

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    // Never UIScreen.main.bounds or a static let: read the container on every layout pass.
    let widthClass = LayoutWidthClass(width: view.bounds.width)
    guard widthClass != appliedWidthClass else { return }
    appliedWidthClass = widthClass
    updateGrid(columns: widthClass.columnCount(compact: 3))   // 3 / 4 / 6
}

Buckets: compact ≤ 500 < medium ≤ 800 < expanded. Cap single-column text at LayoutWidthClass.maxSingleColumnContentWidth (466 pt). Pattern 1–3.

private let railGuide = SideRailAvoidingLayoutGuide()

override func viewDidLoad() {
    super.viewDidLoad()
    railGuide.install(in: view)
    NSLayoutConstraint.activate([
        // Pin both edges: the rail is on the left in a left split view.
        tableView.leftAnchor.constraint(equalTo: railGuide.leftAnchor),
        tableView.rightAnchor.constraint(equalTo: railGuide.rightAnchor),
        tableView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
        tableView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
    ])
}

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    railGuide.update()
}

Uses the resting position, so a push or a back swipe does not make the inset jump. A centered card that never reaches the rail gets 0. Pattern 4.

guard let rail = view.windowSideRail, let window = view.window else { return }
NSLayoutConstraint.deactivate(railConstraints)
railConstraints = SideRail.stripConstraints(for: railView, host: view, side: rail.side)
NSLayoutConstraint.activate(railConstraints)

let clearance = SideRail.statusBarClearance(in: view)   // window coordinates
let top = railView.convert(CGPoint(x: 0, y: clearance.firstUsableY), from: window).y
railButtonTop.constant = top + 16

Folded: below the 156 pt status bar. Left split view: .railWithoutStatusBar, align to y 34. Pattern 5.

private let leftGuide = OcclusionAvoidingLayoutGuide(side: .left)
private let rightGuide = OcclusionAvoidingLayoutGuide(side: .right)

// in setup
leftGuide.install(in: headerView)
rightGuide.install(in: headerView)
backButton.leftAnchor.constraint(greaterThanOrEqualTo: leftGuide.leftAnchor, constant: 16).isActive = true
actionButton.rightAnchor.constraint(equalTo: rightGuide.rightAnchor, constant: -16).isActive = true

// in layoutSubviews, after the row has its frame
if leftGuide.update(row: titleRow.frame) || rightGuide.update(row: titleRow.frame) {
    setNeedsLayout()
}

Only rows that overlap the status bar move; rows below it keep the full width. Pattern 6.

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    sheetHeight.constant = min(contentHeight, SheetLayout.maxHeight(in: view))
}

Keeps the sheet 12 pt below the status bar icons wherever they are. Content that does not fit scrolls. Pattern 7.

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    syncLeftSideRailSafeArea()
}

Tops up additionalSafeAreaInsets.left only by the missing amount and removes it when the rail leaves. Pattern 8.

private let hinge = HingeDivisionObserver()

override func viewDidLoad() {
    super.viewDidLoad()
    hinge.onChange = { [weak self] _ in self?.view.setNeedsLayout() }
    hinge.attach(to: view)
}

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    hinge.evaluate()   // size changes do not always produce a hinge update
    if let division = hinge.division, division.isVertical {
        leadingWidth.constant = division.leadingColumnMaxX
        trailingLeft.constant = division.trailingColumnMinX
    } else {
        // single-column layout
    }
}

Enters at 140° or less, leaves above 150°, waits 0.6 s for the hinge to settle. Do not compare UIHinge.Status yourself. Pattern 10.

private let tent = TentPostureObserver()

override func viewDidLoad() {
    super.viewDidLoad()
    tent.onChange = { [weak self] state in
        guard state.hasHingeReading else { return }   // unknown is not "left the tent"
        state.isTent ? self?.showTentContent() : self?.hideTentContent()
    }
    tent.attach(to: view)
}

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    tent.evaluate()
}

The tent page locks to TentOrientation.interfaceOrientationMask(for:). The portrait orientation lock hides the tent. Pattern 11.

let window = someView.window ?? WindowLookup.primaryWindow()
let scene = someView.window?.windowScene ?? WindowLookup.activeWindowScene()

Prefer view.window. The lookups skip auxiliary key windows but guess with several scenes. Pattern 12.