Innovative iOS Patterns from Signal’s Codebase

1// Measure system load by monitoring display link performance2private func updateDisplayLinkFrequency(displayLinkDuration: TimeInterval) {3    // If display link is firing slower than expected, system is under load4    let actualFrequency = 1.0 / displayLinkDuration56    // Use linear interpolation to determine target update interval7    let displayLinkAlpha = recentDisplayLinkFrequency.inverseLerp(8        lightDisplayLinkFrequency,  // 60 fps (system responsive)9        heavyDisplayLinkFrequency,  // 20 fps (system struggling)10        shouldClamp: true11    )1213    // Slow down updates when system is struggling14    let targetInterval = displayLinkAlpha.lerp(15        fastUpdateInterval,  // 0.05s when responsive16        slowUpdateInterval   // 0.5s when struggling17    )1819    self.targetUpdateInterval = targetInterval20}

1// Updates immediately - causes jank with rapid changes2NotificationCenter.default.addObserver { _ in3    self.tableView.reloadData()4}56// Fixed timer - not adaptive7Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true) { _ in8    self.updateUI()9}

1// Adaptive throttling based on real device performance2// Fast devices: ~50ms updates (responsive)3// Slow devices under load: ~500ms updates (prevents jank)4// Automatically adjusts based on actual display link fire rate

1class AdaptiveUpdateManager {2    private var displayLink: CADisplayLink?3    private var lastUpdateTime: CFTimeInterval = 04    private var targetUpdateInterval: TimeInterval = 0.0556    func startMonitoring() {7        displayLink = CADisplayLink(target: self, selector: #selector(displayLinkFired))8        displayLink?.add(to: .main, forMode: .common)9    }1011    @objc private func displayLinkFired(_ link: CADisplayLink) {12        let duration = link.timestamp - lastUpdateTime13        lastUpdateTime = link.timestamp1415        // Measure how long between display link fires16        // If > 16.67ms (60fps), system is under load17        let actualFPS = 1.0 / duration1819        // Adjust update frequency based on load20        if actualFPS < 40 {21            targetUpdateInterval = 0.5  // Slow down22        } else if actualFPS > 55 {23            targetUpdateInterval = 0.05  // Speed up24        }2526        performPendingUpdatesIfNeeded()27    }28}

1extension UIWindow.Level {2    // Below everything - used for background blur3    static let _background = UIWindow.Level(rawValue: -1)45    // Normal app content6    static let _normal = UIWindow.Level.normal78    // Return-to-call banner (above normal UI)9    static let _returnToCall = UIWindow.Level(rawValue: UIWindow.Level.normal.rawValue + 2)1011    // Active call view (above banner)12    static let _callView = UIWindow.Level(rawValue: UIWindow.Level.normal.rawValue + 3)1314    // Screen blocking for privacy (above calls)15    static let _screenBlocking = UIWindow.Level(rawValue: UIWindow.Level.alert.rawValue + 5)16}1718class WindowManager {19    private lazy var callViewWindow: UIWindow = {20        let window = UIWindow()21        window.windowLevel = ._callView22        window.isHidden = false23        window.backgroundColor = .clear24        return window25    }()2627    private lazy var returnToCallWindow: UIWindow = {28        let window = UIWindow()29        window.windowLevel = ._returnToCall30        window.isHidden = false31        window.backgroundColor = .clear32        return window33    }()3435    func startCall(call: SignalCall) {36        let callVC = CallViewController(call: call)37        callViewWindow.rootViewController = callVC38        ensureWindowState()  // Bring to appropriate level39    }4041    // Uses window level changes instead of hidden property42    // to avoid UIKit layout bugs43    private func ensureWindowState() {44        if hasActiveCall {45            callViewWindow.windowLevel = ._callView46        } else {47            callViewWindow.windowLevel = ._background  // Hide by lowering48        }49    }50}

1// Presenting modally - can be dismissed, interferes with navigation2present(callViewController, animated: true)34// Adding subview to window - wrong z-order, rotation issues5UIApplication.shared.keyWindow?.addSubview(callView)

1// Dedicated windows with precise z-ordering2// Each window has its own view controller hierarchy3// No interference between different UI concerns4callViewWindow.windowLevel = ._callView5screenBlockingWindow.windowLevel = ._screenBlocking

1// iOS has a rotation bug with certain window configurations2// Access private API safely using encoded selector names3private func applyRotationWorkaround() {4    let selectorData = Data(base64Encoded: "X3VwZGF0ZVRvSW50ZXJmYWNlT3JpZW50YXRpb246")!5    let selectorString = String(data: selectorData, encoding: .utf8)!6    let selector = NSSelectorFromString(selectorString)78    if window.responds(to: selector) {9        window.perform(selector, with: nil)10    }11}

1class OverlayWindowManager {2    // Main app window (set by system)3    // Level: UIWindow.Level.normal45    // Custom overlay window6    private lazy var overlayWindow: UIWindow = {7        let window = UIWindow(frame: UIScreen.main.bounds)8        window.windowLevel = UIWindow.Level(rawValue: UIWindow.Level.normal.rawValue + 1)9        window.backgroundColor = .clear10        window.isHidden = false11        return window12    }()1314    func showOverlay(_ viewController: UIViewController) {15        overlayWindow.rootViewController = viewController16        overlayWindow.windowLevel = UIWindow.Level(rawValue: UIWindow.Level.normal.rawValue + 1)17        overlayWindow.makeKey()  // Receives touch events18    }1920    func hideOverlay() {21        overlayWindow.windowLevel = UIWindow.Level(rawValue: -1)22        // Don't set isHidden - can cause layout bugs23    }24}

1public class DebouncedEvent {2    public enum Mode {3        /// Waits before firing - good for batching operations4        /// Example: Search API calls while user types5        case lastOnly67        /// Fires immediately on first request, then throttles subsequent8        /// Example: UI updates during scrolling (responsive but not excessive)9        case firstLast10    }1112    private let mode: Mode13    private let maxFrequencySeconds: TimeInterval14    private let queue: DispatchQueue1516    private var timer: Timer?17    private var pendingRequest: (() -> Void)?18    private var lastFireDate: Date?1920    public init(21        mode: Mode,22        maxFrequencySeconds: TimeInterval,23        queue: DispatchQueue = .main24    ) {25        self.mode = mode26        self.maxFrequencySeconds = maxFrequencySeconds27        self.queue = queue28    }2930    public func request(_ callback: @escaping () -> Void) {31        pendingRequest = callback3233        switch mode {34        case .lastOnly:35            // Cancel existing timer, restart countdown36            timer?.invalidate()37            timer = Timer.scheduledTimer(38                withTimeInterval: maxFrequencySeconds,39                repeats: false40            ) { [weak self] _ in41                self?.fireIfNeeded()42            }4344        case .firstLast:45            // Fire immediately if enough time has passed46            let now = Date()47            let shouldFireImmediately: Bool4849            if let lastFireDate = lastFireDate {50                let timeSinceLast = now.timeIntervalSince(lastFireDate)51                shouldFireImmediately = timeSinceLast >= maxFrequencySeconds52            } else {53                shouldFireImmediately = true  // First request54            }5556            if shouldFireImmediately {57                fireIfNeeded()58            } else {59                // Schedule for later60                timer?.invalidate()61                timer = Timer.scheduledTimer(62                    withTimeInterval: maxFrequencySeconds,63                    repeats: false64                ) { [weak self] _ in65                    self?.fireIfNeeded()66                }67            }68        }69    }7071    private func fireIfNeeded() {72        guard let pendingRequest = pendingRequest else { return }73        lastFireDate = Date()74        self.pendingRequest = nil75        queue.async {76            pendingRequest()77        }78    }79}

1class ConversationViewController: UIViewController {2    // User is scrolling - we want immediate first update,3    // then throttle subsequent updates to avoid jank4    private lazy var contentInsetDebouncer = DebouncedEvent(5        mode: .firstLast,  // ← Responsive!6        maxFrequencySeconds: 0.05  // Max 20 updates/sec7    )89    func scrollViewDidScroll(_ scrollView: UIScrollView) {10        contentInsetDebouncer.request { [weak self] in11            self?.updateContentInsets()  // Expensive operation12        }13    }14}

1class DatabaseChangeObserver {2    // Cross-process database changes - we want to batch them3    private lazy var crossProcessDebouncer = DebouncedEvent(4        mode: .lastOnly,  // ← Batching!5        maxFrequencySeconds: 0.2  // Wait 200ms of silence6    )78    func handleDarwinNotification() {9        // Another process (NSE) changed database10        crossProcessDebouncer.request { [weak self] in11            self?.reloadFromDatabase()  // Expensive12        }13    }14}

1// Only supports "last only" mode2class SimpleDebouncer {3    func debounce(_ action: @escaping () -> Void) {4        timer?.invalidate()5        timer = Timer.scheduledTimer(...) { action() }6    }7}89// Using it for scrolling feels laggy (waits before first update)10debouncer.debounce { updateUI() }  // 300ms delay before first update

1// Choose the right mode for the use case2let searchDebouncer = DebouncedEvent(mode: .lastOnly, maxFrequencySeconds: 0.3)3let scrollDebouncer = DebouncedEvent(mode: .firstLast, maxFrequencySeconds: 0.05)45// Searching: waits for user to stop typing6searchDebouncer.request { performSearch() }78// Scrolling: immediate first update, then throttled9scrollDebouncer.request { updateUI() }  // Fires immediately!

1class SmartDebouncer {2    enum Mode {3        case waitForSilence  // Traditional debounce4        case immediateAndThrottle  // Responsive throttle5    }67    private let mode: Mode8    private let delay: TimeInterval9    private var timer: Timer?10    private var lastExecutionTime: Date?1112    func trigger(_ action: @escaping () -> Void) {13        switch mode {14        case .waitForSilence:15            // Cancel and restart timer16            timer?.invalidate()17            timer = Timer.scheduledTimer(withTimeInterval: delay, repeats: false) { _ in18                action()19            }2021        case .immediateAndThrottle:22            let now = Date()23            let shouldExecuteNow = lastExecutionTime == nil ||24                                   now.timeIntervalSince(lastExecutionTime!) >= delay2526            if shouldExecuteNow {27                lastExecutionTime = now28                action()29            } else {30                // Schedule for later31                timer?.invalidate()32                timer = Timer.scheduledTimer(withTimeInterval: delay, repeats: false) { [weak self] _ in33                    self?.lastExecutionTime = Date()34                    action()35                }36            }37        }38    }39}4041// Usage42let searchDebouncer = SmartDebouncer(mode: .waitForSilence, delay: 0.3)43let scrollDebouncer = SmartDebouncer(mode: .immediateAndThrottle, delay: 0.05)

1class Counter {2    private let lock = NSLock()3    private var _value: Int = 045    var value: Int {6        lock.lock()7        defer { lock.unlock() }8        return _value9    }10}1112// Error: 'Counter' doesn't conform to 'Sendable'13Task {14    let counter = Counter()15    await someAsyncFunction(counter)16}

1@propertyWrapper2public struct Atomic<Value>: @unchecked Sendable {3    private let lock = UnfairLock()4    private var _value: Value56    public init(wrappedValue: Value) {7        self._value = wrappedValue8    }910    public var wrappedValue: Value {11        get { lock.withLock { _value } }12        set { lock.withLock { _value = newValue } }13    }1415    // Read and modify atomically16    public func update<T>(_ block: (inout Value) -> T) -> T {17        lock.withLock {18            block(&_value)19        }20    }2122    // State machine transitions23    public func transition(from: Value, to: Value) throws where Value: Equatable {24        try lock.withLock {25            guard _value == from else {26                throw AtomicError.transitionFailed27            }28            _value = to29        }30    }31}3233// Specialized atomic types34public final class AtomicBool: @unchecked Sendable {35    private let lock = UnfairLock()36    private var _value: Bool3738    public init(_ value: Bool) {39        self._value = value40    }4142    // Try to set flag atomically43    public func tryToSetFlag() -> Bool {44        lock.withLock {45            guard !_value else { return false }46            _value = true47            return true48        }49    }5051    public func tryToClearFlag() -> Bool {52        lock.withLock {53            guard _value else { return false }54            _value = false55            return true56        }57    }58}5960// Atomic collections61public final class AtomicArray<Element>: @unchecked Sendable {62    private let lock = UnfairLock()63    private var _value: [Element]6465    public func append(_ element: Element) {66        lock.withLock {67            _value.append(element)68        }69    }7071    public func removeAll() -> [Element] {72        lock.withLock {73            let old = _value74            _value = []75            return old76        }77    }78}7980public final class AtomicDictionary<Key: Hashable, Value>: @unchecked Sendable {81    private let lock = UnfairLock()82    private var _value: [Key: Value]8384    public subscript(key: Key) -> Value? {85        get { lock.withLock { _value[key] } }86        set { lock.withLock { _value[key] = newValue } }87    }88}

1public final class CancellableContinuation<T>: Sendable {2    private enum State {3        case initial4        case waiting(CheckedContinuation<T, Error>)5        case completed(Result<T, Error>)6        case consumed7    }89    @Atomic private var state: State = .initial1011    public func waitForResult() async throws -> T {12        // Use atomic transition to move from .initial → .waiting13        try await withCheckedThrowingContinuation { continuation in14            do {15                try $state.transition(from: .initial, to: .waiting(continuation))16            } catch {17                // Already completed - resume immediately18                if case .completed(let result) = state {19                    continuation.resume(with: result)20                }21            }22        }23    }2425    public func resume(returning value: T) {26        $state.update { state in27            switch state {28            case .waiting(let continuation):29                continuation.resume(returning: value)30                state = .consumed31            case .initial:32                state = .completed(.success(value))33            default:34                break  // Already consumed35            }36        }37    }38}

1class MessageProcessor {2    @Atomic private var isProcessing: Bool = false34    func processMessages() async {5        // Try to set flag - prevents concurrent processing6        guard $isProcessing.tryToSetFlag() else {7            return  // Already processing8        }910        defer {11            _ = $isProcessing.tryToClearFlag()12        }1314        // Process messages...15    }16}

1actor MessageSender {2    @Atomic private var messagesSent: UInt = 034    func sendMessage(_ message: Message) async {5        // Send message...67        // Atomic increment8        $messagesSent.update { $0 += 1 }9    }1011    func getStats() -> UInt {12        messagesSent  // Thread-safe read13    }14}

1// Data race - multiple threads can read/write simultaneously2class UnsafeCounter: @unchecked Sendable {3    var count = 0  // Not thread-safe!45    func increment() {6        count += 1  // Race condition7    }8}

1// Works but verbose and error-prone2class VerboseCounter: @unchecked Sendable {3    private let lock = NSLock()4    private var _count = 056    func increment() {7        lock.lock()8        _count += 19        lock.unlock()  // Easy to forget!10    }11}

1// Clean, safe, and Sendable2class CleanCounter: Sendable {3    @Atomic private var count = 045    func increment() {6        $count.update { $0 += 1 }7    }89    func tryToSetFlag() -> Bool {10        do {11            try $count.transition(from: 0, to: 1)12            return true13        } catch {14            return false15        }16    }17}

1// Signal's lock primitive (faster than NSLock)2struct UnfairLock {3    private var _lock = os_unfair_lock()45    mutating func withLock<T>(_ block: () throws -> T) rethrows -> T {6        os_unfair_lock_lock(&_lock)7        defer { os_unfair_lock_unlock(&_lock) }8        return try block()9    }10}

1@propertyWrapper2struct Atomic<Value>: @unchecked Sendable {3    private var lock = NSLock()4    private var value: Value56    init(wrappedValue: Value) {7        self.value = wrappedValue8    }910    var wrappedValue: Value {11        get {12            lock.lock()13            defer { lock.unlock() }14            return value15        }16        set {17            lock.lock()18            defer { lock.unlock() }19            value = newValue20        }21    }2223    var projectedValue: Atomic<Value> { self }2425    mutating func update<T>(_ block: (inout Value) -> T) -> T {26        lock.lock()27        defer { lock.unlock() }28        return block(&value)29    }30}3132// Usage33class NetworkManager: Sendable {34    @Atomic private var requestCount = 035    @Atomic private var activeRequests: Set<UUID> = []3637    func performRequest() async {38        let requestId = UUID()3940        $activeRequests.update { $0.insert(requestId) }41        $requestCount.update { $0 += 1 }4243        defer {44            $activeRequests.update { $0.remove(requestId) }45        }4647        // Perform request...48    }49}

1let start = Date()2// User adjusts clock backward 1 hour3let end = Date()4let duration = end.timeIntervalSince(start)  // Negative value!

1/// A date/time value that is monotonically increasing and immune to system clock changes.2/// Perfect for measuring durations and timeouts within a single app session.3public struct MonotonicDate: Comparable, Sendable, Codable {4    /// System uptime in nanoseconds (never decreases)5    private let uptimeNanos: UInt6467    /// Creates a MonotonicDate representing "now"8    public init() {9        self.uptimeNanos = clock_gettime_nsec_np(CLOCK_MONOTONIC_RAW)10    }1112    /// Creates a MonotonicDate representing a future point in time13    public init(fromNow interval: TimeInterval) {14        let nowNanos = clock_gettime_nsec_np(CLOCK_MONOTONIC_RAW)15        let intervalNanos = UInt64(interval * 1_000_000_000)16        self.uptimeNanos = nowNanos + intervalNanos17    }1819    /// Time interval since another MonotonicDate20    /// Always returns a sensible positive value (or zero)21    public func timeIntervalSince(_ other: MonotonicDate) -> TimeInterval {22        let deltaNanos = self.uptimeNanos - other.uptimeNanos23        return TimeInterval(deltaNanos) / 1_000_000_00024    }2526    /// Has this date/time passed yet?27    public var isBeforeNow: Bool {28        let nowNanos = clock_gettime_nsec_np(CLOCK_MONOTONIC_RAW)29        return uptimeNanos < nowNanos30    }3132    /// Comparable conformance - allows sorting33    public static func < (lhs: MonotonicDate, rhs: MonotonicDate) -> Bool {34        lhs.uptimeNanos < rhs.uptimeNanos35    }36}

1class IndividualCall {2    private let callConnectedTime: MonotonicDate?34    var callDuration: TimeInterval {5        guard let connectedTime = callConnectedTime else {6            return 07        }8        return MonotonicDate().timeIntervalSince(connectedTime)9    }1011    func markConnected() {12        callConnectedTime = MonotonicDate()  // Record connection time13    }14}

1class MessageSender {2    func sendMessage(_ message: Message) async throws {3        let deadline = MonotonicDate(fromNow: 30.0)  // 30-second timeout45        while !deadline.isBeforeNow {6            // Try to send...7            if messageSent {8                return9            }1011            try await Task.sleep(nanoseconds: 1_000_000_000)  // 1 second12        }1314        throw MessageSendError.timeout15    }16}

1class RateLimiter {2    private var lastRequestTime: MonotonicDate?3    private let minimumInterval: TimeInterval = 1.045    func canMakeRequest() -> Bool {6        guard let lastTime = lastRequestTime else {7            return true  // First request8        }910        let elapsed = MonotonicDate().timeIntervalSince(lastTime)11        return elapsed >= minimumInterval12    }1314    func recordRequest() {15        lastRequestTime = MonotonicDate()16    }17}

1// Breaks when clock changes2class BrokenTimer {3    let startTime = Date()45    func checkTimeout() -> Bool {6        let elapsed = Date().timeIntervalSince(startTime)7        return elapsed > 30  // Can be negative! Can jump hours!8    }9}1011// Scenario:12// 1. Start timer at 3:00 PM13// 2. User sets clock back to 2:00 PM14// 3. elapsed becomes -3600 seconds (negative!)

1// Always increases, immune to clock changes2class ReliableTimer {3    let startTime = MonotonicDate()45    func checkTimeout() -> Bool {6        let elapsed = MonotonicDate().timeIntervalSince(startTime)7        return elapsed > 30  // Always positive, always accurate8    }9}

1// Copy Signal's implementation or create simplified version2struct MonotonicTime: Comparable {3    private let nanos: UInt6445    init() {6        self.nanos = clock_gettime_nsec_np(CLOCK_MONOTONIC_RAW)7    }89    static var now: MonotonicTime { MonotonicTime() }1011    func elapsed() -> TimeInterval {12        let nowNanos = clock_gettime_nsec_np(CLOCK_MONOTONIC_RAW)13        return TimeInterval(nowNanos - nanos) / 1_000_000_00014    }1516    static func < (lhs: MonotonicTime, rhs: MonotonicTime) -> Bool {17        lhs.nanos < rhs.nanos18    }19}2021// Usage examples22class Examples {23    // Timeout24    func withTimeout<T>(_ duration: TimeInterval, _ work: () async throws -> T) async throws -> T {25        let start = MonotonicTime()2627        let result = try await work()2829        guard start.elapsed() < duration else {30            throw TimeoutError()31        }3233        return result34    }3536    // Debouncing37    class Debouncer {38        var lastTriggerTime = MonotonicTime()39        let interval: TimeInterval = 0.34041        func shouldTrigger() -> Bool {42            let elapsed = lastTriggerTime.elapsed()43            if elapsed >= interval {44                lastTriggerTime = MonotonicTime()45                return true46            }47            return false48        }49    }5051    // Performance measurement52    func measurePerformance<T>(_ label: String, _ work: () -> T) -> T {53        let start = MonotonicTime()54        let result = work()55        let duration = start.elapsed()56        print("\(label): \(duration * 1000)ms")57        return result58    }59}

1// Before2class OldCode {3    var startTime: Date?45    func startTimer() {6        startTime = Date()7    }89    func checkElapsed() -> TimeInterval {10        guard let start = startTime else { return 0 }11        return Date().timeIntervalSince(start)  // BROKEN12    }13}

1// After2class NewCode {3    var startTime: MonotonicTime?45    func startTimer() {6        startTime = MonotonicTime()7    }89    func checkElapsed() -> TimeInterval {10        guard let start = startTime else { return 0 }11        return start.elapsed()  // RELIABLE12    }13}

1/// Custom UICollectionView that protects against UIKit's temporary zero-frame assignments2/// which can cause layout calculation crashes in conversation view3class ConversationCollectionView: UICollectionView {45    // MARK: - Frame Protection67    /// Override frame setter to reject invalid values8    /// UIKit sometimes temporarily sets frames to zero during complex transitions,9    /// which causes our custom layout to crash when calculating cell sizes10    override var frame: CGRect {11        get {12            super.frame13        }14        set {15            let hasInvalidWidth = newValue.width <= 0 || newValue.width.isNaN16            let hasInvalidHeight = newValue.height <= 0 || newValue.height.isNaN1718            if hasInvalidWidth || hasInvalidHeight {19                // Log for debugging but don't crash20                Logger.warn("Rejecting invalid frame: \(newValue)")2122                // Maintain current valid frame23                return24            }2526            // Only allow valid frames27            super.frame = newValue28        }29    }3031    /// Also protect bounds (UIKit can set this independently)32    override var bounds: CGRect {33        get {34            super.bounds35        }36        set {37            let hasInvalidWidth = newValue.width <= 0 || newValue.width.isNaN38            let hasInvalidHeight = newValue.height <= 0 || newValue.height.isNaN3940            if hasInvalidWidth || hasInvalidHeight {41                Logger.warn("Rejecting invalid bounds: \(newValue)")42                return43            }4445            super.bounds = newValue46        }47    }4849    /// Invariant: Collection view should always have a reasonable size50    /// This is critical for our custom layout which calculates cell widths as percentages51    private func assertValidSize() {52        assert(frame.width > 0 && frame.height > 0,53               "ConversationCollectionView should always have valid size")54    }55}

1class MessageCellLayout {2    func calculateCellSize(for message: Message,3                          collectionViewWidth: CGFloat) -> CGSize {4        // Calculate width as percentage of collection view5        let maxWidth = collectionViewWidth * 0.8  // 80% of screen67        // If collectionViewWidth is 0, this crashes!8        let textWidth = min(message.textWidth, maxWidth)910        return CGSize(width: textWidth, height: message.height)11    }12}

1*** Terminating app due to uncaught exception 'NSInternalInconsistencyException'2*** Reason: 'Invalid layout attributes from -[UICollectionViewLayout layoutAttributesForItemAtIndexPath:]'34Stack trace:5- UICollectionView layoutSubviews6- Custom layout calculating cell size with width=07- NaN propagation through layout calculations8- Assertion failure

1// Crashes during complex transitions2class NormalCollectionView: UICollectionView {3    // Uses default frame setter4    // UIKit can set frame = .zero temporarily5    // Layout crashes6}

1// Maintains layout integrity2class ProtectedCollectionView: UICollectionView {3    override var frame: CGRect {4        get { super.frame }5        set {6            guard newValue.width > 0 && newValue.height > 0 else {7                return  // Reject invalid frames8            }9            super.frame = newValue10        }11    }12}

1// UIKit internally during complex transitions:2func performComplexTransition() {3    // 1. Save current frame4    let savedFrame = view.frame56    // 2. Temporarily set to zero for measurement7    view.frame = .zero  //89    // 3. Ask for intrinsic content size10    let size = view.intrinsicContentSize1112    // 4. Calculate new frame13    let newFrame = calculateFrame(from: size)1415    // 5. Set final frame16    view.frame = newFrame1718    // Problem: If a layout pass happens during step 2-3,19    // your view has zero size!20}

1class ProtectedCollectionView: UICollectionView {2    override var frame: CGRect {3        get { super.frame }4        set {5            // Reject zero or negative dimensions6            guard newValue.width > 0, newValue.height > 0 else {7                print("Rejecting invalid frame: \(newValue)")8                return9            }10            super.frame = newValue11        }12    }1314    override var bounds: CGRect {15        get { super.bounds }16        set {17            guard newValue.width > 0, newValue.height > 0 else {18                print("Rejecting invalid bounds: \(newValue)")19                return20            }21            super.bounds = newValue22        }23    }24}

1class RobustCollectionView: UICollectionView {2    override var frame: CGRect {3        get { super.frame }4        set {5            guard isValid(rect: newValue) else {6                Logger.warn("Rejecting invalid frame: \(newValue)")7                return8            }9            super.frame = newValue10        }11    }1213    private func isValid(rect: CGRect) -> Bool {14        // Check for zero or negative15        guard rect.width > 0, rect.height > 0 else {16            return false17        }1819        // Check for NaN (can happen with bad calculations)20        guard !rect.width.isNaN, !rect.height.isNaN else {21            return false22        }2324        // Check for infinity (another edge case)25        guard rect.width.isFinite, rect.height.isFinite else {26            return false27        }2829        return true30    }31}

1class DebuggableCollectionView: UICollectionView {2    override var frame: CGRect {3        get { super.frame }4        set {5            let oldValue = super.frame67            guard isValid(rect: newValue) else {8                // Capture stack trace to find who's setting invalid frame9                let stackTrace = Thread.callStackSymbols10                Logger.error("Invalid frame rejected. Old: \(oldValue), New: \(newValue)\n\(stackTrace)")11                return12            }1314            super.frame = newValue1516            // Log significant changes for debugging17            if abs(newValue.width - oldValue.width) > 100 {18                Logger.debug("Large frame change: \(oldValue) → \(newValue)")19            }20        }21    }22}

1// Rigid initialization - all dependencies needed upfront2class MessageProcessor {3    private let database: Database4    private let networkManager: NetworkManager5    private let accountManager: AccountManager67    init(database: Database,8         networkManager: NetworkManager,9         accountManager: AccountManager) {10        self.database = database11        self.networkManager = networkManager12        self.accountManager = accountManager13    }14}1516// Creating this requires creating everything first17let processor = MessageProcessor(18    database: createDatabase(),  // Expensive19    networkManager: createNetworkManager(),  // Expensive20    accountManager: createAccountManager()  // Expensive21)

1class DatabaseMigratorRunner {2    // Dependencies are closures, not concrete instances3    private let backgroundMessageFetcherFactory: () -> BackgroundMessageFetcherFactory4    private let remoteConfigManager: () -> any RemoteConfigManager5    private let tsAccountManager: () -> TSAccountManager67    init(8        backgroundMessageFetcherFactory: @escaping () -> BackgroundMessageFetcherFactory,9        remoteConfigManager: @escaping () -> any RemoteConfigManager,10        tsAccountManager: @escaping () -> TSAccountManager11    ) {12        self.backgroundMessageFetcherFactory = backgroundMessageFetcherFactory13        self.remoteConfigManager = remoteConfigManager14        self.tsAccountManager = tsAccountManager15    }1617    func run() async throws {18        // Dependencies are created only when needed19        let remoteConfig = remoteConfigManager()20        try await remoteConfig.refreshIfNeeded()2122        // This might not even be called in some code paths23        if needsBackgroundFetch {24            let fetcher = backgroundMessageFetcherFactory()25            await fetcher.fetchMessages()26        }2728        // Each call gets the current instance (could change between calls)29        let account = tsAccountManager()30        if account.isRegistered {31            // ...32        }33    }34}3536// Usage - pass closures instead of instances37let runner = DatabaseMigratorRunner(38    backgroundMessageFetcherFactory: { AppEnvironment.shared.backgroundMessageFetcherFactory },39    remoteConfigManager: { SSKEnvironment.shared.remoteConfigManager },40    tsAccountManager: { DependenciesBridge.shared.tsAccountManager }41)

1// Database needs account info to decrypt2// Account manager needs database to read settings3// Network manager needs both45class AppSetup {6    func initializeServices() {7        // Create database first (but don't use account yet)8        let database = Database()910        // Create account manager with lazy database access11        let accountManager = AccountManager(12            database: { database }  // Closure - can be called later13        )1415        // Create network manager with lazy access to both16        let networkManager = NetworkManager(17            database: { database },18            accountManager: { accountManager }19        )2021        // Now they can all reference each other without circular init issues22    }23}

1class MessageProcessor {2    // Expensive to create - only create if needed3    private let attachmentDownloader: () -> AttachmentDownloader45    func processMessage(_ message: Message) async {6        if message.hasAttachments {7            // Only create downloader when actually needed8            let downloader = attachmentDownloader()9            await downloader.downloadAttachments(for: message)10        }11        // If no attachments, downloader never created12    }13}

1class FeatureFlag {2    // The implementation can change at runtime3    private let accountManager: () -> TSAccountManager45    var isEnabled: Bool {6        // Always get current instance7        let manager = accountManager()8        return manager.currentAccount != nil9    }10}1112// In tests, you can swap out the account manager13func testFeature() {14    let mockManager = MockAccountManager()1516    // Closure returns different instance for testing17    let flag = FeatureFlag(18        accountManager: { mockManager }19    )2021    XCTAssertTrue(flag.isEnabled)22}

1// All dependencies created upfront2class Traditional {3    let heavy1: HeavyService4    let heavy2: HeavyService5    let heavy3: HeavyService67    init(heavy1: HeavyService, heavy2: HeavyService, heavy3: HeavyService) {8        self.heavy1 = heavy1  // Created even if never used9        self.heavy2 = heavy210        self.heavy3 = heavy311    }12}1314// Creating this is expensive15let obj = Traditional(16    heavy1: HeavyService(),  // 100ms17    heavy2: HeavyService(),  // 100ms18    heavy3: HeavyService()   // 100ms - might not even be used!19)

1// Dependencies created only when needed2class Lazy {3    let heavy1: () -> HeavyService4    let heavy2: () -> HeavyService5    let heavy3: () -> HeavyService67    init(heavy1: @escaping () -> HeavyService,8         heavy2: @escaping () -> HeavyService,9         heavy3: @escaping () -> HeavyService) {10        self.heavy1 = heavy1  // Just storing closure - instant11        self.heavy2 = heavy212        self.heavy3 = heavy313    }1415    func doSomething() {16        if condition {17            heavy1().doWork()  // Created only if condition met18        }19    }20}2122// Initialization is instant23let obj = Lazy(24    heavy1: { HeavyService() },  // Not created yet25    heavy2: { HeavyService() },26    heavy3: { HeavyService() }27)

1class Service {2    private let dependency: () -> Dependency34    init(dependency: @escaping () -> Dependency = { DefaultDependency.shared }) {5        self.dependency = dependency6    }7}

1class Service {2    private let dependencyFactory: () -> Dependency3    private lazy var dependency: Dependency = dependencyFactory()45    init(dependency: @escaping () -> Dependency) {6        self.dependencyFactory = dependency7    }89    func doWork() {10        // Only created once, then cached11        dependency.performAction()12    }13}

1class Service {2    private let dependency: () -> any DependencyProtocol34    init(dependency: @escaping () -> any DependencyProtocol) {5        self.dependency = dependency6    }7}

1class MyViewController: UIViewController {2    // Instead of: private let database: Database3    private let database: () -> Database45    // Instead of: private let networkManager: NetworkManager6    private let networkManager: () -> NetworkManager78    init(9        database: @escaping () -> Database,10        networkManager: @escaping () -> NetworkManager11    ) {12        self.database = database13        self.networkManager = networkManager14        super.init(nibName: nil, bundle: nil)15    }1617    func loadData() async {18        // Create when needed19        let db = database()20        let data = await db.fetch()2122        // Send to server23        let network = networkManager()24        try await network.upload(data)25    }26}2728// Usage29let vc = MyViewController(30    database: { AppDelegate.shared.database },31    networkManager: { AppDelegate.shared.networkManager }32)

1class ServiceA {2    private let serviceB: () -> ServiceB34    init(serviceB: @escaping () -> ServiceB) {5        self.serviceB = serviceB6    }78    func doSomething() {9        serviceB().helperMethod()10    }11}1213class ServiceB {14    private let serviceA: () -> ServiceA1516    init(serviceA: @escaping () -> ServiceA) {17        self.serviceA = serviceA18    }1920    func doSomethingElse() {21        serviceA().helperMethod()22    }23}2425// No circular init issue!26var serviceA: ServiceA!27var serviceB: ServiceB!2829serviceA = ServiceA(serviceB: { serviceB })30serviceB = ServiceB(serviceA: { serviceA })