Timer Patterns Reference

Complete API reference for iOS timer mechanisms. For decision trees and crash prevention, see axiom-timer-patterns.

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Part 1: Timer API

Timer.scheduledTimer (Block-Based)

// Most common — block-based, auto-added to current RunLoop
let timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] _ in
    self?.updateProgress()
}

Key detail: Added to .default RunLoop mode. Stops during scrolling. See Part 1 RunLoop modes table below.

Timer.scheduledTimer (Selector-Based)

// Objective-C style — RETAINS TARGET (leak risk)
let timer = Timer.scheduledTimer(
    timeInterval: 1.0,
    target: self,       // Timer retains self!
    selector: #selector(update),
    userInfo: nil,
    repeats: true
)

Danger: This API retains target. If self also holds the timer, you have a retain cycle. The block-based API with [weak self] is always safer.

Timer.init (Manual RunLoop Addition)

// Create timer without adding to RunLoop
let timer = Timer(timeInterval: 1.0, repeats: true) { [weak self] _ in
    self?.updateProgress()
}

// Add to specific RunLoop mode
RunLoop.current.add(timer, forMode: .common)  // Survives scrolling

timer.tolerance

timer.tolerance = 0.1  // Allow 100ms flexibility for system coalescing

System batches timers with similar fire dates when tolerance is set. Minimum recommended: 10% of interval. Reduces CPU wakes and energy consumption.

RunLoop Modes

Mode	Constant	When Active	Timer Fires?
Default	.default / RunLoop.Mode.default	Normal user interaction	Yes
Tracking	.tracking / RunLoop.Mode.tracking	Scroll/drag gesture active	Only if added to .common
Common	.common / RunLoop.Mode.common	Pseudo-mode (default + tracking)	Yes (always)

timer.invalidate()

timer.invalidate()  // Stops timer, removes from RunLoop
// Timer is NOT reusable after invalidate — create a new one
timer = nil          // Release reference

Key detail: invalidate() must be called from the same thread that created the timer (usually main thread).

timer.isValid

if timer.isValid {
    // Timer is still active
}

Returns false after invalidate() or after a non-repeating timer fires.

Timer.publish (Combine)

Timer.publish(every: 1.0, tolerance: 0.1, on: .main, in: .common)
    .autoconnect()
    .sink { [weak self] _ in
        self?.updateProgress()
    }
    .store(in: &cancellables)

See Part 3 for full Combine timer details.

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Part 2: DispatchSourceTimer API

Creation

// Create timer source on a specific queue
let queue = DispatchQueue(label: "com.app.timer")
let timer = DispatchSource.makeTimerSource(flags: [], queue: queue)

flags: Usually empty ([]). Use .strict for precise timing (disables system coalescing, higher energy cost).

Schedule

// Relative deadline (monotonic clock)
timer.schedule(
    deadline: .now() + 1.0,     // First fire
    repeating: .seconds(1),     // Interval
    leeway: .milliseconds(100)  // Tolerance (like Timer.tolerance)
)

// Wall clock deadline (survives device sleep)
timer.schedule(
    wallDeadline: .now() + 1.0,
    repeating: .seconds(1),
    leeway: .milliseconds(100)
)

deadline vs wallDeadline: deadline uses monotonic clock (pauses when device sleeps). wallDeadline uses wall clock (continues across sleep). Use deadline for most cases.

Event Handler

timer.setEventHandler { [weak self] in
    self?.performWork()
}

Before cancel: Set handler to nil to break retain cycles:

timer.setEventHandler(handler: nil)
timer.cancel()

Lifecycle Methods

timer.activate()   // Start — can only call ONCE (idle → running)
timer.suspend()    // Pause (running → suspended)
timer.resume()     // Unpause (suspended → running)
timer.cancel()     // Stop permanently (must NOT be suspended)

State Machine Lifecycle

                    activate()
        idle ──────────────► running
                               │  ▲
                    suspend()  │  │  resume()
                               ▼  │
                            suspended
                               │
                    resume() + cancel()
                               │
                               ▼
                           cancelled

Critical rules:

• activate() can only be called once (idle → running)
• cancel() requires non-suspended state (resume first if suspended)
• cancelled is terminal — no further operations allowed
• Dealloc requires non-suspended state (cancel first if needed)

Leeway (Tolerance)

// Leeway values
timer.schedule(deadline: .now(), repeating: 1.0, leeway: .milliseconds(100))
timer.schedule(deadline: .now(), repeating: 1.0, leeway: .seconds(1))
timer.schedule(deadline: .now(), repeating: 1.0, leeway: .never)  // Strict — high energy

Leeway is the DispatchSourceTimer equivalent of Timer.tolerance. Allows system to coalesce timer firings for energy efficiency.

End-to-End Example

Complete DispatchSourceTimer lifecycle in one block:

let queue = DispatchQueue(label: "com.app.polling")
let timer = DispatchSource.makeTimerSource(queue: queue)
timer.schedule(deadline: .now() + 1.0, repeating: .seconds(5), leeway: .milliseconds(500))
timer.setEventHandler { [weak self] in
    self?.fetchUpdates()
}
timer.activate()  // idle → running

// Later — pause:
timer.suspend()   // running → suspended

// Later — resume:
timer.resume()    // suspended → running

// Cleanup — MUST resume before cancel if suspended:
timer.setEventHandler(handler: nil)  // Break retain cycles
timer.resume()    // Ensure non-suspended state
timer.cancel()    // running → cancelled (terminal)

For a safe wrapper that prevents all crash patterns, see axiom-timer-patterns Part 4: SafeDispatchTimer.

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Part 3: Combine Timer

Timer.publish

import Combine

// Create publisher — RunLoop mode matters here too
let publisher = Timer.publish(
    every: 1.0,          // Interval
    tolerance: 0.1,      // Optional tolerance
    on: .main,           // RunLoop
    in: .common          // Mode — use .common to survive scrolling
)

.autoconnect()

// Starts immediately when first subscriber attaches
Timer.publish(every: 1.0, on: .main, in: .common)
    .autoconnect()
    .sink { date in
        print("Fired at \(date)")
    }
    .store(in: &cancellables)

.connect() (Manual Start)

// Manual control over when timer starts
let timerPublisher = Timer.publish(every: 1.0, on: .main, in: .common)
let cancellable = timerPublisher
    .sink { date in
        print("Fired at \(date)")
    }

// Start later
let connection = timerPublisher.connect()

// Stop
connection.cancel()

Cancellation

// Via AnyCancellable storage — cancelled when Set is cleared or object deallocs
private var cancellables = Set<AnyCancellable>()

// Manual cancellation
cancellables.removeAll()  // Cancels all subscriptions

SwiftUI Integration

class TimerViewModel: ObservableObject {
    @Published var elapsed: Int = 0
    private var cancellables = Set<AnyCancellable>()

    func start() {
        Timer.publish(every: 1.0, tolerance: 0.1, on: .main, in: .common)
            .autoconnect()
            .sink { [weak self] _ in
                self?.elapsed += 1
            }
            .store(in: &cancellables)
    }

    func stop() {
        cancellables.removeAll()
    }
}

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Part 4: AsyncTimerSequence (Swift Concurrency)

ContinuousClock.timer

// Monotonic clock — does NOT pause when app suspends
for await _ in ContinuousClock().timer(interval: .seconds(1)) {
    await updateData()
}
// Loop exits when task is cancelled

SuspendingClock.timer

// Suspending clock — pauses when app suspends
for await _ in SuspendingClock().timer(interval: .seconds(1)) {
    await processItem()
}

ContinuousClock vs SuspendingClock:

• ContinuousClock: Time keeps advancing during app suspension. Use for absolute timing.
• SuspendingClock: Time pauses when app suspends. Use for "user-perceived" timing.

Task Cancellation

// Timer automatically stops when task is cancelled
let timerTask = Task {
    for await _ in ContinuousClock().timer(interval: .seconds(1)) {
        await fetchLatestData()
    }
}

// Later: cancel the timer
timerTask.cancel()

Background Polling with Structured Concurrency

func startPolling() async {
    do {
        for try await _ in ContinuousClock().timer(interval: .seconds(30)) {
            try Task.checkCancellation()
            let data = try await api.fetchUpdates()
            await MainActor.run { updateUI(with: data) }
        }
    } catch is CancellationError {
        // Clean exit
    } catch {
        // Handle fetch error
    }
}

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Part 5: Task.sleep Alternatives

One-Shot Delay

// Simple delay — NOT a timer
try await Task.sleep(for: .seconds(1))

// Deadline-based
try await Task.sleep(until: .now + .seconds(1), clock: .continuous)

When to Use Sleep vs Timer

Need	Use
One-shot delay before action	Task.sleep(for:)
Repeating action	ContinuousClock().timer(interval:)
Delay with cancellation	Task.sleep(for:) in a Task
Retry with backoff	Task.sleep(for:) in a loop

Retry with Exponential Backoff

func fetchWithRetry(maxAttempts: Int = 3) async throws -> Data {
    var delay: Duration = .seconds(1)
    for attempt in 1...maxAttempts {
        do {
            return try await api.fetch()
        } catch where attempt < maxAttempts {
            try await Task.sleep(for: delay)
            delay *= 2  // Exponential backoff
        }
    }
    throw FetchError.maxRetriesExceeded
}

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Part 6: LLDB Timer Inspection

Timer (NSTimer) Commands

# Check if timer is still valid
po timer.isValid

# See next fire date
po timer.fireDate

# See timer interval
po timer.timeInterval

# Force RunLoop iteration (may trigger timer)
expression -l objc -- (void)[[NSRunLoop mainRunLoop] run]

DispatchSourceTimer Commands

# Inspect dispatch source
po timer

# Break on dispatch source cancel (all sources)
breakpoint set -n dispatch_source_cancel

# Break on EXC_BAD_INSTRUCTION to catch timer crashes
# (Xcode does this automatically for Swift runtime errors)

# Check if a DispatchSource is cancelled
expression -l objc -- (long)dispatch_source_testcancel((void*)timer)

General Timer Debugging

# List all timers on the main RunLoop
expression -l objc -- (void)CFRunLoopGetMain()

# Break when any Timer fires
breakpoint set -S "scheduledTimerWithTimeInterval:target:selector:userInfo:repeats:"

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Part 7: Platform Availability Matrix

API	iOS	macOS	watchOS	tvOS
Timer	2.0+	10.0+	2.0+	9.0+
DispatchSourceTimer	8.0+ (GCD)	10.10+	2.0+	9.0+
Timer.publish (Combine)	13.0+	10.15+	6.0+	13.0+
AsyncTimerSequence	16.0+	13.0+	9.0+	16.0+
Task.sleep	13.0+	10.15+	6.0+	13.0+

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Related Skills

• axiom-timer-patterns — Decision trees, crash patterns, SafeDispatchTimer wrapper
• axiom-energy — Timer tolerance as energy optimization (Pattern 1)
• axiom-energy-ref — Timer efficiency APIs with WWDC code examples
• axiom-memory-debugging — Timer as Pattern 1 memory leak

Resources

Skills: axiom-timer-patterns, axiom-energy-ref, axiom-memory-debugging