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Swift Dictionary

Timer

Swift's Timer class lets you execute code after a delay or at regular intervals. It is commonly used for periodic UI updates and animation triggers.

Method List

Method / Property	Description
Timer.scheduledTimer(withTimeInterval:repeats:block:)	Creates a timer and adds it to the default RunLoop.
Timer.scheduledTimer(timeInterval:target:selector:userInfo:repeats:)	Creates a timer using a selector (Objective-C style).
Timer.invalidate()	Stops the timer. It cannot be restarted.
timer.isValid	Returns whether the timer is still active.
timer.fire()	Fires the timer immediately.
Task.sleep(nanoseconds:)	Used for delayed execution in Swift Concurrency (recommended).

Sample Code

import Foundation

// A timer that repeats every 1 second
var count = 0
let timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { timer in
    count += 1
    print("Elapsed: \(count) second(s)")
    if count >= 3 {
        timer.invalidate()  // Stop the timer
        print("Timer stopped")
    }
}

// Run the RunLoop to actually fire the timer (for command-line apps)
RunLoop.current.run(until: Date(timeIntervalSinceNow: 3.5))

// A one-shot timer that fires only once
let oneShot = Timer.scheduledTimer(withTimeInterval: 2.0, repeats: false) { _ in
    print("Fired once after 2 seconds")
}

RunLoop.current.run(until: Date(timeIntervalSinceNow: 2.5))

// Delayed execution with Swift Concurrency (recommended)
Task {
    print("Task started")
    try await Task.sleep(nanoseconds: 1_000_000_000)  // Wait 1 second
    print("Processing after 1 second")

    try await Task.sleep(nanoseconds: 500_000_000)    // Wait 0.5 seconds
    print("Processing after another 0.5 seconds")
}

RunLoop.current.run(until: Date(timeIntervalSinceNow: 2.0))

Overview

Timer operates by being attached to a RunLoop. scheduledTimer is automatically added to the default RunLoop. When using a timer on a background thread, you must add it to a RunLoop manually.

In environments that support Swift Concurrency, using Task.sleep(nanoseconds:) or Task.sleep(for:) instead of Timer is simpler. If you forget to stop a Timer, the RunLoop will continue to hold a reference to it, causing a memory leak. Always call invalidate() when the timer is no longer needed.

For NotificationCenter, see NotificationCenter.

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