Playgrounds, Timers, and Async Callbacks

This is a simple example of using Timer in Swift. I’ve seen some Stack Overflow advice that says Timer performs badly but had no backup to support this assertion.

Swift 4.1
import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

let t = Timer.scheduledTimer(withTimeInterval: 5.0, repeats: false) { (timer) in
    print("TIMER FIRED")

You must invoke scheduledTimer on the main thread. If you don’t, iOS will quietly never fire the timer.

Useful reading: