uilabel.text在最后一次循环执行之前未更改

DispatchQueue.global().async {
    let countdown = ["3", "2", "1", "GO !"]

    for i in 0..<countdown.count {
        DispatchQueue.main.async {
            self.countdownStatus.text = countdown[i]
        }
        sleep(1)
    }
}

let myTimer : Timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(self.performCountdown), userInfo: nil, repeats: false)

let countdown = ["3", "2", "1", "GO !"]
var i = 0
func performCountdown() {
    while i < 4{
        print(countdown[i])
        i = i+1
    }

import Foundation
import UIKit
let countdown = ["3", "2", "1", "GO !"]
DispatchQueue.global(qos: .background).async {
    for i in 0..<countdown.count {
        print(countdown[i])
        sleep(1)
    }

}