do-one-loop-at-a-time-async.swift

import Foundation

let smp = DispatchSemaphore(value: 1)
let smp1 = DispatchSemaphore(value: 0)

func doSomething() {
    for i in (0..<4) {
        smp.wait()
        print("[\(i * 3)]")
        defer {
            smp.signal()
        }
        if i % 3 == 2 {
            return // continue
        }
        DispatchQueue.global().asyncAfter(deadline: .now() + .milliseconds(50)) {
            print("[\(i * 3 + 1)] ok")
            smp1.signal()
        }

        smp1.wait()

        print("[\(i * 3 + 2)] after wait")
    }
}

doSomething()

RunLoop.main.run(until: Date(timeIntervalSinceNow: 1))