Final solution for medium post about DispatchGroup and DispatchSemaphore synchronization

main.swift

import Foundation

func fetchData(_ data: Int, delay: Double, completionHandler: @escaping (String)->()) {
    DispatchQueue.main.asyncAfter(deadline: .now() + delay) {
        completionHandler("\(data)")
    }
}

DispatchQueue.global().async {
    let semaphore = DispatchSemaphore(value: 0)
    var text = ""
    for i in 0..<20 {
        fetchData(i, delay: Double.random(in: 0...0.2)) {
            text += "\($0) - "
            semaphore.signal()
        }
        semaphore.wait()
    }
    print(text)
    exit(0)
}
RunLoop.current.run()