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

var text = ""
for i in 0..<20 {
    fetchData(i, delay: Double.random(in: 0...0.2)) {
        text += "\($0) - "
    }
}
print("text:", text)

RunLoop.current.run()