main.swift

func combineAsyncCalls(completionHandler: @escaping (String)->()) {
    var text = ""
    let semaphore = DispatchSemaphore(value: 0)
    DispatchQueue.global().async {
        fetchData(0, delay: 0.4) {
            text += $0
            semaphore.signal()
        }
        fetchData(1, delay: 0.2) {
            text += $0
            semaphore.signal()
        }
        semaphore.wait()
        semaphore.wait()

        completionHandler(text)
    }
}