问异步延迟

EN

func first()  { print("First") }
func second() { print("Second") }
func third()  { print("Third") }

// Since we will block the queue while wait for all three functions to complete,
// dispatch it to a background queue. Don't block the main queue
DispatchQueue.global(qos: .background).async {
    let queue = OperationQueue()
    queue.addOperation(first)
    queue.addOperation(second)
    queue.addOperation(third)

    queue.waitUntilAllOperationsAreFinished()

    // Now all your functions are complete
}