https://stackoverflow.com/a/49924716/1271826

GroupOrSemaphore.swift

func performUsingGroup() {
    let dq1 = DispatchQueue.global(qos: .default)
    let dq2 = DispatchQueue.global(qos: .default)
    let group = DispatchGroup()
    
    for i in 1...3 {
        dq1.async(group: group) {
            print("\(#function) DispatchQueue 1: \(i)")
        }
    }
    for i in 1...3 {
        dq2.async(group: group) {
            print("\(#function) DispatchQueue 2: \(i)")
        }
    }
    
    group.notify(queue: .main) {
        print("done by group")
    }
}

func performUsingSemaphore() {
    let dq1 = DispatchQueue.global(qos: .default)
    let dq2 = DispatchQueue.global(qos: .default)
    let semaphore = DispatchSemaphore(value: 1)
    
    DispatchQueue.global().async {
        for i in 1...3 {
            _ = semaphore.wait(timeout: .distantFuture)
            dq1.async {
                print("\(#function) DispatchQueue 1: \(i)")
                semaphore.signal()
            }
        }
        for i in 1...3 {
            _ = semaphore.wait(timeout: .distantFuture)
            dq2.async {
                print("\(#function) DispatchQueue 2: \(i)")
                semaphore.signal()
            }
        }
    }
    
}