serial_concurent.swift

let concurent = DispatchQueue.init(label: "defaultQos", attributes: .concurrent)

concurent.sync {
    print("sync defaultQos exec")
    // 1
}
concurent.sync {
    print("sync defaultQos exec")
    // 2
}


DispatchQueue.init(label: "defaultQos").async {
    print("async defaultQos exec")
    // 4
}

print("sync main exec")
// 3

/* console output
sync defaultQos exec
sync defaultQos exec
sync main exec
async defaultQos exec
*/