group.swift

let group = DispatchGroup()

let workItemFirst = DispatchWorkItem {
    print("exec 1st WI")
    // 1
    group.leave()
}

let workItemSecond = DispatchWorkItem {
    print("exec 2nd WI")
    // 2
    group.leave()
}

let queue = DispatchQueue(label: "queue")
queue.async(execute: workItemFirst)
group.enter()
queue.async(execute: workItemSecond)
group.enter()
group.notify(queue: .main) {
    print("exec group notify in main queue")
    // 4
}

print("main exec")
// 2

/* console output
 exec 1st WI
 main exec
 exec 2nd WI
 exec group notify in main queue
*/