work_item.swift

let workItemFirst = DispatchWorkItem {
    print("exec 1st WI")
    // 2
}

let workItemSecond = DispatchWorkItem {
    print("exec 1st WI")
    // Not called
}

let queue = DispatchQueue(label: "queue")

queue.async(execute: workItemFirst)
queue.async(execute: workItemSecond)
// Cancel second item
workItemSecond.cancel()

print("main exec")
// 1

/* console output
 main exec
 exec 1st WI
*/