多线程递归

var arr = [1,2,3,4]
 let queue = DispatchQueue(label: "queue")

 func removeArr() {
    if arr.count > 0 {
        queue.async {
            sleep(2)
            arr.remove(at: 0)
            print(arr.count)
            removeArr()
        }
    } else {
        queue.async {
            print("remove all")
        }
    }
 }
 print("remove start")
 print(arr.count)
 removeArr()


let queue = DispatchQueue(label: "queue", attributes: .concurrent)

func requestData(dict: Dictionary<String,Any>, block: @escaping (()->())) {
    let arr = dict["list"] as! Array<String>
    let key = dict["key"] as! String
    var num = arr.count
    let lock = NSLock()
    for str in arr {
        queue.async {
            sleep(3)
            lock.lock()
            num -= 1
            print("\(str) finished")
            if num == 0 {
                print("\(key) all finished")
                block()
            }
            lock.unlock()
        }
    }
}


var arr = [["key":"task1","list":["task 1.1","task 1.2","task 1.3"]],["key":"task2","list":["task 2.1","task 2.2"]],["key":"task3","list":["task 3.1","task 3.2","task 3.3"]]]
var num = arr.count
let lock = NSLock()
print("all task start")
for sub in arr {
    queue.async {
        requestData(dict: sub, block: {
            lock.lock()
            num -= 1
            if num == 0 {
                print("all task finished")
            }
            lock.unlock()
        })
    }
}