派遣信号量

DispatchSemaphore 提供传统计数信号量的有效实现，可用于控制跨多个执行上下文的资源访问。

何时使用信号量的情况可能是，如果你正在进行一些文件读/写，如果多个任务同时尝试从文件读取和写入，则可以提高性能以使每个任务等待轮到所以不要让 I / O 控制器负担过重。

Swift 3

func do2TasksAtATime () {
    print("starting long running tasks (2 at a time)")
    let sem = DispatchSemaphore(value: 2)            //this semaphore only allows 2 tasks to run at the same time (the resource count)
    for i in 0...7 {                                 //launch a bunch of tasks
        DispatchQueue.global().async {               //run tasks on a background thread
            sem.wait()                               //wait here if no resources available
            sleep(2)                                 //do some long task eg file access (here we are just sleeping for a 2 seconds for demonstration purposes)
            print("long task \(i) done! \(Date())")
            sem.signal()                             //let the semaphore know this resource is now available
        }
    }
}

示例输出:(注意时间戳）

starting long running tasks (2 at a time)
long task 0 done! 2017-02-16 07:11:53 +0000
long task 1 done! 2017-02-16 07:11:53 +0000
long task 2 done! 2017-02-16 07:11:55 +0000
long task 3 done! 2017-02-16 07:11:55 +0000
long task 5 done! 2017-02-16 07:11:57 +0000
long task 4 done! 2017-02-16 07:11:57 +0000
long task 6 done! 2017-02-16 07:11:59 +0000
long task 7 done! 2017-02-16 07:11:59 +0000

有关详细信息，请参阅 Apple Docs