文章目录
  1. 1. 创建一个信号量
  2. 2. 提高信号量
  3. 3. 等待降低信号量
  4. 4. 举例分析

dispatch_semaphore（信号量）就是控制访问资源的数量，比如系统有两个资源可以被利用，同时有三个线程要访问，只能允许两个线程访问，第三个应当等待资源被释放后再访问。

注意：在GCD中，只有调度的线程在信号量不足的时候才会进入内核态进行线程阻塞

创建一个信号量

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func dispatch_semaphore_create(_ value: Int) ->  dispatch_semaphore_t !

其中value为信号量的初值，如果小于0则会返回NULL

提高信号量

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func dispatch_semaphore_signal(_ dsema: dispatch_semaphore_t!) -> Int

等待降低信号量

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func dispatch_semaphore_wait(_ dsema: dispatch_semaphore_t!,  
                           _ timeout: dispatch_time_t) -> Int

注意：正常的使用顺序是先降低然后再提高，这两个函数通常成对使用。

举例分析

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import UIKit
class ViewController: UIViewController {
    var semaphore:dispatch_semaphore_t;
    required init(coder aDecoder: NSCoder) {
        self.semaphore = dispatch_semaphore_create(1)
        super.init(coder: aDecoder)
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), {() -> Void  in
             self.task_first()
        })
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), { () -> Void in
            self.task_second()
        })
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), { () -> Void in
            self.task_third()
        })
        // Do any additional setup after loading the view, typically from a nib.
    }
    func task_first(){
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER)
        NSLog("%@","First task starting")
        sleep(1)
        NSLog("%@", "First task is done")
        dispatch_semaphore_signal(self.semaphore)
    }
    func task_second(){
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER)
        NSLog("%@","Second task starting")
        sleep(1)
        NSLog("%@", "Second task is done")
        dispatch_semaphore_signal(self.semaphore)
    }
    func task_third(){
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER)
        NSLog("%@","Thrid task starting")
        sleep(1)
        NSLog("%@", "Thrid task is done")
        dispatch_semaphore_signal(self.semaphore)
    }
}

这段代码模拟提交三个任务，提交到全局队列（并行队列）

当信号量的初始值为 2 时候：

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2015-08-10 17:42:01.963 TestExample[632:11631] First task starting
2015-08-10 17:42:01.964 TestExample[632:11630] Second task starting
2015-08-10 17:42:02.971 TestExample[632:11630] Second task is done
2015-08-10 17:42:02.971 TestExample[632:11631] First task is done
2015-08-10 17:42:02.971 TestExample[632:11633] Thrid task starting
2015-08-10 17:42:03.974 TestExample[632:11633] Thrid task is done

当信号量的初始值为 3 时候：

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2015-08-10 17:42:49.912 TestExample[666:12259] First task starting
2015-08-10 17:42:49.912 TestExample[666:12258] Second task starting
2015-08-10 17:42:49.912 TestExample[666:12260] Thrid task starting
2015-08-10 17:42:50.915 TestExample[666:12259] First task is done
2015-08-10 17:42:50.915 TestExample[666:12260] Thrid task is done
2015-08-10 17:42:50.915 TestExample[666:12258] Second task is done

当信号量的初始值为 1 时候：

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2015-08-10 17:43:35.140 TestExample[694:12768] First task starting
2015-08-10 17:43:36.145 TestExample[694:12768] First task is done
2015-08-10 17:43:36.145 TestExample[694:12771] Second task starting
2015-08-10 17:43:37.146 TestExample[694:12771] Second task is done
2015-08-10 17:43:37.146 TestExample[694:12769] Thrid task starting
2015-08-10 17:43:38.150 TestExample[694:12769] Thrid task is done