/////-------------注意----------------
这篇文章是在Swift 1.0时代写的，已经不适合当前的语法。关于Swift最新版本的GCD，参见我的这篇博客
/////-------------注意----------------
一 何为信号量？
简单来说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：再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
二 如何使用信号量
三个主要函数
创建一个信号量
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
func dispatch_semaphore_signal(_ dsema: dispatch_semaphore_t!) -> Int
等待降低信号量
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。
三 举例分析
<pre name="code" class="objc">//// ViewController.swift// SwiftTestExample//// Created by huangwenchen on 15/1/6.// Copyright (c) 2015年 huangwenchen. All rights reserved.//import UIKitclass 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) } override func didReceiveMemoryWarning() { super.didReceiveMemoryWarning() // Dispose of any resources that can be recreated. }}
这段代码模拟提交三个任务，提交到全局队列（并行队列）
当信号量的初初始为2时候
输出
2015-01-06 19:42:01.963 SwiftTestExample[632:11631] First task starting2015-01-06 19:42:01.964 SwiftTestExample[632:11630] Second task starting2015-01-06 19:42:02.971 SwiftTestExample[632:11630] Second task is done2015-01-06 19:42:02.971 SwiftTestExample[632:11631] First task is done2015-01-06 19:42:02.971 SwiftTestExample[632:11633] Thrid task starting2015-01-06 19:42:03.974 SwiftTestExample[632:11633] Thrid task is done当信号量为3的时候
2015-01-06 19:42:49.912 SwiftTestExample[666:12259] First task starting2015-01-06 19:42:49.912 SwiftTestExample[666:12258] Second task starting2015-01-06 19:42:49.912 SwiftTestExample[666:12260] Thrid task starting2015-01-06 19:42:50.915 SwiftTestExample[666:12259] First task is done2015-01-06 19:42:50.915 SwiftTestExample[666:12260] Thrid task is done2015-01-06 19:42:50.915 SwiftTestExample[666:12258] Second task is done
当信号量为1的时候
2015-01-06 19:43:35.140 SwiftTestExample[694:12768] First task starting2015-01-06 19:43:36.145 SwiftTestExample[694:12768] First task is done2015-01-06 19:43:36.145 SwiftTestExample[694:12771] Second task starting2015-01-06 19:43:37.146 SwiftTestExample[694:12771] Second task is done2015-01-06 19:43:37.146 SwiftTestExample[694:12769] Thrid task starting2015-01-06 19:43:38.150 SwiftTestExample[694:12769] Thrid task is done