完整详解swift GCD系列（四）dispatch_semaphore（信号量）

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http://blog.csdn.net/hello_hwc?viewmode=contents

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

提高信号量

copy

funcdispatch_semaphore_signal(_dsema:dispatch_semaphore_t!)->Int

copy

funcdispatch_semaphore_wait(_dsema:dispatch_semaphore_t!,
1. _timeout:dispatch_time_t)->Int

三 举例分析

<prename="code"class="objc">//
1. //ViewController.swift
2. //SwiftTestExample
3. //
4. //Createdbyhuangwenchenon15/1/6.
5. //Copyright(c)2015年huangwenchen.Allrightsreserved.
6. //
8. importUIKit
9. classViewController:UIViewController{
10. varsemaphore:dispatch_semaphore_t;
11. requiredinit(coderaDecoder:NSCoder){
12. self.semaphore=dispatch_semaphore_create(1)
13. super.init(coder:aDecoder)
14. }
15. overridefuncviewDidLoad(){
16. super.viewDidLoad()
17. dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),{()->Voidin
18. self.task_first()
19. })
20. dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,{()->Voidin
21. self.task_second()
22. })
23. self.task_third()
24. //Doanyadditionalsetupafterloadingtheview,typicallyfromanib.
25. }
26. functask_first(){
27. dispatch_semaphore_wait(self.semaphore,DISPATCH_TIME_FOREVER)
28. NSLog("%@","Firsttaskstarting")
29. sleep(1)
31. dispatch_semaphore_signal(self.semaphore)
32. functask_second(){
33. dispatch_semaphore_wait(self.semaphore,DISPATCH_TIME_FOREVER)
34. NSLog("%@","Secondtaskstarting")
35. sleep(1)
37. dispatch_semaphore_signal(self.semaphore)
38. functask_third(){
41. overridefuncdidReceiveMemoryWarning(){
42. super.didReceiveMemoryWarning()
43. //DispoSEOfanyresourcesthatcanberecreated.
45. }

当信号量的初初始为2时候

输出

2015-01-0619:42:01.963SwiftTestExample[632:11631]Firsttaskstarting
1. 2015-01-0619:42:01.964SwiftTestExample[632:11630]Secondtaskstarting
2. 19:42:02.971SwiftTestExample[632:11630]Secondtaskisdone
3. 19:42:02.971SwiftTestExample[632:11631]Firsttaskisdone
4. 19:42:02.971SwiftTestExample[632:11633]Thridtaskstarting
5. 19:42:03.974SwiftTestExample[632:11633]Thridtaskisdone

copy

19:42:49.912SwiftTestExample[666:12259]Firsttaskstarting
1. 19:42:49.912SwiftTestExample[666:12258]Secondtaskstarting
2. 19:42:49.912SwiftTestExample[666:12260]Thridtaskstarting
3. 19:42:50.915SwiftTestExample[666:12259]Firsttaskisdone
4. 19:42:50.915SwiftTestExample[666:12260]Thridtaskisdone
5. 19:42:50.915SwiftTestExample[666:12258]Secondtaskisdone

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19:43:35.140SwiftTestExample[694:12768]Firsttaskstarting
1. 19:43:36.145SwiftTestExample[694:12768]Firsttaskisdone
2. 19:43:36.145SwiftTestExample[694:12771]Secondtaskstarting
3. 19:43:37.146SwiftTestExample[694:12771]Secondtaskisdone
4. 19:43:37.146SwiftTestExample[694:12769]Thridtaskstarting
5. 19:43:38.150SwiftTestExample[694:12769]Thridtaskisdone

（编辑：李大同）

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