完整详解GCD系列（四）dispatch_semaphore（信号量）
原创Blog，转载请注明出处
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
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 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)
}
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 starting 2015-01-06 19:42:01.964 SwiftTestExample[632:11630] Second task starting 2015-01-06 19:42:02.971 SwiftTestExample[632:11630] Second task is done 2015-01-06 19:42:02.971 SwiftTestExample[632:11631] First task is done 2015-01-06 19:42:02.971 SwiftTestExample[632:11633] Thrid task starting 2015-01-06 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 starting 2015-01-06 19:42:49.912 SwiftTestExample[666:12258] Second task starting 2015-01-06 19:42:49.912 SwiftTestExample[666:12260] Thrid task starting 2015-01-06 19:42:50.915 SwiftTestExample[666:12259] First task is done 2015-01-06 19:42:50.915 SwiftTestExample[666:12260] Thrid task is done 2015-01-06 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 starting 2015-01-06 19:43:36.145 SwiftTestExample[694:12768] First task is done 2015-01-06 19:43:36.145 SwiftTestExample[694:12771] Second task starting 2015-01-06 19:43:37.146 SwiftTestExample[694:12771] Second task is done 2015-01-06 19:43:37.146 SwiftTestExample[694:12769] Thrid task starting 2015-01-06 19:43:38.150 SwiftTestExample[694:12769] Thrid task is done