完整詳解GCD系列（四）dispatch_semaphore（訊號量），dispatchsemaphore

完整詳解GCD系列（四）dispatch_semaphore（訊號量），dispatchsemaphore

原創Blog，轉載請註明出處

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

一 何為訊號量？

  簡單來說就是控制訪問資源的數量，比如系統有兩個資源可以被利用，同時有三個線程要訪問，只能允許兩個線程訪問，第三個應當等待資源被釋放後再訪問。

注意：再GCD中，只有調度的線程在訊號量不足的時候才會進入核心態進行線程阻塞

二 如何使用訊號量

三個主要函數

建立一個訊號量

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

其中value為訊號量的初值，如果小於0則會返回NULL

提高訊號量

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

等待降低訊號量

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

注意，正常的使用順序是先降低然後再提高，這兩個函數通常成對使用。

三 舉例分析

<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.    }}

這段代碼類比提交三個任務，提交到全域隊列（並行隊列）

當訊號量的初初始為2時候

輸出

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

當訊號量為3的時候

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

當訊號量為1的時候

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