完整詳解GCD系列（四）dispatch
////-------------注意----------------
這篇文章是在Swift 1.0時代寫的，已經不適合當前的語法。關於Swift最新版本的GCD，參見我的這篇博客
GCD精講
/////-------------注意----------------
一 何為信號量？
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，只有調度的線程在信號量不足的時候才會進入內核態進行線程阻塞
二 如何使用信號量
三個主要函數
創建一個信號量
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func dispatch_semaphore_create(_ value: Int) -> dispatch_semaphore_t !
其中value為信號量的初值，如果小於0則會返回NULL
提高信號量
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func dispatch_semaphore_signal(_ dsema: dispatch_semaphore_t!) -> Int
等待降低信號量
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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
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//
// 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.
}
}
這段代碼模擬提交三個任務，提交到全局隊列（並行隊列）
當信號量的初初始為2時候
輸出
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215-1-619:42:1.963SwiftTestExample[632:11631] First task starting
215-1-619:42:1.964SwiftTestExample[632:1163] Second task starting
215-1-619:42:2.971SwiftTestExample[632:1163] Second task is done
215-1-619:42:2.971SwiftTestExample[632:11631] First task is done
215-1-619:42:2.971SwiftTestExample[632:11633] Thrid task starting
215-1-619:42:3.974SwiftTestExample[632:11633] Thrid task is done
當信號量為3的時候
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215-1-619:42:49.912SwiftTestExample[666:12259] First task starting
215-1-619:42:49.912SwiftTestExample[666:12258] Second task starting
215-1-619:42:49.912SwiftTestExample[666:1226] Thrid task starting
215-1-619:42:5.915SwiftTestExample[666:12259] First task is done
215-1-619:42:5.915SwiftTestExample[666:1226] Thrid task is done
215-1-619:42:5.915SwiftTestExample[666:12258] Second task is done
當信號量為1的時候
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215-1-619:43:35.140SwiftTestExample[694:12768] First task starting
215-1-619:43:36.145SwiftTestExample[694:12768] First task is done
215-1-619:43:36.145SwiftTestExample[694:12771] Second task starting
215-1-619:43:37.146SwiftTestExample[694:12771] Second task is done
215-1-619:43:37.146SwiftTestExample[694:12769] Thrid task starting
215-1-619:43:38.150SwiftTestExample[694:12769] Thrid task is done
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