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背景

擔(dān)心了兩周的我終于輪到去醫(yī)院做胃鏡檢查了！去的時(shí)候我都想好了最壞的可能（胃癌），之前在網(wǎng)上查的癥狀都很相似。最后檢查結(jié)果出來(lái)終于安心了，診斷結(jié)果：慢性非萎縮性胃炎（胃竇為主）

我是一個(gè)心里素質(zhì)不過(guò)關(guān)的人，所以說(shuō)對(duì)待問(wèn)題的時(shí)候可能會(huì)有一種悲觀的想法。朋友說(shuō)我本來(lái)可能沒(méi)病都被自己嚇出病了，這是一個(gè)心態(tài)問(wèn)題。

你們可能問(wèn)我做胃鏡什么感覺(jué)？我只能告訴你一個(gè)字：真爽，具體只能自己去感受。

自己眼中的自己.jpg

保持樂(lè)觀的心態(tài)

看完下面的笑話就要開(kāi)始我們的裝逼之旅了^_

一個(gè)大學(xué)生去公司實(shí)習(xí)，老板讓他先從掃地開(kāi)始。大學(xué)生：“我可是大學(xué)生哎……”老板：“哦，對(duì)了，我差點(diǎn)忘了你是大學(xué)生，來(lái)來(lái)來(lái)，我教你怎么掃地”

一天，老師讓同學(xué)們寫(xiě)作文，題目是《我的理想》。
小明在作文里寫(xiě)道：我長(zhǎng)大了要去搶銀行，然后把錢(qián)分給窮苦老百姓。
第二天老師改完了，寫(xiě)給小明的評(píng)語(yǔ)是這樣的：很不錯(cuò)的理想，分錢(qián)的時(shí)候不要忘了老師，但你要注意你的同桌，他說(shuō)他長(zhǎng)大了要去當(dāng)警察。

GCD基本介紹

GCD（Grand Central Dispatch）iOS 4.0開(kāi)始引入的新多線程編程功能,
GCD（Grand Central Dispatch）是異步執(zhí)行任務(wù)的技術(shù)之一。一般將應(yīng)用程序中記述的線程管理用的代碼在系統(tǒng)級(jí)中實(shí)現(xiàn)。開(kāi)發(fā)者只需要定義想執(zhí)行的任務(wù)并追加到適當(dāng)?shù)腄ispatch Queue中，GCD就能生成必要的線程并計(jì)劃執(zhí)行任務(wù)。
GCD（Grand Central Dispatch）是基于C語(yǔ)言開(kāi)發(fā)的一套多線程開(kāi)發(fā)機(jī)制，是完全面向過(guò)程的。

GCD基本概念

這就需要上一篇博客里的基本知識(shí)了（不清楚去看下）iOS開(kāi)發(fā)之多線程編程總結(jié)（一）

任務(wù)和隊(duì)列

任務(wù)：就是執(zhí)行操作的意思，換句話說(shuō)就是你在線程中執(zhí)行的那段代碼。在GCD中是放在block中的。執(zhí)行任務(wù)有兩種方式：同步執(zhí)行和異步執(zhí)行。兩者的主要區(qū)別是：是否具備開(kāi)啟新線程的能力。
1. 同步執(zhí)行（sync）：只能在當(dāng)前線程中執(zhí)行任務(wù)，不具備開(kāi)啟新線程的能力
- 必須等待當(dāng)前語(yǔ)句執(zhí)行完畢，才會(huì)執(zhí)行下一條語(yǔ)句
- 不會(huì)開(kāi)啟線程
- 在當(dāng)前主線程執(zhí)行 block 的任務(wù)
- dispatch_sync(queue, block);
1. ** 異步執(zhí)行（async）**：可以在新的線程中執(zhí)行任務(wù)，具備開(kāi)啟新線程的能力
- 不用等待當(dāng)前語(yǔ)句執(zhí)行完畢，就可以執(zhí)行下一條語(yǔ)句
- 會(huì)開(kāi)啟線程執(zhí)行 block 的任務(wù)
- 異步是多線程的代名詞
- dispatch_async(queue, block);
隊(duì)列：這里的隊(duì)列指任務(wù)隊(duì)列，即用來(lái)存放任務(wù)的隊(duì)列。隊(duì)列是一種特殊的線性表，采用FIFO（先進(jìn)先出）的原則，即新任務(wù)總是被插入到隊(duì)列的末尾，而讀取任務(wù)的時(shí)候總是從隊(duì)列的頭部開(kāi)始讀取。每讀取一個(gè)任務(wù)，則從隊(duì)列中釋放一個(gè)任務(wù)。在GCD中有四種隊(duì)列：串行隊(duì)列、并發(fā)隊(duì)列、主隊(duì)列、全局隊(duì)列。
1. 串行隊(duì)列（Serial Dispatch Queue）：讓任務(wù)一個(gè)接著一個(gè)地執(zhí)行（一個(gè)任務(wù)執(zhí)行完畢后，再執(zhí)行下一個(gè)任務(wù)）
  - 一次只能"調(diào)度"一個(gè)任務(wù)
  - dispatch_queue_create("queue", NULL);
    或者dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
2. 并發(fā)隊(duì)列（Concurrent Dispatch Queue）：可以讓多個(gè)任務(wù)并發(fā)（同時(shí)）執(zhí)行（自動(dòng)開(kāi)啟多個(gè)線程同時(shí)執(zhí)行任務(wù)），
  - 一次可以"調(diào)度"多個(gè)任務(wù)
  - 并發(fā)功能只有在異步（dispatch_async）函數(shù)下才有效
  - dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
3. 主隊(duì)列
  - 專(zhuān)門(mén)用來(lái)在主線程上調(diào)度任務(wù)的隊(duì)列
  - 不會(huì)開(kāi)啟線程
  - 在主線程空閑時(shí)才會(huì)調(diào)度隊(duì)列中的任務(wù)在主線程執(zhí)行
  - dispatch_get_main_queue();
4. 全局隊(duì)列
  - 執(zhí)行過(guò)程和并發(fā)隊(duì)列一致，參考并發(fā)隊(duì)列
  - dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

小結(jié): 在以后的使用中，記住下面的就可以了！

開(kāi)不開(kāi)線程由執(zhí)行任務(wù)的函數(shù)決定

異步開(kāi)，異步是多線程的代名詞
同步不開(kāi)

開(kāi)幾條線程由隊(duì)列決定

串行隊(duì)列開(kāi)一條線程（GCD會(huì)開(kāi)一條，NSOperation Queue最大并發(fā)數(shù)為1時(shí)也可能開(kāi)多條）
并發(fā)隊(duì)列開(kāi)多條線程，具體能開(kāi)的線程數(shù)量由底層線程池決定

GCD的使用

今天我們學(xué)習(xí)下面圖片的相關(guān)知識(shí)點(diǎn)，Demo下載鏈接會(huì)在文章最后給出來(lái)

GCD知識(shí)點(diǎn).png

簡(jiǎn)單來(lái)看一段代碼：異步下載圖片

#pragma mark - 1.異步下載圖片
- (IBAction)downLoadAction:(UIButton *)sender {
    self.imageView.image = nil;
    //獲取全局隊(duì)列
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        //異步下載圖片
        NSURL *url=[NSURL URLWithString:@"https://p1.bpimg.com/524586/475bc82ff016054ds.jpg"];
        //將資源轉(zhuǎn)換為二進(jìn)制
        NSData *data=[NSData dataWithContentsOfURL:url];
        //將二進(jìn)制轉(zhuǎn)化為圖片
        UIImage *image=[UIImage imageWithData:data];
        
        //獲取主隊(duì)列,更新UI
        dispatch_async(dispatch_get_main_queue(), ^{
            //給圖片控件賦值
            self.imageView.image=image;
        });
    });
}

和NSThread對(duì)比可以發(fā)現(xiàn)

所有的代碼寫(xiě)在一起的，讓代碼更加簡(jiǎn)單，易于閱讀和維護(hù)
NSThread 通過(guò) @selector 指定要執(zhí)行的方法，代碼分散
GCD 通過(guò) block 指定要執(zhí)行的代碼，代碼集中
使用 GCD 不需要管理線程的創(chuàng)建/銷(xiāo)毀/復(fù)用的過(guò)程！程序員不用關(guān)心線程的生命周期
如果要開(kāi)多個(gè)線程 NSThread 必須實(shí)例化多個(gè)線程對(duì)象或者使用分類(lèi)方法
NSThread 靠 NSObject 的分類(lèi)方法實(shí)現(xiàn)的線程間通訊，GCD 靠 block
dispatch_async(queue, block);就是異步執(zhí)行一個(gè)隊(duì)列里面的任務(wù)block。每個(gè)block之間是異步執(zhí)行的，但是block里面的代碼是順序執(zhí)行的！
dispatch_sync(queue, block);就是同步執(zhí)行一個(gè)隊(duì)列里面的任務(wù)block

1. 串行隊(duì)列（Serial Dispatch Queue）

串行隊(duì)列的創(chuàng)建：

 dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
 dispatch_queue_t queue = dispatch_queue_create("queue", NULL);

串行隊(duì)列同步和異步執(zhí)行Demo：

 #pragma mark - 串行隊(duì)列同步和串行隊(duì)列異步
 //串行隊(duì)列同步
 - (void)serialQueueSyncMethod{
     //創(chuàng)建隊(duì)列
     dispatch_queue_t queue = dispatch_queue_create("serialQueueSyncMethod", DISPATCH_QUEUE_SERIAL);
     //執(zhí)行任務(wù)
     for (int i = 0; i < 6; i++) {
         NSLog(@"mainThread--->%d",i);
         dispatch_sync(queue, ^{
             NSLog(@"Current Thread=%@---->%d-----",[NSThread currentThread],i);
         });
     }
     NSLog(@"串行隊(duì)列同步end");
  }

 //串行隊(duì)列異步
  - (void)serialQueueAsyncMethod{
     dispatch_queue_t queue = dispatch_queue_create("serialQueueAsyncMethod", DISPATCH_QUEUE_SERIAL);
     for (int i = 0; i < 6; i++) {
         NSLog(@"mainThread--->%d",i);
         dispatch_async(queue, ^{
             NSLog(@"Current Thread=%@---->%d-----",[NSThread currentThread],i);
         });
     }
     NSLog(@"串行隊(duì)列異步end");
  }

串行隊(duì)列同步執(zhí)行結(jié)果：

2016-11-03 17:16:35.794 ThreadDemo[27088:5268309] mainThread--->0
2016-11-03 17:16:35.795 ThreadDemo[27088:5268309] Current Thread=<NSThread: 0x60800007ae40>{number = 1, name = main}---->0-----
2016-11-03 17:16:35.795 ThreadDemo[27088:5268309] mainThread--->1
2016-11-03 17:16:35.795 ThreadDemo[27088:5268309] Current Thread=<NSThread: 0x60800007ae40>{number = 1, name = main}---->1-----
2016-11-03 17:16:35.795 ThreadDemo[27088:5268309] mainThread--->2
2016-11-03 17:16:35.795 ThreadDemo[27088:5268309] Current Thread=<NSThread: 0x60800007ae40>{number = 1, name = main}---->2-----
2016-11-03 17:16:35.795 ThreadDemo[27088:5268309] mainThread--->3
2016-11-03 17:16:35.796 ThreadDemo[27088:5268309] Current Thread=<NSThread: 0x60800007ae40>{number = 1, name = main}---->3-----
2016-11-03 17:16:35.796 ThreadDemo[27088:5268309] mainThread--->4
2016-11-03 17:16:35.796 ThreadDemo[27088:5268309] Current Thread=<NSThread: 0x60800007ae40>{number = 1, name = main}---->4-----
2016-11-03 17:16:35.796 ThreadDemo[27088:5268309] mainThread--->5
2016-11-03 17:16:35.796 ThreadDemo[27088:5268309] Current Thread=<NSThread: 0x60800007ae40>{number = 1, name = main}---->5-----
2016-11-03 17:16:35.796 ThreadDemo[27088:5268309] 串行隊(duì)列同步end

串行隊(duì)列異步執(zhí)行結(jié)果：

2016-11-03 17:22:25.074 ThreadDemo[27122:5273206] mainThread--->0
2016-11-03 17:22:25.074 ThreadDemo[27122:5273206] mainThread--->1
2016-11-03 17:22:25.074 ThreadDemo[27122:5273252] Current Thread=<NSThread: 0x60800007f380>{number = 5, name = (null)}---->0-----
2016-11-03 17:22:25.074 ThreadDemo[27122:5273206] mainThread--->2
2016-11-03 17:22:25.074 ThreadDemo[27122:5273252] Current Thread=<NSThread: 0x60800007f380>{number = 5, name = (null)}---->1-----
2016-11-03 17:22:25.074 ThreadDemo[27122:5273206] mainThread--->3
2016-11-03 17:22:25.075 ThreadDemo[27122:5273252] Current Thread=<NSThread: 0x60800007f380>{number = 5, name = (null)}---->2-----
2016-11-03 17:22:25.075 ThreadDemo[27122:5273206] mainThread--->4
2016-11-03 17:22:25.075 ThreadDemo[27122:5273252] Current Thread=<NSThread: 0x60800007f380>{number = 5, name = (null)}---->3-----
2016-11-03 17:22:25.075 ThreadDemo[27122:5273206] mainThread--->5
2016-11-03 17:22:25.075 ThreadDemo[27122:5273252] Current Thread=<NSThread: 0x60800007f380>{number = 5, name = (null)}---->4-----
2016-11-03 17:22:25.075 ThreadDemo[27122:5273206] 串行隊(duì)列異步end
2016-11-03 17:22:25.075 ThreadDemo[27122:5273252] Current Thread=<NSThread: 0x60800007f380>{number = 5, name = (null)}---->5-----

小結(jié)：

從串行隊(duì)列同步執(zhí)行結(jié)果看出打印是交替執(zhí)行的！從打印中看到number=1，說(shuō)明線程block任務(wù)是在主線程中執(zhí)行的。因?yàn)?strong>同步是不會(huì)開(kāi)辟線程的，所有當(dāng)前只有一個(gè)主線程MainThread。也就是說(shuō)串行隊(duì)列同步執(zhí)行不會(huì)開(kāi)辟線程，所有block任務(wù)之間是同步執(zhí)行的
從串行隊(duì)列異步執(zhí)行結(jié)果看出打印并不是交替執(zhí)行的！從打印中看到number=5，說(shuō)明線程block的任務(wù)是在一個(gè)全新的線程中執(zhí)行的。因?yàn)?strong>異步是會(huì)開(kāi)辟線程的，所有當(dāng)前有主線程MainThread和子線程number=5。也就是說(shuō)串行隊(duì)列異步執(zhí)行會(huì)僅會(huì)開(kāi)辟一個(gè)新的線程，所有block任務(wù)之間是同步執(zhí)行的
以先進(jìn)先出的方式,順序調(diào)度隊(duì)列中的任務(wù)執(zhí)行
無(wú)論隊(duì)列中指定的任務(wù)函數(shù)是同步還是異步,都會(huì)等待前一個(gè)任務(wù)執(zhí)行完畢以后,再調(diào)度后面的任務(wù)

2. 并發(fā)隊(duì)列（Concurrent Dispatch Queue）

并發(fā)隊(duì)列創(chuàng)建

dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);

并發(fā)隊(duì)列同步和異步執(zhí)行Demo

 #pragma mark - 并行隊(duì)列同步和并行隊(duì)列異步
 //并行隊(duì)列同步
 - (void)concurrentQueueSyncMethod{
     dispatch_queue_t queue = dispatch_queue_create("concurrentQueueSyncMethod", DISPATCH_QUEUE_CONCURRENT);
    
     for (int i = 0; i < 6; i++) {
         dispatch_sync(queue, ^{
             NSLog(@"Current Thread=%@---->%d-----",[NSThread currentThread],i);
         });
     }
     NSLog(@"并行隊(duì)列同步end");
  }

 //并行隊(duì)列異步
- (void)concurrentQueueAsyncMethod{
     dispatch_queue_t queue = dispatch_queue_create("concurrentQueueAsyncMethod", DISPATCH_QUEUE_CONCURRENT);

     for (int i = 0; i < 6; i++) {
         dispatch_async(queue, ^{
             NSLog(@"Current Thread=%@---->%d-----",[NSThread currentThread],i);
         });
     }
    
     NSLog(@"并行隊(duì)列異步end");
  }

并發(fā) 隊(duì)列同步執(zhí)行結(jié)果：

2016-11-03 17:49:33.850 ThreadDemo[27176:5290096] Current Thread=<NSThread: 0x60000007bf00>{number = 1, name = main}---->0-----
2016-11-03 17:49:33.851 ThreadDemo[27176:5290096] Current Thread=<NSThread: 0x60000007bf00>{number = 1, name = main}---->1-----
2016-11-03 17:49:33.851 ThreadDemo[27176:5290096] Current Thread=<NSThread: 0x60000007bf00>{number = 1, name = main}---->2-----
2016-11-03 17:49:33.851 ThreadDemo[27176:5290096] Current Thread=<NSThread: 0x60000007bf00>{number = 1, name = main}---->3-----
2016-11-03 17:49:33.851 ThreadDemo[27176:5290096] Current Thread=<NSThread: 0x60000007bf00>{number = 1, name = main}---->4-----
2016-11-03 17:49:33.851 ThreadDemo[27176:5290096] Current Thread=<NSThread: 0x60000007bf00>{number = 1, name = main}---->5-----
2016-11-03 17:49:33.851 ThreadDemo[27176:5290096] 并行隊(duì)列同步end

并發(fā)隊(duì)列異步執(zhí)行結(jié)果：

2016-11-03 18:33:32.794 ThreadDemo[27283:5311953] 并行隊(duì)列異步end
2016-11-03 18:33:32.794 ThreadDemo[27283:5312009] Current Thread=<NSThread: 0x60800026b8c0>{number = 3, name = (null)}---->0-----
2016-11-03 18:33:32.794 ThreadDemo[27283:5312006] Current Thread=<NSThread: 0x600000269180>{number = 4, name = (null)}---->1-----
2016-11-03 18:33:32.794 ThreadDemo[27283:5312003] Current Thread=<NSThread: 0x600000268f80>{number = 6, name = (null)}---->3-----
2016-11-03 18:33:32.794 ThreadDemo[27283:5312174] Current Thread=<NSThread: 0x60800026b740>{number = 8, name = (null)}---->5-----
2016-11-03 18:33:32.794 ThreadDemo[27283:5312004] Current Thread=<NSThread: 0x60800026b7c0>{number = 5, name = (null)}---->2-----
2016-11-03 18:33:32.794 ThreadDemo[27283:5312173] Current Thread=<NSThread: 0x60800026b800>{number = 7, name = (null)}---->4-----

小結(jié)：

并發(fā)隊(duì)列同步執(zhí)行和串行隊(duì)列同步執(zhí)行一樣，都不會(huì)開(kāi)辟新線程，block任務(wù)之間是同步執(zhí)行的！
并發(fā)隊(duì)列異步執(zhí)行結(jié)果中看到開(kāi)辟了多個(gè)線程，并且執(zhí)行順序也不是順序執(zhí)行。因?yàn)?strong>異步開(kāi)多線程的代名詞，并發(fā)是開(kāi)多條線程的代名詞
有多個(gè)線程，操作進(jìn)來(lái)之后它會(huì)將這些隊(duì)列安排在可用的處理器上，同時(shí)保證先進(jìn)來(lái)的任務(wù)優(yōu)先處理。
以先進(jìn)先出的方式，并發(fā)調(diào)度隊(duì)列中的任務(wù)執(zhí)行
如果當(dāng)前調(diào)度的任務(wù)是同步執(zhí)行的，會(huì)等待任務(wù)執(zhí)行完成后，再調(diào)度后續(xù)的任務(wù)
如果當(dāng)前調(diào)度的任務(wù)是異步執(zhí)行的，同時(shí)底層線程池有可用的線程資源，會(huì)再新的線程調(diào)度后續(xù)任務(wù)的執(zhí)行

3. 全局隊(duì)列（Global Dispatch Queue）

全局隊(duì)列基本知識(shí)

dispatch_get_global_queue函數(shù)來(lái)獲取
全局隊(duì)列是所有應(yīng)用程序都能夠使用的并發(fā)隊(duì)列（Concurrent Dispatch Queue），沒(méi)必要通過(guò)dispatch_queue_create函數(shù)逐個(gè)生成并發(fā)隊(duì)列，只需要獲取Global Dispatch Queue即可
是系統(tǒng)為了方便程序員開(kāi)發(fā)提供的，其工作表現(xiàn)與并發(fā)隊(duì)列一致、其工作表現(xiàn)與并發(fā)隊(duì)列一致、其工作表現(xiàn)與并發(fā)隊(duì)列一致
Global Dispatch Queue有4個(gè)優(yōu)先級(jí)，分別是高優(yōu)先級(jí)、默認(rèn)優(yōu)先級(jí)、低優(yōu)先級(jí)、后臺(tái)優(yōu)先級(jí)！因?yàn)閄NU內(nèi)核用于Global Dispatch Queue的線程并不能保證實(shí)時(shí)性，因此執(zhí)行優(yōu)先級(jí)知識(shí)大概的判斷和區(qū)分。

  #define DISPATCH_QUEUE_PRIORITY_HIGH 2
  #define DISPATCH_QUEUE_PRIORITY_DEFAULT 0
  #define DISPATCH_QUEUE_PRIORITY_LOW (-2)
  #define DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN

全局隊(duì)列同步和異步執(zhí)行Demo

 #pragma mark -全局隊(duì)列同步和全局隊(duì)列異步(工作表現(xiàn)與并發(fā)隊(duì)列一致)
 //全局隊(duì)列同步
 - (void)globalSyncMethod{
     //獲取全局隊(duì)列
       dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
     //執(zhí)行任務(wù)
     for (int i = 0; i < 10; ++i) {
         dispatch_sync(queue, ^{
             NSLog(@"global_queue_sync%@---->%d----",[NSThread currentThread],i);
         });
     }
     NSLog(@"global_queue_sync_end");
 }

 //全局隊(duì)列異步
 - (void)globalAsyncMethod{
     //獲取全局隊(duì)列
      dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
     //執(zhí)行任務(wù)
     for (int i = 0; i < 10; ++i) {
         dispatch_async(queue, ^{
             NSLog(@"global_queue_async%@---->%d----",[NSThread currentThread],i);
         });
     }
     NSLog(@"global_queue_async_end");
 }

全局隊(duì)列同步執(zhí)行結(jié)果：

2016-11-03 19:06:02.650 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->0----
2016-11-03 19:06:02.651 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->1----
2016-11-03 19:06:02.651 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->2----
2016-11-03 19:06:02.651 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->3----
2016-11-03 19:06:02.651 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->4----
2016-11-03 19:06:02.651 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->5----
2016-11-03 19:06:02.652 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->6----
2016-11-03 19:06:02.652 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->7----
2016-11-03 19:06:02.652 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->8----
2016-11-03 19:06:02.652 ThreadDemo[27347:5324064] global_queue_sync<NSThread: 0x600000072200>{number = 1, name = main}---->9----
2016-11-03 19:06:02.652 ThreadDemo[27347:5324064] global_queue_sync_end

全局隊(duì)列異步執(zhí)行結(jié)果：

2016-11-03 19:12:00.242 ThreadDemo[27430:5333537] global_queue_async_end
2016-11-03 19:12:00.242 ThreadDemo[27430:5334057] global_queue_async<NSThread: 0x608000266100>{number = 5, name = (null)}---->0----
2016-11-03 19:12:00.242 ThreadDemo[27430:5334053] global_queue_async<NSThread: 0x600000263c80>{number = 6, name = (null)}---->1----
2016-11-03 19:12:00.242 ThreadDemo[27430:5334056] global_queue_async<NSThread: 0x600000263cc0>{number = 7, name = (null)}---->2----
2016-11-03 19:12:00.242 ThreadDemo[27430:5334063] global_queue_async<NSThread: 0x608000268500>{number = 8, name = (null)}---->3----
2016-11-03 19:12:00.242 ThreadDemo[27430:5334064] global_queue_async<NSThread: 0x600000263b80>{number = 9, name = (null)}---->4----
2016-11-03 19:12:00.243 ThreadDemo[27430:5334057] global_queue_async<NSThread: 0x608000266100>{number = 5, name = (null)}---->5----
2016-11-03 19:12:00.243 ThreadDemo[27430:5334053] global_queue_async<NSThread: 0x600000263c80>{number = 6, name = (null)}---->7----
2016-11-03 19:12:00.243 ThreadDemo[27430:5334065] global_queue_async<NSThread: 0x600000073600>{number = 10, name = (null)}---->6----
2016-11-03 19:12:00.243 ThreadDemo[27430:5334056] global_queue_async<NSThread: 0x600000263cc0>{number = 7, name = (null)}---->8----
2016-11-03 19:12:00.243 ThreadDemo[27430:5334066] global_queue_async<NSThread: 0x608000267d40>{number = 11, name = (null)}---->9----

4. 主隊(duì)列（Main Dispatch Queue）

主隊(duì)列基本知識(shí)

dispatch_get_main_queue()函數(shù)來(lái)獲取
專(zhuān)門(mén)用來(lái)在主線程上調(diào)度任務(wù)的隊(duì)列
不會(huì)開(kāi)啟線程
以先進(jìn)先出的方式，在主線程空閑時(shí)才會(huì)調(diào)度隊(duì)列中的任務(wù)在主線程執(zhí)行
如果當(dāng)前主線程正在有任務(wù)執(zhí)行，那么無(wú)論主隊(duì)列中當(dāng)前被添加了什么任務(wù)，都不會(huì)被調(diào)度

主隊(duì)列同步和異步執(zhí)行Demo

 #pragma mark -主隊(duì)列同步和主隊(duì)列異步
 //主隊(duì)列同步
 - (void)mainSyncMethod{
     //獲取主隊(duì)列
     dispatch_queue_t queue = dispatch_get_main_queue();
     //執(zhí)行任務(wù)
     for (int i = 0; i < 10; ++i) {
         dispatch_sync(queue, ^{
             NSLog(@"main_queue_sync%@---->%d----",[NSThread currentThread],i);
         });
     }
     NSLog(@"main_queue_sync_end");
 }

 //主隊(duì)列異步
 - (void)mainAsyncMethod{
     //獲取主隊(duì)列
     dispatch_queue_t queue = dispatch_get_main_queue();
     //執(zhí)行任務(wù)
     for (int i = 0; i < 10; ++i) {
         dispatch_async(queue, ^{
            NSLog(@"main_queue_async%@---->%d----",[NSThread currentThread],i);
         });
     }
     NSLog(@"main_queue_async_end");
 }

主隊(duì)列同步執(zhí)行結(jié)果:

主線程和主隊(duì)列相互等待造成死鎖，程序會(huì)直接卡死！
原因：源代碼在Main Dispatch Queue 即主隊(duì)列中執(zhí)行指定的block任務(wù)，并等待其結(jié)束。而其實(shí)在主線程中正在執(zhí)行這些源代碼，所以無(wú)法執(zhí)行追加到Main Dispatch Queue 的block任務(wù)。
下面例子也一樣：
dispatch_queue_t queue = dispatch_get_main_queue();
    dispatch_async(queue, ^{
        dispatch_sync(queue, ^{
            NSLog(@"main_queue_sync%@----",[NSThread currentThread]);
        });
    });

主隊(duì)列異步執(zhí)行結(jié)果:

2016-11-03 19:45:38.154 ThreadDemo[27501:5349956] main_queue_async_end
2016-11-03 19:45:38.155 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->0----
2016-11-03 19:45:38.155 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->1----
2016-11-03 19:45:38.155 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->2----
2016-11-03 19:45:38.155 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->3----
2016-11-03 19:45:38.155 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->4----
2016-11-03 19:45:38.156 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->5----
2016-11-03 19:45:38.156 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->6----
2016-11-03 19:45:38.156 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->7----
2016-11-03 19:45:38.156 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->8----
2016-11-03 19:45:38.156 ThreadDemo[27501:5349956] main_queue_async<NSThread: 0x608000070880>{number = 1, name = main}---->9----

5.延遲執(zhí)行（dispatch_after）

在GCD中我們使用dispatch_after()函數(shù)來(lái)延遲執(zhí)行隊(duì)列中的任務(wù), dispatch_after()是異步執(zhí)行隊(duì)列中的任務(wù)的，也就是說(shuō)使用dispatch_after()來(lái)執(zhí)行隊(duì)列中的任務(wù)不會(huì)阻塞當(dāng)前任務(wù)。等到延遲時(shí)間到了以后就會(huì)開(kāi)辟一個(gè)新的線程然后執(zhí)行隊(duì)列中的任務(wù)。
dispatch_after(dispatch_time_t when,dispatch_queue_t queue,dispatch_block_t block);來(lái)創(chuàng)建
經(jīng)過(guò)我的猜測(cè)測(cè)試（看不到源碼），你們也思考一下這個(gè)延遲函數(shù)的實(shí)現(xiàn)過(guò)程。底層實(shí)現(xiàn)應(yīng)該是dispath_async函數(shù)追加block到Main Dispatch Queue等相應(yīng)隊(duì)列，偽代碼如下(以Main Dispatch Queue為例):

 dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
 delay(2){
 dispatch_async(queue, ^{
     dispatch_async(dispatch_get_main_queue(), ^{
     add task to mainQueue
    });
  });
 }

延遲執(zhí)行（dispatch_after）Demo

#pragma mark -  延遲執(zhí)行
 - (void)GCDAfterRunMethod{
     // 循環(huán)5次
     for (int i =0; i < 10000; i++ ) {
         NSLog(@"let's go %d",i);
         // 設(shè)置2秒后執(zhí)行block
         dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC));
         dispatch_after(time, dispatch_get_main_queue(), ^{
             NSLog(@"This is my %d number!%@",i,[NSThread currentThread]);
         });
     }
       //等價(jià)于下面實(shí)現(xiàn)
 //    dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
 //    for (int i = 0; i < 10000; i++) {  //        NSLog(@"let's go %d",i);
 //        dispatch_async(queue, ^{
 //           dispatch_async(dispatch_get_main_queue(), ^{  //                NSLog(@"This is my %d number!%@",i,[NSThread currentThread]);
 //           });
 //        });
 //    }
 }

注意：

dispatch_after(dispatch_time_t when,dispatch_queue_t queue,dispatch_block_t block);
第二個(gè)參數(shù)：指定要追加處理的Dispatch Queue
第三個(gè)參數(shù)：指定要執(zhí)行處理的Block
第一個(gè)參數(shù)：指定時(shí)間用的dispatch_time_t類(lèi)型的值，該值是使用dispatch_time函數(shù)或者dispatch_walltime函數(shù)生成
1.dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC));
里面有兩個(gè)參數(shù)：

  參數(shù)1獲取指定的時(shí)間開(kāi)始，通常為DISPATCH_TIME_NOW，定義：
  #define DISPATCH_TIME_NOW (0ull)//現(xiàn)在
  #define DISPATCH_TIME_FOREVER (~0ull)//永遠(yuǎn)，意味著你用它你的任務(wù)永遠(yuǎn)不會(huì)執(zhí)行了！

參數(shù)2是從參數(shù)1延遲指定的時(shí)間來(lái)執(zhí)行任務(wù)，也是一個(gè)時(shí)間
#define NSEC_PER_SEC 1000000000ull
#define NSEC_PER_MSEC 1000000ull
#define USEC_PER_SEC 1000000ull
#define NSEC_PER_USEC 1000ull
ull是C語(yǔ)言的數(shù)值字面量，是顯示表明類(lèi)型是使用的字符串（表示“unsigned long long”）
NSEC_PER_SEC 是秒的單位數(shù)量級(jí) 等價(jià)于1億納秒
上面代碼中提到的(int64_t)(2 * NSEC_PER_SEC))就是2秒了

2.使用dispatch_walltime函數(shù)生成dispatch_time_t,用于計(jì)算絕對(duì)時(shí)間，例如在dispatch_after函數(shù)中指定2016年11月3日21時(shí)45分1秒這一絕對(duì)時(shí)間，可粗略的鬧鐘功能來(lái)使用。也就是到指定時(shí)間就會(huì)執(zhí)行任務(wù)

//由NSDate類(lèi)對(duì)象獲取dispatch_time_t傳遞給dispatch_after函數(shù)使用
dispatch_time_t  getDispatchTimeByDate(NSDate *date){
NSTimeInterval interval;
double second,subSecond;
struct timespec time;
dispatch_time_t milestone;
   
interval = [date timeIntervalSince1970];
   
subSecond = modf(interval, &second);
time.tv_sec = second;
time.tv_nsec = subSecond * NSEC_PER_SEC;

milestone = dispatch_walltime(&time, 0);
return milestone;
}

6. 更改優(yōu)先級(jí)（dispatch_set_target_queue）

dispatch_queue_create函數(shù)生成的Dispatch Queue不管是 串行隊(duì)列（Serial Dispatch Queue） 還是 并發(fā)隊(duì)列（Concurrent Dispatch Queue） ，都是用與 默認(rèn)優(yōu)先級(jí)全局隊(duì)列（Global Dispatch Queue）相同執(zhí)行優(yōu)先級(jí)的線程。而全局隊(duì)列工作方式與并發(fā)隊(duì)列工作方式完全一致！
dispatch_set_target_queue(dispatch_object_t object,dispatch_queue_t _Nullable queue);函數(shù)來(lái)更改隊(duì)列優(yōu)先級(jí)
- 第一個(gè)參數(shù)：指定要變更優(yōu)先級(jí)的隊(duì)列（要更改隊(duì)列）
- 第二個(gè)參數(shù)：指定第一個(gè)參數(shù)（隊(duì)列）要和我們預(yù)期隊(duì)列執(zhí)行相同優(yōu)先級(jí)的隊(duì)列（目標(biāo)隊(duì)列）

1. dispatch_set_target_queue第一個(gè)Demo：

 - (void)GCDSetTargetQueueMethod{
    
     dispatch_queue_t targetQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    
     dispatch_queue_t queue1 = dispatch_queue_create("queue1", DISPATCH_QUEUE_SERIAL);
     dispatch_queue_t queue2 = dispatch_queue_create("queue2", DISPATCH_QUEUE_CONCURRENT);
      //更改優(yōu)先級(jí)
     dispatch_set_target_queue(queue1, targetQueue);
    
     for (int i = 0; i < 6; i++) {
         dispatch_async(queue1, ^{
         NSLog(@"queue1-currentThread = %@-->%d",[NSThread currentThread],i);
         });
     }
     for (int i = 0; i < 6; i++) {
          dispatch_async(queue2, ^{
          NSLog(@"queue2-----currentThread = %@----->%d",[NSThread currentThread],i);
          });
     }
 }

打印結(jié)果：

2016-11-04 11:14:57.034 ThreadDemo[28477:5530919] queue2-----currentThread = <NSThread: 0x60000007e0c0>{number = 11, name = (null)}----->1
2016-11-04 11:14:57.034 ThreadDemo[28477:5530920] queue2-----currentThread = <NSThread: 0x60000007a140>{number = 12, name = (null)}----->2
2016-11-04 11:14:57.034 ThreadDemo[28477:5530918] queue2-----currentThread = <NSThread: 0x60000007a100>{number = 10, name = (null)}----->0
2016-11-04 11:14:57.034 ThreadDemo[28477:5530919] queue2-----currentThread = <NSThread: 0x60000007e0c0>{number = 11, name = (null)}----->3
2016-11-04 11:14:57.034 ThreadDemo[28477:5530912] queue1-currentThread = <NSThread: 0x60000007ec00>{number = 9, name = (null)}-->0
2016-11-04 11:14:57.034 ThreadDemo[28477:5530920] queue2-----currentThread = <NSThread: 0x60000007a140>{number = 12, name = (null)}----->5
2016-11-04 11:14:57.034 ThreadDemo[28477:5530921] queue2-----currentThread = <NSThread: 0x60000007c340>{number = 13, name = (null)}----->4
2016-11-04 11:14:57.035 ThreadDemo[28477:5530912] queue1-currentThread = <NSThread: 0x60000007ec00>{number = 9, name = (null)}-->1
2016-11-04 11:14:57.036 ThreadDemo[28477:5530912] queue1-currentThread = <NSThread: 0x60000007ec00>{number = 9, name = (null)}-->2
2016-11-04 11:14:57.036 ThreadDemo[28477:5530912] queue1-currentThread = <NSThread: 0x60000007ec00>{number = 9, name = (null)}-->3
2016-11-04 11:14:57.036 ThreadDemo[28477:5530912] queue1-currentThread = <NSThread: 0x60000007ec00>{number = 9, name = (null)}-->4
2016-11-04 11:14:57.037 ThreadDemo[28477:5530912] queue1-currentThread = <NSThread: 0x60000007ec00>{number = 9, name = (null)}-->5

打印解釋?zhuān)?/strong>

queue1和queue2設(shè)置的目標(biāo)隊(duì)列是全局隊(duì)列（并發(fā)）,也就是允許要更改的隊(duì)列可以開(kāi)辟多條線程（可以超過(guò)一條）

代碼中queue1變更和低優(yōu)先級(jí)的全局隊(duì)列一樣優(yōu)先級(jí)的串行隊(duì)列，queue2是dispatch_queue_create創(chuàng)建和默認(rèn)優(yōu)先級(jí)的全局隊(duì)列一樣優(yōu)先級(jí)的并發(fā)隊(duì)列。

queue1和queue2都是異步執(zhí)行，都會(huì)開(kāi)辟線程，queue2是并發(fā)隊(duì)列所以打印中看到有多個(gè)線程，并且任務(wù)之間也不是順序執(zhí)行。queue1是串行隊(duì)列，所以只會(huì)開(kāi)辟一個(gè)線程，任務(wù)會(huì)順序執(zhí)行。

queue2隊(duì)列的優(yōu)先級(jí)比queue1隊(duì)列的優(yōu)先級(jí)要高，從打印中可以看到queue2的確比queue1任務(wù)要普遍先執(zhí)行。為什么要說(shuō)普遍呢，因?yàn)閺慕Y(jié)果上看到queue1有一個(gè)任務(wù)是在queue2里任務(wù)沒(méi)有執(zhí)行完畢也執(zhí)行了，為什么會(huì)出現(xiàn)這個(gè)問(wèn)題呢？？

在上面全局隊(duì)列中說(shuō)到了優(yōu)先級(jí)的事情因?yàn)閄NU內(nèi)核用于Global Dispatch Queue的線程并不能保證實(shí)時(shí)性，因此執(zhí)行優(yōu)先級(jí)知識(shí)大概的判斷和區(qū)分，所有我們不能完全依賴這個(gè)優(yōu)先級(jí)來(lái)做隊(duì)列的順序事情，否則會(huì)出現(xiàn)問(wèn)題！切記

2. dispatch_set_target_queue第一個(gè)Demo：

dispatch_set_target_queue除了能用來(lái)設(shè)置隊(duì)列的優(yōu)先級(jí)之外，還能夠創(chuàng)建隊(duì)列的執(zhí)行層次
，當(dāng)我們想讓不同隊(duì)列中的任務(wù)同步的執(zhí)行時(shí)，我們可以創(chuàng)建一個(gè)串行隊(duì)列，然后將這些隊(duì)列的target指向新創(chuàng)建的隊(duì)列即可，比如：

隊(duì)列的執(zhí)行層次.png

- (void)GCDSetTargetQueueMethod{ dispatch_queue_t targetQueue = dispatch_queue_create("targetQueue", DISPATCH_QUEUE_SERIAL);//目標(biāo)隊(duì)列 dispatch_queue_t queue1 = dispatch_queue_create("queue1", DISPATCH_QUEUE_SERIAL);//串行隊(duì)列 dispatch_queue_t queue2 = dispatch_queue_create("queue2", DISPATCH_QUEUE_CONCURRENT);//并發(fā)隊(duì)列 //設(shè)置參考 dispatch_set_target_queue(queue1, targetQueue); dispatch_set_target_queue(queue2, targetQueue); for (int i = 0; i < 6; i++) { dispatch_async(queue1, ^{ NSLog(@"queue1-currentThread = %@-->%d",[NSThread currentThread],i); }); } for (int i = 0; i < 6; i++) { dispatch_async(queue2, ^{ NSLog(@"queue2-currentThread = %@-->%d",[NSThread currentThread],i); }); } }

打印結(jié)果：

2016-11-04 11:34:34.722 ThreadDemo[28551:5540844] queue1-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->0 2016-11-04 11:34:34.723 ThreadDemo[28551:5540844] queue1-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->1 2016-11-04 11:34:34.723 ThreadDemo[28551:5540844] queue1-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->2 2016-11-04 11:34:34.723 ThreadDemo[28551:5540844] queue1-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->3 2016-11-04 11:34:34.723 ThreadDemo[28551:5540844] queue1-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->4 2016-11-04 11:34:34.723 ThreadDemo[28551:5540844] queue1-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->5 2016-11-04 11:34:34.724 ThreadDemo[28551:5540844] queue2-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->0 2016-11-04 11:34:34.724 ThreadDemo[28551:5540844] queue2-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->1 2016-11-04 11:34:34.724 ThreadDemo[28551:5540844] queue2-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->2 2016-11-04 11:34:34.724 ThreadDemo[28551:5540844] queue2-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->3 2016-11-04 11:34:34.724 ThreadDemo[28551:5540844] queue2-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->4 2016-11-04 11:34:34.725 ThreadDemo[28551:5540844] queue2-currentThread = <NSThread: 0x60000007b640>{number = 3, name = (null)}-->5

打印解釋?zhuān)?/strong>

queue1和queue2設(shè)置的目標(biāo)隊(duì)列是串行隊(duì)列,也就是允許要更改的隊(duì)列可以開(kāi)辟一條線程。

queue1和queue2優(yōu)先級(jí)一樣，他兩執(zhí)行順序相當(dāng)于一個(gè)串行隊(duì)列異步執(zhí)行！

queue1和queue2隊(duì)列以targetQueue隊(duì)列為參照對(duì)象，那么queue1和queue2中的任務(wù)將按照targetQueue的隊(duì)列處理。

適用場(chǎng)景：一般都是把一個(gè)任務(wù)放到一個(gè)串行的queue中，如果這個(gè)任務(wù)被拆分了，被放置到多個(gè)串行的queue中，但實(shí)際還是需要這個(gè)任務(wù)同步執(zhí)行，那么就會(huì)有問(wèn)題，因?yàn)槎鄠€(gè)串行queue之間是并行的。這時(shí)候dispatch_set_target_queue將起到作用。

** dispatch_set_target_queue小結(jié)：**

dispatch_set_target_queue可以更改Dispatch Queue優(yōu)先級(jí)。

dispatch_set_target_queue可以更改隊(duì)列的執(zhí)行層次，隊(duì)列里的任務(wù)將會(huì)按照目標(biāo)隊(duì)列（target Queue）的隊(duì)列來(lái)處理

7. 任務(wù)組Dispatch Group

GCD的任務(wù)組在開(kāi)發(fā)中是經(jīng)常被使用到，當(dāng)你一組任務(wù)結(jié)束后再執(zhí)行一些操作時(shí)，使用任務(wù)組在合適不過(guò)了。dispatch_group的職責(zé)就是當(dāng)隊(duì)列中的所有任務(wù)都執(zhí)行完畢后在去做一些操作，也就是說(shuō)在任務(wù)組中執(zhí)行的隊(duì)列，當(dāng)隊(duì)列中的所有任務(wù)都執(zhí)行完畢后就會(huì)發(fā)出一個(gè)通知來(lái)告訴用戶任務(wù)組中所執(zhí)行的隊(duì)列中的任務(wù)執(zhí)行完畢了。關(guān)于將隊(duì)列放到任務(wù)組中執(zhí)行有兩種方式，一種是使用dispatch_group_async()函數(shù)，將隊(duì)列與任務(wù)組進(jìn)行關(guān)聯(lián)并自動(dòng)執(zhí)行隊(duì)列中的任務(wù)。另一種方式是手動(dòng)的將隊(duì)列與組進(jìn)行關(guān)聯(lián)然后使用異步將隊(duì)列進(jìn)行執(zhí)行，也就是dispatch_group_enter()與dispatch_group_leave()方法的使用。下方就給出詳細(xì)的介紹。

1.隊(duì)列與組自動(dòng)關(guān)聯(lián)并執(zhí)行

首先我們來(lái)介紹dispatch_group_async()函數(shù)的使用方式，該函數(shù)會(huì)將隊(duì)列與相應(yīng)的任務(wù)組進(jìn)行關(guān)聯(lián)，并且自動(dòng)執(zhí)行。當(dāng)與任務(wù)組關(guān)聯(lián)的隊(duì)列中的任務(wù)都執(zhí)行完畢后，會(huì)通過(guò)dispatch_group_notify()函數(shù)發(fā)出通知告訴用戶任務(wù)組中的所有任務(wù)都執(zhí)行完畢了。使用通知的方式是不會(huì)阻塞當(dāng)前線程的，如果你使用dispatch_group_wait()函數(shù)，那么就會(huì)阻塞當(dāng)前線程，直到任務(wù)組中的所有任務(wù)都執(zhí)行完畢。

//自動(dòng)執(zhí)行任務(wù)組 - (void)GCDAutoDispatchGroupMethod{ dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_group_t group = dispatch_group_create(); for (int i = 0; i < 6; i++) { dispatch_group_async(group, queue, ^{ NSLog(@"current Thread = %@----->%d",[NSThread currentThread],i); }); } dispatch_group_notify(group, dispatch_get_main_queue(), ^{ NSLog(@"current Thread = %@----->這是最后執(zhí)行",[NSThread currentThread]); }); }

打印結(jié)果：

2016-11-04 15:11:34.010 ThreadDemo[29225:5641180] current Thread = <NSThread: 0x600000468200>{number = 5, name = (null)}----->2 2016-11-04 15:11:34.010 ThreadDemo[29225:5641193] current Thread = <NSThread: 0x60800007f900>{number = 3, name = (null)}----->0 2016-11-04 15:11:34.011 ThreadDemo[29225:5641438] current Thread = <NSThread: 0x608000467980>{number = 8, name = (null)}----->5 2016-11-04 15:11:34.010 ThreadDemo[29225:5641178] current Thread = <NSThread: 0x608000467640>{number = 6, name = (null)}----->3 2016-11-04 15:11:34.010 ThreadDemo[29225:5641177] current Thread = <NSThread: 0x6080004675c0>{number = 4, name = (null)}----->1 2016-11-04 15:11:34.010 ThreadDemo[29225:5641437] current Thread = <NSThread: 0x600000467f40>{number = 7, name = (null)}----->4 2016-11-04 15:11:34.011 ThreadDemo[29225:5641137] current Thread = <NSThread: 0x60800007e200>{number = 1, name = main}----->這是最后執(zhí)行

上面的函數(shù)就是使用dispatch_group_async()函數(shù)將隊(duì)列與任務(wù)組進(jìn)行關(guān)聯(lián)并執(zhí)行。首先我們創(chuàng)建了一個(gè)全局隊(duì)列(并發(fā))，然后又創(chuàng)建了一個(gè)類(lèi)型為dispatch_group_t的任務(wù)組group。使用dispatch_group_async()函數(shù)將兩者進(jìn)行關(guān)聯(lián)并執(zhí)行。使用dispatch_group_notify()函數(shù)進(jìn)行監(jiān)聽(tīng)group中隊(duì)列的執(zhí)行結(jié)果，如果執(zhí)行完畢后，我們就在主線程中對(duì)結(jié)果進(jìn)行處理。

dispatch_group_notify()函數(shù)有兩個(gè)參數(shù)一個(gè)是發(fā)送通知的group，另一個(gè)是處理返回結(jié)果的隊(duì)列。

dispatch_group_async(dispatch_group_t group,dispatch_queue_t queue,dispatch_block_t block);函數(shù)與dispatch_async函數(shù)相同，都追加block到指定的Dispatch Queue中，與dispatch_async不同的是指定生成的Dispatch Group為第一個(gè)參數(shù)。指定的block屬于指定的Dispatch Group，不是Dispatch Queue，切記！

無(wú)論向什么樣的Dispatch Queue中追加處理，使用Dispatch Group都可監(jiān)視這些處理執(zhí)行的結(jié)束。一旦檢測(cè)到所有處理執(zhí)行結(jié)束，就可將結(jié)束的處理追加到Dispatch Queue中。這就是使用Dispatch Group的原因。

在追加到Dispatch Group中的處理全部執(zhí)行結(jié)束時(shí)，代碼中使用的dispatch_group_notify函數(shù)會(huì)將執(zhí)行的Block追加到Dispatch Queue中，將第一個(gè)參數(shù)指定為要監(jiān)視的Dispatch Group。在追加到該Dispatch Group的全部處理執(zhí)行結(jié)束時(shí)，將第三個(gè)參數(shù)的Block追加到第二個(gè)參數(shù)的Dispatch Queue中。在dispatch_group_notify函數(shù)中不管指定什么樣的Dispatch Queue，屬于Dispatch Group的全部處理在追加指定的Block時(shí)都已執(zhí)行結(jié)束。

另外，在Dispatch Group中也可以使用dispatch_group_wait函數(shù)僅等待全部處理執(zhí)行結(jié)束。

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_group_t group = dispatch_group_create(); dispatch_group_async(group, queue, ^{NSLog(@"blk0");}); dispatch_group_async(group, queue, ^{NSLog(@"blk1");}); dispatch_group_async(group, queue, ^{NSLog(@"blk2");}); dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

dispatch_group_wait函數(shù)的第二個(gè)參數(shù)指定為等待時(shí)間(超時(shí))。它屬于dispatch_time_t類(lèi)型的值。代碼中使用的DISPATCH_TIME_FOREVER，意味著永久等待。只要屬于Dispatch Group的操作尚未執(zhí)行結(jié)束，就會(huì)一直等待，中途不能取消。

指定等待時(shí)間為1微秒時(shí)，應(yīng)做如下處理：

//等待group處理結(jié)束 // dispatch_group_wait(group, DISPATCH_TIME_FOREVER); dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1*USEC_PER_SEC));//1微秒 long result = dispatch_group_wait(group, time); if (result == 0) { //屬于Dispatch Group 的block任務(wù)全部處理結(jié)束 NSLog(@"Dispatch Group全部處理完畢"); }else{ //屬于Dispatch Group 的block任務(wù)還在處理中 NSLog(@"Dispatch Group正在處理"); }

如果dispatch_group_wait函數(shù)的返回值不為0，就意味著雖然經(jīng)過(guò)了指定的時(shí)間，但屬于Dispatch Group的某一個(gè)處理還在執(zhí)行中。如果返回值為0，那么全部處理執(zhí)行結(jié)束。當(dāng)?shù)却龝r(shí)間為DISPATCH_TIME_FOREVER，由dispatch_group_wait函數(shù)返回時(shí)，屬于Dispatch Group的處理必定全部執(zhí)行結(jié)束，因此返回值恒為0。

這里的“等待”是什么意思？這意味著一旦調(diào)用dispatch_group_wait函數(shù)，該函數(shù)就處于調(diào)用的狀態(tài)而不返回。即執(zhí)行dispatch_group_wait函數(shù)的現(xiàn)在的線程（當(dāng)前線程）停止。在經(jīng)過(guò)dispatch_group_wait函數(shù)中指定的時(shí)間或?qū)儆谥付―ispatch Group的處理全部執(zhí)行結(jié)束之前，執(zhí)行該函數(shù)的線程停止，屬于阻塞狀態(tài)。

指定DISPACH_TIME_NOW，則不用任何等待即可判定屬于Dispatch Group的處理是否執(zhí)行結(jié)束。

long result = diaptach_group_wait(group, DISPACH_TIME_NOW);

在主線程的RunLoop的每次循環(huán)中，可檢查執(zhí)行是否結(jié)束，從而不消耗多余的等待時(shí)間，雖然這樣有額可以，但一般在這種情況下，還是推薦用dispatch_group_notify函數(shù)追加結(jié)束處理到Main Dispatch Queue中。這是因?yàn)?code>dispatch_group_notify函數(shù)可以簡(jiǎn)化代碼。并且如果你用了diaptach_group_wait等待時(shí)間過(guò)長(zhǎng)，中間不能取消隊(duì)列任務(wù)這就很坑了！

2. 隊(duì)列與組手動(dòng)關(guān)聯(lián)并執(zhí)行

接下來(lái)我們將手動(dòng)的管理任務(wù)組與隊(duì)列中的關(guān)系，也就是不使用dispatch_group_async()函數(shù)。我們使用dispatch_group_enter()與dispatch_group_leave()函數(shù)將隊(duì)列中的每次任務(wù)加入到到任務(wù)組中。首先我們使用dispatch_group_enter()函數(shù)進(jìn)入到任務(wù)組中，然后異步執(zhí)行隊(duì)列中的任務(wù)，最后使用dispatch_group_leave()函數(shù)離開(kāi)任務(wù)組即可。下面的函數(shù)中我們使用了dispatch_group_wait()函數(shù)，該函數(shù)的職責(zé)就是阻塞當(dāng)前線程，來(lái)等待任務(wù)組中的任務(wù)執(zhí)行完畢。該函數(shù)的第一個(gè)參數(shù)是所要等待的group，第二個(gè)參數(shù)是等待超時(shí)時(shí)間，此處我們?cè)O(shè)置的是DISPATCH_TIME_FOREVER，就說(shuō)明等待任務(wù)組的執(zhí)行永不超時(shí)，直到任務(wù)組中所有任務(wù)執(zhí)行完畢。

//手動(dòng)執(zhí)行任務(wù)組 - (void)GCDManualDispatchGroupMethod{ dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_group_t group = dispatch_group_create(); for (int i = 0; i < 6; i++) { dispatch_group_enter(group);//進(jìn)入隊(duì)列組 dispatch_async(queue, ^{ NSLog(@"current Thread = %@----->%d",[NSThread currentThread],i); dispatch_group_leave(group);//離開(kāi)隊(duì)列組 }); } long result = dispatch_group_wait(group, DISPATCH_TIME_FOREVER);//阻塞當(dāng)前線程，直到所有任務(wù)執(zhí)行完畢才會(huì)繼續(xù)往下執(zhí)行 if (result == 0) { //屬于Dispatch Group 的block任務(wù)全部處理結(jié)束 NSLog(@"Dispatch Group全部處理完畢"); }else{ //屬于Dispatch Group 的block任務(wù)還在處理中 NSLog(@"Dispatch Group正在處理"); } dispatch_group_notify(group, dispatch_get_main_queue(), ^{ NSLog(@"current Thread = %@----->這是最后執(zhí)行",[NSThread currentThread]); }); }

打印結(jié)果：dispatch_group_wait()函數(shù)下方的print()函數(shù)在所有任務(wù)執(zhí)行完畢之前是不會(huì)被調(diào)用的，因?yàn)?code>dispatch_group_wait()會(huì)將當(dāng)前線程進(jìn)行阻塞。當(dāng)然雖然是手動(dòng)的將隊(duì)列與任務(wù)組進(jìn)行關(guān)聯(lián)的，感覺(jué)display_group_notify()函數(shù)還是好用的。

2016-11-04 16:19:13.802 ThreadDemo[29402:5678525] current Thread = <NSThread: 0x600000271780>{number = 5, name = (null)}----->2 2016-11-04 16:19:13.802 ThreadDemo[29402:5678527] current Thread = <NSThread: 0x608000077300>{number = 3, name = (null)}----->0 2016-11-04 16:19:13.802 ThreadDemo[29402:5678524] current Thread = <NSThread: 0x60800007d840>{number = 4, name = (null)}----->1 2016-11-04 16:19:13.803 ThreadDemo[29402:5678760] current Thread = <NSThread: 0x600000271cc0>{number = 8, name = (null)}----->5 2016-11-04 16:19:13.802 ThreadDemo[29402:5678545] current Thread = <NSThread: 0x60800007d000>{number = 6, name = (null)}----->3 2016-11-04 16:19:13.802 ThreadDemo[29402:5678544] current Thread = <NSThread: 0x60800007d040>{number = 7, name = (null)}----->4 2016-11-04 16:19:13.803 ThreadDemo[29402:5678489] Dispatch Group全部處理完畢 2016-11-04 16:19:13.804 ThreadDemo[29402:5678489] current Thread = <NSThread: 0x600000069dc0>{number = 1, name = main}----->這是最后執(zhí)行

8. 柵欄任務(wù)Dispatch_barrier_async

barrier顧名思義柵欄、障礙物的意思！
在訪問(wèn)數(shù)據(jù)庫(kù)或文件時(shí)，使用Serial Dispatch Queue可避免數(shù)據(jù)競(jìng)爭(zhēng)的問(wèn)題。

寫(xiě)入處理確實(shí)不可與其他的寫(xiě)入處理以及包含讀取處理的其他某些處理并行執(zhí)行。但是如果讀取處理只是與讀取處理并行執(zhí)行，那么多個(gè)并行執(zhí)行就不會(huì)發(fā)生問(wèn)題。

也就是說(shuō)，為了高效率地進(jìn)行訪問(wèn)，讀取處理追加到Concurrent Dispatch Queue中，寫(xiě)入出路在任何一個(gè)讀取處理沒(méi)有執(zhí)行的狀態(tài)下，追加到Serial Dispatch Queue中即可(在寫(xiě)入處理結(jié)束之前，讀取處理不可執(zhí)行)。

雖然利用Dispatch Group和dispatch_set_target_queue函數(shù)也可實(shí)現(xiàn)，但代碼會(huì)很復(fù)雜。有興趣的可以自己嘗試寫(xiě)寫(xiě)！
CD為我們提供了更為聰明的解決辦法——dispatch_barrier_async函數(shù)。該函數(shù)同dispatch_queue_create函數(shù)生成的Concurrent Dispatch Queue一起使用。

#pragma mark - Dispatch_barrier_async - (void)GCDBarrierAsyncMethod{ dispatch_queue_t concurrentQueue = dispatch_queue_create("concurrentQueue", DISPATCH_QUEUE_CONCURRENT); void(^blk1_reading)(void) = ^{ NSLog(@"blk1---reading"); }; void(^blk2_reading)(void) = ^{ NSLog(@"blk2---reading"); }; void(^blk3_reading)(void) = ^{ NSLog(@"blk3---reading"); }; void(^blk4_reading)(void) = ^{ NSLog(@"blk4---reading"); }; void(^blk_writing)(void) = ^{ NSLog(@"blk---writing"); }; dispatch_async(concurrentQueue, blk1_reading); dispatch_async(concurrentQueue, blk2_reading); //添加追加操作,,會(huì)等待b1和b2全部執(zhí)行結(jié)束，執(zhí)行完成追加操作b，才會(huì)繼續(xù)并發(fā)執(zhí)行下面操作 dispatch_barrier_async(concurrentQueue, blk_writing); dispatch_async(concurrentQueue, blk3_reading); dispatch_async(concurrentQueue, blk4_reading); }

打印結(jié)果：

2016-11-04 17:02:13.202 ThreadDemo[29492:5700974] blk2---reading 2016-11-04 17:02:13.202 ThreadDemo[29492:5700972] blk1---reading 2016-11-04 17:02:13.203 ThreadDemo[29492:5700972] blk---writing 2016-11-04 17:02:13.203 ThreadDemo[29492:5700972] blk3---reading 2016-11-04 17:02:13.203 ThreadDemo[29492:5700974] blk4---reading

使用Concurrent Dispatch Queue和dispatch_barrier_async函數(shù)可實(shí)現(xiàn)高效率的數(shù)據(jù)庫(kù)訪問(wèn)和文件訪問(wèn),dispatch_barrier_async函數(shù)還是比較好理解的。

9. 循環(huán)執(zhí)行dispatch_apply

dispatch_apply()函數(shù)是用來(lái)循環(huán)來(lái)執(zhí)行隊(duì)列中的任務(wù)的，使用方式為：dispatch_apply(循環(huán)次數(shù), 任務(wù)所在的隊(duì)列) { 要循環(huán)執(zhí)行的任務(wù) }。使用該函數(shù)循環(huán)執(zhí)行并行隊(duì)列中的任務(wù)時(shí)，會(huì)開(kāi)辟新的線程，不過(guò)有可能會(huì)在當(dāng)前線程中執(zhí)行一些任務(wù)。而使用dispatch_apply()執(zhí)行串行隊(duì)列中的任務(wù)時(shí)，會(huì)在當(dāng)前線程中執(zhí)行。無(wú)論是使用并行隊(duì)列還是串行隊(duì)列，dispatch_apply()都會(huì)阻塞當(dāng)前執(zhí)行函數(shù)線程。

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_apply(10, queue, ^(size_t index){ NSLog(@"%zu", index); }); NSLog(@"done");

2016-11-04 17:24:25.819 ThreadDemo[29598:5715955] 2 2016-11-04 17:24:25.819 ThreadDemo[29598:5715904] 0 2016-11-04 17:24:25.819 ThreadDemo[29598:5716588] 1 2016-11-04 17:24:25.819 ThreadDemo[29598:5716640] 3 2016-11-04 17:24:25.820 ThreadDemo[29598:5715955] 4 2016-11-04 17:24:25.820 ThreadDemo[29598:5715904] 5 2016-11-04 17:24:25.820 ThreadDemo[29598:5716588] 6 2016-11-04 17:24:25.820 ThreadDemo[29598:5716640] 7 2016-11-04 17:24:25.820 ThreadDemo[29598:5715904] 9 2016-11-04 17:24:25.820 ThreadDemo[29598:5715955] 8 2016-11-04 17:24:25.820 ThreadDemo[29598:5715904] done

輸出結(jié)果中最后的done必定在最后的位置上，這是因?yàn)閐ispatch_apply函數(shù)會(huì)等待全部處理執(zhí)行結(jié)束，也就是阻塞當(dāng)前執(zhí)行函數(shù)線程。

另外，由于dispatch_apply函數(shù)也與dispatch_sync函數(shù)相同，會(huì)等待處理執(zhí)行結(jié)束(阻塞)，因此推薦在dispatch_async函數(shù)中非同步地執(zhí)行dispatch_apply函數(shù)。

#pragma mark - Dispatch_apply - (void)GCDDispatchApplyMethod{ dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); void(^blk1_reading)(void) = ^{ NSLog(@"blk1---reading"); }; void(^blk2_reading)(void) = ^{ NSLog(@"blk2---reading"); }; void(^blk3_reading)(void) = ^{ NSLog(@"blk3---reading"); }; void(^blk_writing)(void) = ^{ NSLog(@"blk---writing"); }; NSMutableArray *array = [NSMutableArray new]; [array addObject:blk1_reading]; [array addObject:blk2_reading]; [array addObject:blk3_reading]; [array addObject:blk_writing]; dispatch_async(queue, ^{ dispatch_apply(array.count, queue, ^(size_t index) { void (^blk)(void) = [array objectAtIndex:index]; blk(); NSLog(@"%zu====%@",index,[array objectAtIndex:index]); }); NSLog(@"全部執(zhí)行結(jié)束"); dispatch_async(dispatch_get_main_queue(), ^{ //在main Dispatch queue中執(zhí)行處理，更新用戶界面等待 NSLog(@"done"); }); }); }

打印結(jié)果：

2016-11-04 17:36:02.258 ThreadDemo[29635:5723967] blk3---reading 2016-11-04 17:36:02.258 ThreadDemo[29635:5723940] blk2---reading 2016-11-04 17:36:02.258 ThreadDemo[29635:5723929] blk1---reading 2016-11-04 17:36:02.258 ThreadDemo[29635:5723968] blk---writing 2016-11-04 17:36:02.259 ThreadDemo[29635:5723940] 1====<__NSGlobalBlock__: 0x102895af0> 2016-11-04 17:36:02.259 ThreadDemo[29635:5723967] 2====<__NSGlobalBlock__: 0x102895b30> 2016-11-04 17:36:02.259 ThreadDemo[29635:5723929] 0====<__NSGlobalBlock__: 0x102895ab0> 2016-11-04 17:36:02.259 ThreadDemo[29635:5723968] 3====<__NSGlobalBlock__: 0x102895b70> 2016-11-04 17:36:02.259 ThreadDemo[29635:5723940] 全部執(zhí)行結(jié)束 2016-11-04 17:36:02.259 ThreadDemo[29635:5723499] done

10. 隊(duì)列的掛起和喚醒

隊(duì)列的掛起與喚醒相對(duì)較為簡(jiǎn)單，如果你想對(duì)一個(gè)隊(duì)列中的任務(wù)的執(zhí)行進(jìn)行掛起，那么你就使用dispatch_suspend()函數(shù)即可。如果你要喚醒某個(gè)掛起的隊(duì)列，那么你就可以使用dispatch_resum()函數(shù)。這兩個(gè)函數(shù)所需的參數(shù)都是你要掛起或者喚醒的隊(duì)列，鑒于知識(shí)點(diǎn)的簡(jiǎn)單性就不做過(guò)多的贅述了。

#pragma mark -Dispatch_suspend/Dispatch_resume - (void)GCDDispatch_suspend_resume{ //系統(tǒng)默認(rèn)生成的，所以無(wú)法調(diào)用dispatch_resume()和dispatch_suspend()來(lái)控制執(zhí)行繼續(xù)或中斷。 dispatch_queue_t queue1 = dispatch_queue_create("queue1", 0); dispatch_queue_t queue2 = dispatch_queue_create("queue2", 0); dispatch_group_t group = dispatch_group_create(); dispatch_async(queue1, ^{ for (int i = 0; i < 5; i++) { NSLog(@"%@-------%d",[NSThread currentThread],i); sleep(1); } }); dispatch_async(queue2, ^{ NSLog(@"task2"); }); dispatch_group_async(group, queue1, ^{ NSLog(@"task1 finished!"); }); dispatch_group_async(group, queue2, ^{ dispatch_suspend(queue1);//掛起 NSLog(@"task2 finished!掛起queue1"); [NSThread sleepForTimeInterval:20.0]; dispatch_resume(queue1);//喚醒隊(duì)列 }); dispatch_group_wait(group, DISPATCH_TIME_FOREVER); dispatch_async(queue1, ^{ NSLog(@"task3"); }); dispatch_async(queue2, ^{ NSLog(@"task4"); }); }

打印結(jié)果：可以先思考一下打印結(jié)果

2016-11-04 17:59:41.219 ThreadDemo[29749:5738141] task2 2016-11-04 17:59:41.219 ThreadDemo[29749:5738138] <NSThread: 0x60000027c380>{number = 4, name = (null)}-------0 2016-11-04 17:59:41.220 ThreadDemo[29749:5738141] task2 finished!掛起queue1 2016-11-04 17:59:42.220 ThreadDemo[29749:5738138] <NSThread: 0x60000027c380>{number = 4, name = (null)}-------1 2016-11-04 17:59:43.223 ThreadDemo[29749:5738138] <NSThread: 0x60000027c380>{number = 4, name = (null)}-------2 2016-11-04 17:59:44.225 ThreadDemo[29749:5738138] <NSThread: 0x60000027c380>{number = 4, name = (null)}-------3 2016-11-04 17:59:45.230 ThreadDemo[29749:5738138] <NSThread: 0x60000027c380>{number = 4, name = (null)}-------4 2016-11-04 18:00:01.220 ThreadDemo[29749:5738141] task1 finished! 2016-11-04 18:00:01.220 ThreadDemo[29749:5738141] task3 2016-11-04 18:00:01.221 ThreadDemo[29749:5738769] task4

11. 信號(hào)量Dispatch Semaphore

當(dāng)并行執(zhí)行的處理更新數(shù)據(jù)時(shí)，會(huì)產(chǎn)出數(shù)據(jù)不一致的情況，有時(shí)應(yīng)用就會(huì)異常退出?？梢允褂肧erial Dispatch Queue 和 dispatch_barrier也可以處理這種情況，我們也可以使用信號(hào)量Dispatch Semaphor來(lái)更好的處理線程安全和同步的問(wèn)題。

Dispatch Semaphore是持有計(jì)數(shù)的信號(hào)。使用計(jì)數(shù)來(lái)實(shí)現(xiàn)該等待還是運(yùn)行功能。計(jì)數(shù)為0時(shí)等待，計(jì)數(shù)為1或者大于1時(shí)，減去1而不是等待，就可以運(yùn)行。

其實(shí)信號(hào)量就是根據(jù)pv操作來(lái)實(shí)現(xiàn)的

p操作-1 v操作+1

信號(hào)量就是一個(gè)資源計(jì)數(shù)器，對(duì)信號(hào)量有兩個(gè)操作來(lái)達(dá)到互斥，分別是P和V操作。

一般情況是這樣進(jìn)行臨界訪問(wèn)或互斥訪問(wèn)的：設(shè)信號(hào)量值為1，當(dāng)一個(gè)進(jìn)程1運(yùn)行時(shí)，使用資源，進(jìn)行P操作，即對(duì)信號(hào)量值減1，也就是資源數(shù)少了1個(gè)。這是信號(hào)量值為0。系統(tǒng)中規(guī)定當(dāng)信號(hào)量值為0是，必須等待，直到信號(hào)量值不為零才能繼續(xù)操作。這時(shí)如果進(jìn)程2想要運(yùn)行，那么也必須進(jìn)行P操作，但是此時(shí)信號(hào)量為0，所以無(wú)法減1，即不能P操作，也就阻塞。這樣就到到了進(jìn)程1排他訪問(wèn)。當(dāng)進(jìn)程1運(yùn)行結(jié)束后，釋放資源，進(jìn)行V操作。資源數(shù)重新加1，這是信號(hào)量的值變?yōu)?. 這時(shí)進(jìn)程2發(fā)現(xiàn)資源數(shù)不為0，信號(hào)量能進(jìn)行P操作了，立即執(zhí)行P操作。信號(hào)量值又變?yōu)?.次數(shù)進(jìn)程2咱有資源，排他訪問(wèn)資源。這就是信號(hào)量來(lái)控制互斥的原理

信號(hào)量為0則阻塞線程，大于0則不會(huì)阻塞。則我們通過(guò)改變信號(hào)量的值，來(lái)控制是否阻塞線程，從而達(dá)到線程同步。

- (void)GCDDispatchSemaphore{ dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); //創(chuàng)建一個(gè)信號(hào)量dispatch dispatch_semaphore_t semaphore = dispatch_semaphore_create(1); NSMutableArray *array = [NSMutableArray new]; for (int i = 0; i < 10; i++) { dispatch_async(queue, ^{ //一直等待信號(hào)量大于0 dispatch_semaphore_wait(semaphore, dispatch_time(DISPATCH_TIME_NOW, DISPATCH_TIME_FOREVER)); //進(jìn)行排他控制的處理代碼... //將dispatch semphore的計(jì)數(shù)器減1 [array addObject:[NSNumber numberWithInteger:i]]; //排他控制處理結(jié)束 //將dispatch semphore的計(jì)數(shù)器加1 //如果有通過(guò)dispatch_semaphore_wait函數(shù)dispatch semphore的計(jì)數(shù)值增加的線程，等待就由最先等待的線程執(zhí)行 dispatch_semaphore_signal(semaphore); }); NSLog(@"%@",array); } }

創(chuàng)建一個(gè)信號(hào)量dispatch dispatch_semaphore_t dispatch_semaphore_create(long value);

等待信號(hào)量longdispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout);返回值和dispatch_group_wait函數(shù)相同，也可以通過(guò)下面進(jìn)行分支處理：

long result = dispatch_semaphore_wait(semaphore, dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10*NSEC_PER_SEC))); if (result == 0){ //進(jìn)行排他控制的處理代碼... //將dispatch semphore的計(jì)數(shù)器減1 [array addObject:[NSNumber numberWithInteger:i]]; //排他控制處理結(jié)束 //將dispatch semphore的計(jì)數(shù)器加1 //如果有通過(guò)dispatch_semaphore_wait函數(shù)dispatch semphore的計(jì)數(shù)值增加的線程，等待就由最先等待的線程執(zhí)行 dispatch_semaphore_signal(semaphore); }else{ //這個(gè)時(shí)候計(jì)數(shù)值為0，在達(dá)到指定時(shí)間為止還是等待狀態(tài)。 //處理邏輯.... }

在進(jìn)行排他處理結(jié)束時(shí)，通過(guò)dispatch_semaphore_signal來(lái)講計(jì)數(shù)值加1

GCD總結(jié)

開(kāi)不開(kāi)線程由執(zhí)行任務(wù)的函數(shù)決定

異步開(kāi)，異步是多線程的代名詞

同步不開(kāi)

開(kāi)幾條線程由隊(duì)列決定

串行隊(duì)列開(kāi)一條線程（GCD會(huì)開(kāi)一條，NSOperation Queue最大并發(fā)數(shù)為1時(shí)也可能開(kāi)多條）

并發(fā)隊(duì)列開(kāi)多條線程，具體能開(kāi)的線程數(shù)量由底層線程池決定

GCD的使用步驟

1.創(chuàng)建block任務(wù)

2.創(chuàng)建GCD隊(duì)列（串行和并發(fā)）

3.把block任務(wù)放到GCD隊(duì)列里，以異步或者同步方式執(zhí)行GCD隊(duì)列

結(jié)尾：

能看到這里的小伙伴都是真愛(ài)啊，內(nèi)容太多了。學(xué)完這個(gè)你們多思考一下里面方法的聯(lián)系以及實(shí)現(xiàn)，好了今天的GCD就講到這里,中間有什么錯(cuò)誤歡迎你們批評(píng)指出！下一篇博客將帶你走進(jìn)NSOperation。

喜歡的話就請(qǐng)點(diǎn)個(gè)喜歡加個(gè)關(guān)注^_（樓主好無(wú)恥?。??

ThreadDemo下載鏈接

參考書(shū)籍：Objective-C高級(jí)編程iOS與OS X多線程和內(nèi)存管理

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iOS開(kāi)發(fā)之多線程編程總結(jié)（二）

背景

GCD基本介紹

GCD基本概念

任務(wù)和隊(duì)列

GCD的使用

1. 串行隊(duì)列（Serial Dispatch Queue）

串行隊(duì)列的創(chuàng)建：

串行隊(duì)列同步和異步執(zhí)行Demo：

串行隊(duì)列 同步執(zhí)行結(jié)果：

串行隊(duì)列 異步執(zhí)行結(jié)果：

小結(jié)：

2. 并發(fā)隊(duì)列（Concurrent Dispatch Queue）

并發(fā)隊(duì)列創(chuàng)建

并發(fā)隊(duì)列同步和異步執(zhí)行Demo

并發(fā) 隊(duì)列同步執(zhí)行結(jié)果：

并發(fā)隊(duì)列 異步執(zhí)行結(jié)果：

小結(jié)：

3. 全局隊(duì)列（Global Dispatch Queue）

全局隊(duì)列基本知識(shí)

全局隊(duì)列同步和異步執(zhí)行Demo

全局隊(duì)列 同步執(zhí)行結(jié)果：

全局隊(duì)列 異步執(zhí)行結(jié)果：

4. 主隊(duì)列（Main Dispatch Queue）

主隊(duì)列基本知識(shí)

主隊(duì)列同步和異步執(zhí)行Demo

主隊(duì)列 同步執(zhí)行結(jié)果:

主隊(duì)列 異步執(zhí)行結(jié)果:

5.延遲執(zhí)行（dispatch_after）

延遲執(zhí)行（dispatch_after）Demo

注意：

6. 更改優(yōu)先級(jí)（dispatch_set_target_queue）

1. dispatch_set_target_queue第一個(gè)Demo：

打印結(jié)果：

打印解釋?zhuān)?/strong>

2. dispatch_set_target_queue第一個(gè)Demo：

打印結(jié)果：

打印解釋?zhuān)?/strong>

** dispatch_set_target_queue小結(jié)：**

7. 任務(wù)組Dispatch Group

1.隊(duì)列與組自動(dòng)關(guān)聯(lián)并執(zhí)行

打印結(jié)果：

2. 隊(duì)列與組手動(dòng)關(guān)聯(lián)并執(zhí)行

8. 柵欄任務(wù)Dispatch_barrier_async

打印結(jié)果：

9. 循環(huán)執(zhí)行dispatch_apply

打印結(jié)果：

10. 隊(duì)列的掛起和喚醒

打印結(jié)果：可以先思考一下打印結(jié)果

11. 信號(hào)量Dispatch Semaphore

GCD總結(jié)

結(jié)尾：

相關(guān)閱讀更多精彩內(nèi)容

友情鏈接更多精彩內(nèi)容

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串行隊(duì)列同步執(zhí)行結(jié)果：

串行隊(duì)列異步執(zhí)行結(jié)果：

并發(fā)隊(duì)列異步執(zhí)行結(jié)果：

全局隊(duì)列同步執(zhí)行結(jié)果：

全局隊(duì)列異步執(zhí)行結(jié)果：

主隊(duì)列同步執(zhí)行結(jié)果:

主隊(duì)列異步執(zhí)行結(jié)果:

dispatch_set_target_queue小結(jié)：