文中較詳細(xì)介紹GCD隊(duì)列筹吐，各種GCD使用方法，實(shí)例如何使用Dispatch Source監(jiān)聽系統(tǒng)底層對象淘衙，分析不同鎖的性能對比，實(shí)例GCD死鎖情況昭躺。文中的Demo在這里 https://github.com/ming1016/GCDDemo 對著文章試著來調(diào)demo體會更深哦忌锯，細(xì)細(xì)嚼消化好:)

GCD（Grand Central Dispatch） 介紹

GCD屬于系統(tǒng)級的線程管理，在Dispatch queue中執(zhí)行需要執(zhí)行的任務(wù)性能非常的高领炫。GCD這塊已經(jīng)開源偶垮，地址http://libdispatch.macosforge.org。GCD中的FIFO隊(duì)列稱為dispatch queue帝洪，用來保證先進(jìn)來的任務(wù)先得到執(zhí)行似舵。

GCD概要

• 和operation queue一樣都是基于隊(duì)列的并發(fā)編程API，他們通過集中管理大家協(xié)同使用的線程池葱峡。
• 公開的5個(gè)不同隊(duì)列：運(yùn)行在主線程中的main queue砚哗，3個(gè)不同優(yōu)先級的后臺隊(duì)列（High Priority Queue，Default Priority Queue砰奕，Low Priority Queue）蛛芥，以及一個(gè)優(yōu)先級更低的后臺隊(duì)列Background Priority Queue（用于I/O）
• 可創(chuàng)建自定義隊(duì)列：串行或并列隊(duì)列。自定義一般放在Default Priority Queue和Main Queue里军援。
• 操作是在多線程上還是單線程主要是看隊(duì)列的類型和執(zhí)行方法仅淑，并行隊(duì)列異步執(zhí)行才能在多線程，并行隊(duì)列同步執(zhí)行就只會在主線程執(zhí)行了

基本概念

• 系統(tǒng)標(biāo)準(zhǔn)兩個(gè)隊(duì)列

//全局隊(duì)列胸哥，一個(gè)并行的隊(duì)列
dispatch_get_global_queue
//主隊(duì)列涯竟，主線程中的唯一隊(duì)列，一個(gè)串行隊(duì)列
dispatch_get_main_queue

• 自定義隊(duì)列

//串行隊(duì)列
dispatch_queue_create("com.starming.serialqueue", DISPATCH_QUEUE_SERIAL)
//并行隊(duì)列
dispatch_queue_create("com.starming.concurrentqueue", DISPATCH_QUEUE_CONCURRENT)

• 同步異步線程創(chuàng)建

//同步線程
dispatch_sync(..., ^(block))
//異步線程
dispatch_async(..., ^(block))

隊(duì)列（dispatch queue）

• Serial：又叫private dispatch queues空厌，同時(shí)只執(zhí)行一個(gè)任務(wù)庐船。Serial queue常用于同步訪問特定的資源或數(shù)據(jù)。當(dāng)你創(chuàng)建多個(gè)Serial queue時(shí)嘲更，雖然各自是同步筐钟，但serial queue之間是并發(fā)執(zhí)行。
• Main dispatch queue：全局可用的serial queue赋朦，在應(yīng)用程序主線程上執(zhí)行任務(wù)篓冲。
• Concurrent：又叫g(shù)lobal dispatch queue，可以并發(fā)的執(zhí)行多個(gè)任務(wù)北发，但執(zhí)行完成順序是隨機(jī)的。系統(tǒng)提供四個(gè)全局并發(fā)隊(duì)列喷屋，這四個(gè)隊(duì)列有這對應(yīng)的優(yōu)先級琳拨，用戶是不能夠創(chuàng)建全局隊(duì)列的，只能獲取屯曹。

dipatch_queue_t queue;
queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,0);

• user create queue：創(chuàng)建自己定義的隊(duì)列狱庇，可以用dispatch_queue_create函數(shù)惊畏，函數(shù)有兩個(gè)參數(shù)，第一個(gè)自定義的隊(duì)列名密任，第二個(gè)參數(shù)是隊(duì)列類型颜启，默認(rèn)NULL或者DISPATCH_QUEUE_SERIAL的是串行浪讳，參數(shù)為DISPATCH_QUEUE_CONCURRENT為并行隊(duì)列缰盏。

dispatch_queue_t queue
queue = dispatch_queue_create("com.starming.gcddemo.concurrentqueue", DISPATCH_QUEUE_CONCURRENT);

• 自定義隊(duì)列的優(yōu)先級：可以通過dipatch_queue_attr_make_with_qos_class或dispatch_set_target_queue方法設(shè)置隊(duì)列的優(yōu)先級

//dipatch_queue_attr_make_with_qos_class
dispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_UTILITY, -1);
dispatch_queue_t queue = dispatch_queue_create("com.starming.gcddemo.qosqueue", attr);

//dispatch_set_target_queue
dispatch_queue_t queue = dispatch_queue_create("com.starming.gcddemo.settargetqueue",NULL); //需要設(shè)置優(yōu)先級的queue
dispatch_queue_t referQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0); //參考優(yōu)先級
dispatch_set_target_queue(queue, referQueue); //設(shè)置queue和referQueue的優(yōu)先級一樣

• dispatch_set_target_queue：可以設(shè)置優(yōu)先級，也可以設(shè)置隊(duì)列層級體系淹遵，比如讓多個(gè)串行和并行隊(duì)列在統(tǒng)一一個(gè)串行隊(duì)列里串行執(zhí)行口猜，如下

dispatch_queue_t serialQueue = dispatch_queue_create("com.starming.gcddemo.serialqueue", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t firstQueue = dispatch_queue_create("com.starming.gcddemo.firstqueue", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t secondQueue = dispatch_queue_create("com.starming.gcddemo.secondqueue", DISPATCH_QUEUE_CONCURRENT);

dispatch_set_target_queue(firstQueue, serialQueue);
dispatch_set_target_queue(secondQueue, serialQueue);

dispatch_async(firstQueue, ^{
    NSLog(@"1");
    [NSThread sleepForTimeInterval:3.f];
});
dispatch_async(secondQueue, ^{
    NSLog(@"2");
    [NSThread sleepForTimeInterval:2.f];
});
dispatch_async(secondQueue, ^{
    NSLog(@"3");
    [NSThread sleepForTimeInterval:1.f];
});

隊(duì)列類型

隊(duì)列默認(rèn)是串行的，如果設(shè)置改參數(shù)為NULL會按串行處理透揣，只能執(zhí)行一個(gè)單獨(dú)的block济炎，隊(duì)列也可以是并行的，同一時(shí)間執(zhí)行多個(gè)block

- (id)init;
{
     self = [super init];
     if (self != nil) {
          NSString *label = [NSString stringWithFormat:@"%@.isolation.%p", [self class], self];
          self.isolationQueue = dispatch_queue_create([label UTF8String], 0);

          label = [NSString stringWithFormat:@"%@.work.%p", [self class], self];
          self.workQueue = dispatch_queue_create([label UTF8String], 0);
     }
     return self;
}

5種隊(duì)列辐真，主隊(duì)列（main queue）,四種通用調(diào)度隊(duì)列须尚，自己定制的隊(duì)列。四種通用調(diào)度隊(duì)列為

• QOS_CLASS_USER_INTERACTIVE：user interactive等級表示任務(wù)需要被立即執(zhí)行提供好的體驗(yàn)侍咱，用來更新UI耐床，響應(yīng)事件等。這個(gè)等級最好保持小規(guī)模放坏。
• QOS_CLASS_USER_INITIATED：user initiated等級表示任務(wù)由UI發(fā)起異步執(zhí)行咙咽。適用場景是需要及時(shí)結(jié)果同時(shí)又可以繼續(xù)交互的時(shí)候。
• QOS_CLASS_UTILITY：utility等級表示需要長時(shí)間運(yùn)行的任務(wù)淤年，伴有用戶可見進(jìn)度指示器钧敞。經(jīng)常會用來做計(jì)算，I/O麸粮，網(wǎng)絡(luò)溉苛，持續(xù)的數(shù)據(jù)填充等任務(wù)。這個(gè)任務(wù)節(jié)能弄诲。
• QOS_CLASS_BACKGROUND：background等級表示用戶不會察覺的任務(wù)愚战，使用它來處理預(yù)加載，或者不需要用戶交互和對時(shí)間不敏感的任務(wù)齐遵。

示例：后臺加載顯示圖片

override func viewDidLoad() {
     super.viewDidLoad()

     dispatch_async(dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)) { // 將工作從主線程轉(zhuǎn)移到全局隊(duì)列中寂玲，這是dispatch_async調(diào)用，異步提交保證調(diào)用線程會繼續(xù)執(zhí)行下去梗摇，這樣viewDidLoad在主線程上能夠更早完成拓哟，
          let overlayImage = self.faceOverlayImageFromImage(self.image)
          dispatch_async(dispatch_get_main_queue()) { // 新圖完成，把一個(gè)閉包加入主線程用來更新UIImageView伶授，只有在主線程能操作UIKit断序。
               self.fadeInNewImage(overlayImage) // 更新UI
          }
     }
}

何時(shí)使用何種隊(duì)列類型

• 主隊(duì)列（順序）：隊(duì)列中有任務(wù)完成需要更新UI時(shí)流纹，dispatch_after在這種類型中使用。
• 并發(fā)隊(duì)列：用來執(zhí)行與UI無關(guān)的后臺任務(wù)违诗，dispatch_sync放在這里漱凝，方便等待任務(wù)完成進(jìn)行后續(xù)處理或和dispatch barrier同步。dispatch groups放在這里也不錯(cuò)诸迟。
• 自定義順序隊(duì)列：順序執(zhí)行后臺任務(wù)并追蹤它時(shí)茸炒。這樣做同時(shí)只有一個(gè)任務(wù)在執(zhí)行可以防止資源競爭。dipatch barriers解決讀寫鎖問題的放在這里處理亮蒋。dispatch groups也是放在這里扣典。

可以使用下面的方法簡化QoS等級參數(shù)的寫法

var GlobalMainQueue: dispatch_queue_t {
     return dispatch_get_main_queue()
}
var GlobalUserInteractiveQueue: dispatch_queue_t {
     return dispatch_get_global_queue(Int(QOS_CLASS_USER_INTERACTIVE.value), 0)
}
var GlobalUserInitiatedQueue: dispatch_queue_t {
     return dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)
}
var GlobalUtilityQueue: dispatch_queue_t {
     return dispatch_get_global_queue(Int(QOS_CLASS_UTILITY.value), 0)
}
var GlobalBackgroundQueue: dispatch_queue_t {
     return dispatch_get_global_queue(Int(QOS_CLASS_BACKGROUND.value), 0)
}

//使用起來就是這樣，易讀而且容易看出在使用哪個(gè)隊(duì)列
dispatch_async(GlobalUserInitiatedQueue) {
     let overlayImage = self.faceOverlayImageFromImage(self.image)
     dispatch_async(GlobalMainQueue) {
          self.fadeInNewImage(overlayImage)
     }
}

dispatch_once用法

dispatch_once_t要是全局或static變量慎玖，保證dispatch_once_t只有一份實(shí)例

+ (UIColor *)boringColor;
{
     static UIColor *color;
     //只運(yùn)行一次
     static dispatch_once_t onceToken;
     dispatch_once(&onceToken, ^{
          color = [UIColor colorWithRed:0.380f green:0.376f blue:0.376f alpha:1.000f];
     });
     return color;
}

dispatch_async

設(shè)計(jì)一個(gè)異步的API調(diào)用dispatch_async()贮尖，這個(gè)調(diào)用放在API的方法或函數(shù)中做。讓API的使用者設(shè)置一個(gè)回調(diào)處理隊(duì)列

- (void)processImage:(UIImage *)image completionHandler:(void(^)(BOOL success))handler;
{
     dispatch_async(self.isolationQueue, ^(void){
          // do actual processing here
          dispatch_async(self.resultQueue, ^(void){
               handler(YES);
          });
     });
}

可以避免界面會被一些耗時(shí)的操作卡死趁怔，比如讀取網(wǎng)絡(luò)數(shù)據(jù)湿硝，大數(shù)據(jù)IO，還有大量數(shù)據(jù)的數(shù)據(jù)庫讀寫润努，這時(shí)需要在另一個(gè)線程中處理关斜，然后通知主線程更新界面，GCD使用起來比NSThread和NSOperation方法要簡單方便铺浇。

//代碼框架
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     // 耗時(shí)的操作
     dispatch_async(dispatch_get_main_queue(), ^{
          // 更新界面
     });
});

//下載圖片的示例
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"];
     NSData * data = [[NSData alloc]initWithContentsOfURL:url];
     UIImage *image = [[UIImage alloc]initWithData:data];
     if (data != nil) {
          dispatch_async(dispatch_get_main_queue(), ^{
               self.imageView.image = image;
          });
     }
});

dispatch_after延后執(zhí)行

dispatch_after只是延時(shí)提交block痢畜，不是延時(shí)立刻執(zhí)行。

- (void)foo
{
     double delayInSeconds = 2.0;
     dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t) (delayInSeconds * NSEC_PER_SEC));
     dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
          [self bar];
     });
}

范例鳍侣，實(shí)現(xiàn)一個(gè)推遲出現(xiàn)彈出框提示丁稀，比如說提示用戶評價(jià)等功能。

func showOrHideNavPrompt() {
     let delayInSeconds = 1.0
     let popTime = dispatch_time(DISPATCH_TIME_NOW,
          Int64(delayInSeconds * Double(NSEC_PER_SEC))) // 在這里聲明推遲的時(shí)間
     dispatch_after(popTime, GlobalMainQueue) { // 等待delayInSeconds將閉包異步到主隊(duì)列
          let count = PhotoManager.sharedManager.photos.count
          if count > 0 {
               self.navigationItem.prompt = nil
          } else {
               self.navigationItem.prompt = "Add photos with faces to Googlyify them!"
          }
     }
}

例子中的dispatch time的參數(shù)倚聚，可以先看看函數(shù)原型

dispatch_time_t dispatch_time ( dispatch_time_t when, int64_t delta );

第一個(gè)參數(shù)為DISPATCH_TIME_NOW表示當(dāng)前线衫。第二個(gè)參數(shù)的delta表示納秒，一秒對應(yīng)的納秒為1000000000惑折，系統(tǒng)提供了一些宏來簡化

 #define NSEC_PER_SEC 1000000000ull //每秒有多少納秒
 #define USEC_PER_SEC 1000000ull    //每秒有多少毫秒
 #define NSEC_PER_USEC 1000ull      //每毫秒有多少納秒

這樣如果要表示一秒就可以這樣寫

dispatch_time(DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC);
dispatch_time(DISPATCH_TIME_NOW, 1000 * USEC_PER_SEC);
dispatch_time(DISPATCH_TIME_NOW, USEC_PER_SEC * NSEC_PER_USEC);

dispatch_barrier_async使用Barrier Task方法Dispatch Barrier解決多線程并發(fā)讀寫同一個(gè)資源發(fā)生死鎖

Dispatch Barrier確保提交的閉包是指定隊(duì)列中在特定時(shí)段唯一在執(zhí)行的一個(gè)授账。在所有先于Dispatch Barrier的任務(wù)都完成的情況下這個(gè)閉包才開始執(zhí)行。輪到這個(gè)閉包時(shí)barrier會執(zhí)行這個(gè)閉包并且確保隊(duì)列在此過程不會執(zhí)行其它任務(wù)惨驶。閉包完成后隊(duì)列恢復(fù)白热。需要注意dispatch_barrier_async只在自己創(chuàng)建的隊(duì)列上有這種作用，在全局并發(fā)隊(duì)列和串行隊(duì)列上粗卜，效果和dispatch_sync一樣

//創(chuàng)建隊(duì)列
self.isolationQueue = dispatch_queue_create([label UTF8String], DISPATCH_QUEUE_CONCURRENT);
//改變setter
- (void)setCount:(NSUInteger)count forKey:(NSString *)key
{
     key = [key copy];
     //確保所有barrier都是async異步的
     dispatch_barrier_async(self.isolationQueue, ^(){
          if (count == 0) {
               [self.counts removeObjectForKey:key];
          } else {
               self.counts[key] = @(count);
          }
     });
}
- (void)dispatchBarrierAsyncDemo {
    //防止文件讀寫沖突屋确，可以創(chuàng)建一個(gè)串行隊(duì)列，操作都在這個(gè)隊(duì)列中進(jìn)行，沒有更新數(shù)據(jù)讀用并行乍恐，寫用串行。
    dispatch_queue_t dataQueue = dispatch_queue_create("com.starming.gcddemo.dataqueue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(dataQueue, ^{
        [NSThread sleepForTimeInterval:2.f];
        NSLog(@"read data 1");
    });
    dispatch_async(dataQueue, ^{
        NSLog(@"read data 2");
    });
    //等待前面的都完成测砂，在執(zhí)行barrier后面的
    dispatch_barrier_async(dataQueue, ^{
        NSLog(@"write data 1");
        [NSThread sleepForTimeInterval:1];
    });
    dispatch_async(dataQueue, ^{
        [NSThread sleepForTimeInterval:1.f];
        NSLog(@"read data 3");
    });
    dispatch_async(dataQueue, ^{
        NSLog(@"read data 4");
    });
}

swift示例

//使用dispatch_queue_create初始化一個(gè)并發(fā)隊(duì)列茵烈。第一個(gè)參數(shù)遵循反向DNS命名習(xí)慣，方便描述砌些，第二個(gè)參數(shù)是指出是并發(fā)還是順序呜投。
private let concurrentPhotoQueue = dispatch_queue_create(
"com.raywenderlich.GooglyPuff.photoQueue", DISPATCH_QUEUE_CONCURRENT)

func addPhoto(photo: Photo) {
     dispatch_barrier_async(concurrentPhotoQueue) { // 將寫操作加入到自定義的隊(duì)列。開始執(zhí)行時(shí)這個(gè)就是隊(duì)列中唯一的一個(gè)在執(zhí)行的任務(wù)存璃。
          self._photos.append(photo) // barrier能夠保障不會和其他任務(wù)同時(shí)進(jìn)行仑荐。
          dispatch_async(GlobalMainQueue) { // 涉及到UI所以這個(gè)通知應(yīng)該在主線程中，所以分派另一個(gè)異步任務(wù)到主隊(duì)列中纵东。
               self.postContentAddedNotification()
          }
     }
}

//上面是解決了寫可能發(fā)生死鎖粘招，下面是使用dispatch_sync解決讀時(shí)可能會發(fā)生的死鎖。
var photos: [Photo] {
     var photosCopy: [Photo]!
     dispatch_sync(concurrentPhotoQueue) { // 同步調(diào)度到concurrentPhotoQueue隊(duì)列執(zhí)行讀操作
          photosCopy = self._photos // 保存
     }
     return photosCopy
}
//這樣讀寫問題都解決了偎球。

都用異步處理避免死鎖洒扎，異步的缺點(diǎn)在于調(diào)試不方便，但是比起同步容易產(chǎn)生死鎖這個(gè)副作用還算小的衰絮。

dispatch_apply進(jìn)行快速迭代

類似for循環(huán)袍冷，但是在并發(fā)隊(duì)列的情況下dispatch_apply會并發(fā)執(zhí)行block任務(wù)。

for (size_t y = 0; y < height; ++y) {
     for (size_t x = 0; x < width; ++x) {
          // Do something with x and y here
     }
}
//因?yàn)榭梢圆⑿袌?zhí)行猫牡，所以使用dispatch_apply可以運(yùn)行的更快
- (void)dispatchApplyDemo {
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.starming.gcddemo.concurrentqueue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_apply(10, concurrentQueue, ^(size_t i) {
        NSLog(@"%zu",i);
    });
    NSLog(@"The end"); //這里有個(gè)需要注意的是胡诗，dispatch_apply這個(gè)是會阻塞主線程的。這個(gè)log打印會在dispatch_apply都結(jié)束后才開始執(zhí)行
}

dispatch_apply能避免線程爆炸淌友，因?yàn)镚CD會管理并發(fā)

- (void)dealWiththreadWithMaybeExplode:(BOOL)explode {
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.starming.gcddemo.concurrentqueue",DISPATCH_QUEUE_CONCURRENT);
    if (explode) {
        //有問題的情況煌恢，可能會死鎖
        for (int i = 0; i < 999 ; i++) {
            dispatch_async(concurrentQueue, ^{
                NSLog(@"wrong %d",i);
                //do something hard
            });
        }
    } else {
        //會優(yōu)化很多，能夠利用GCD管理
        dispatch_apply(999, concurrentQueue, ^(size_t i){
            NSLog(@"correct %zu",i);
            //do something hard
        });
    }
}

示例：

func downloadPhotosWithCompletion(completion: BatchPhotoDownloadingCompletionClosure?) {
     var storedError: NSError!
     var downloadGroup = dispatch_group_create()
     let addresses = [OverlyAttachedGirlfriendURLString,
          SuccessKidURLString,
          LotsOfFacesURLString]

     dispatch_apply(UInt(addresses.count), GlobalUserInitiatedQueue) {
          i in
          let index = Int(i)
          let address = addresses[index]
          let url = NSURL(string: address)
          dispatch_group_enter(downloadGroup)
          let photo = DownloadPhoto(url: url!) {
               image, error in
               if let error = error {
                    storedError = error
               }
               dispatch_group_leave(downloadGroup)
          }
          PhotoManager.sharedManager.addPhoto(photo)
     }

     dispatch_group_notify(downloadGroup, GlobalMainQueue) {
          if let completion = completion {
               completion(error: storedError)
          }
     }
}

Block組合Dispatch_groups

dispatch groups是專門用來監(jiān)視多個(gè)異步任務(wù)亩进。dispatch_group_t實(shí)例用來追蹤不同隊(duì)列中的不同任務(wù)症虑。

當(dāng)group里所有事件都完成GCD API有兩種方式發(fā)送通知，第一種是dispatch_group_wait归薛，會阻塞當(dāng)前進(jìn)程谍憔，等所有任務(wù)都完成或等待超時(shí)。第二種方法是使用dispatch_group_notify主籍，異步執(zhí)行閉包习贫，不會阻塞。

第一種使用dispatch_group_wait的swift的例子：

func downloadPhotosWithCompletion(completion: BatchPhotoDownloadingCompletionClosure?) {
     dispatch_async(GlobalUserInitiatedQueue) { // 因?yàn)閐ispatch_group_wait會租塞當(dāng)前進(jìn)程千元，所以要使用dispatch_async將整個(gè)方法要放到后臺隊(duì)列才能夠保證主線程不被阻塞
          var storedError: NSError!
          var downloadGroup = dispatch_group_create() // 創(chuàng)建一個(gè)dispatch group

          for address in [OverlyAttachedGirlfriendURLString,
               SuccessKidURLString,
               LotsOfFacesURLString]
          {
               let url = NSURL(string: address)
               dispatch_group_enter(downloadGroup) // dispatch_group_enter是通知dispatch group任務(wù)開始了苫昌，dispatch_group_enter和dispatch_group_leave是成對調(diào)用，不然程序就崩潰了幸海。
               let photo = DownloadPhoto(url: url!) {
                    image, error in
                    if let error = error {
                         storedError = error
                    }
                    dispatch_group_leave(downloadGroup) // 保持和dispatch_group_enter配對祟身。通知任務(wù)已經(jīng)完成
               }
               PhotoManager.sharedManager.addPhoto(photo)
          }

          dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER) // dispatch_group_wait等待所有任務(wù)都完成直到超時(shí)奥务。如果任務(wù)完成前就超時(shí)了，函數(shù)會返回一個(gè)非零值袜硫，可以通過返回值判斷是否超時(shí)氯葬。也可以用DISPATCH_TIME_FOREVER表示一直等。
          dispatch_async(GlobalMainQueue) { // 這里可以保證所有圖片任務(wù)都完成婉陷，然后在main queue里加入完成后要處理的閉包帚称，會在main queue里執(zhí)行。
               if let completion = completion { // 執(zhí)行閉包內(nèi)容
                    completion(error: storedError)
               }
          }
     }
}

oc例子

- (void)dispatchGroupWaitDemo {
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.starming.gcddemo.concurrentqueue",DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_t group = dispatch_group_create();
    //在group中添加隊(duì)列的block
    dispatch_group_async(group, concurrentQueue, ^{
        [NSThread sleepForTimeInterval:2.f];
        NSLog(@"1");
    });
    dispatch_group_async(group, concurrentQueue, ^{
        NSLog(@"2");
    });
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    NSLog(@"go on");
}

第二種使用dispatch_group_notify的swift的例子：

func downloadPhotosWithCompletion(completion: BatchPhotoDownloadingCompletionClosure?) {
     // 不用加dispatch_async秽澳，因?yàn)闆]有阻塞主進(jìn)程
     var storedError: NSError!
     var downloadGroup = dispatch_group_create()

     for address in [OverlyAttachedGirlfriendURLString,
          SuccessKidURLString,
          LotsOfFacesURLString]
     {
          let url = NSURL(string: address)
          dispatch_group_enter(downloadGroup)
          let photo = DownloadPhoto(url: url!) {
               image, error in
               if let error = error {
                    storedError = error
               }
               dispatch_group_leave(downloadGroup)
          }
          PhotoManager.sharedManager.addPhoto(photo)
     }

     dispatch_group_notify(downloadGroup, GlobalMainQueue) { // dispatch_group_notify和dispatch_group_wait的區(qū)別就是是異步執(zhí)行閉包的闯睹，當(dāng)dispatch groups中沒有剩余的任務(wù)時(shí)閉包才執(zhí)行。這里是指明在主隊(duì)列中執(zhí)行担神。
          if let completion = completion {
               completion(error: storedError)
          }
     }
}

oc例子

//dispatch_group_notify
- (void)dispatchGroupNotifyDemo {
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.starming.gcddemo.concurrentqueue",DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, concurrentQueue, ^{
        NSLog(@"1");
    });
    dispatch_group_async(group, concurrentQueue, ^{
        NSLog(@"2");
    });
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"end");
    });
    NSLog(@"can continue");

}

//dispatch_group_wait
- (void)dispatchGroupWaitDemo {
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.starming.gcddemo.concurrentqueue",DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_t group = dispatch_group_create();
    //在group中添加隊(duì)列的block
    dispatch_group_async(group, concurrentQueue, ^{
        [NSThread sleepForTimeInterval:2.f];
        NSLog(@"1");
    });
    dispatch_group_async(group, concurrentQueue, ^{
        NSLog(@"2");
    });
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    NSLog(@"can continue");
}

如何對現(xiàn)有API使用dispatch_group_t

//給Core Data的-performBlock:添加groups楼吃。組合完成任務(wù)后使用dispatch_group_notify來運(yùn)行一個(gè)block即可。
- (void)withGroup:(dispatch_group_t)group performBlock:(dispatch_block_t)block
{
     if (group == NULL) {
          [self performBlock:block];
     } else {
          dispatch_group_enter(group);
          [self performBlock:^(){
               block();
               dispatch_group_leave(group);
          }];
     }
}

//NSURLConnection也可以這樣做
+ (void)withGroup:(dispatch_group_t)group
     sendAsynchronousRequest:(NSURLRequest *)request
     queue:(NSOperationQueue *)queue
     completionHandler:(void (^)(NSURLResponse*, NSData*, NSError*))handler
{
     if (group == NULL) {
          [self sendAsynchronousRequest:request
               queue:queue
               completionHandler:handler];
     } else {
          dispatch_group_enter(group);
          [self sendAsynchronousRequest:request
                    queue:queue
                    completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){
               handler(response, data, error);
               dispatch_group_leave(group);
          }];
     }
}

注意事項(xiàng)

• dispatch_group_async等價(jià)于dispatch_group_enter() 和 dispatch_group_leave()的組合妄讯。
• dispatch_group_enter() 必須運(yùn)行在 dispatch_group_leave() 之前所刀。
• dispatch_group_enter() 和 dispatch_group_leave() 需要成對出現(xiàn)的

Dispatch Block

隊(duì)列執(zhí)行任務(wù)都是block的方式，

• 創(chuàng)建block

- (void)createDispatchBlock {
    //normal way
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.starming.gcddemo.concurrentqueue",DISPATCH_QUEUE_CONCURRENT);
    dispatch_block_t block = dispatch_block_create(0, ^{
        NSLog(@"run block");
    });
    dispatch_async(concurrentQueue, block);

    //QOS way
    dispatch_block_t qosBlock = dispatch_block_create_with_qos_class(0, QOS_CLASS_USER_INITIATED, -1, ^{
        NSLog(@"run qos block");
    });
    dispatch_async(concurrentQueue, qosBlock);
}

• dispatch_block_wait：可以根據(jù)dispatch block來設(shè)置等待時(shí)間捞挥，參數(shù)DISPATCH_TIME_FOREVER會一直等待block結(jié)束

- (void)dispatchBlockWaitDemo {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.starming.gcddemo.serialqueue", DISPATCH_QUEUE_SERIAL);
    dispatch_block_t block = dispatch_block_create(0, ^{
        NSLog(@"star");
        [NSThread sleepForTimeInterval:5.f];
        NSLog(@"end");
    });
    dispatch_async(serialQueue, block);
    //設(shè)置DISPATCH_TIME_FOREVER會一直等到前面任務(wù)都完成
    dispatch_block_wait(block, DISPATCH_TIME_FOREVER);
    NSLog(@"ok, now can go on");
}

• dispatch_block_notify：可以監(jiān)視指定dispatch block結(jié)束浮创，然后再加入一個(gè)block到隊(duì)列中。三個(gè)參數(shù)分別為砌函，第一個(gè)是需要監(jiān)視的block斩披，第二個(gè)參數(shù)是需要提交執(zhí)行的隊(duì)列，第三個(gè)是待加入到隊(duì)列中的block

- (void)dispatchBlockNotifyDemo {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.starming.gcddemo.serialqueue", DISPATCH_QUEUE_SERIAL);
    dispatch_block_t firstBlock = dispatch_block_create(0, ^{
        NSLog(@"first block start");
        [NSThread sleepForTimeInterval:2.f];
        NSLog(@"first block end");
    });
    dispatch_async(serialQueue, firstBlock);
    dispatch_block_t secondBlock = dispatch_block_create(0, ^{
        NSLog(@"second block run");
    });
    //first block執(zhí)行完才在serial queue中執(zhí)行second block
    dispatch_block_notify(firstBlock, serialQueue, secondBlock);
}

• dispatch_block_cancel：iOS8后GCD支持對dispatch block的取消

- (void)dispatchBlockCancelDemo {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.starming.gcddemo.serialqueue", DISPATCH_QUEUE_SERIAL);
    dispatch_block_t firstBlock = dispatch_block_create(0, ^{
        NSLog(@"first block start");
        [NSThread sleepForTimeInterval:2.f];
        NSLog(@"first block end");
    });
    dispatch_block_t secondBlock = dispatch_block_create(0, ^{
        NSLog(@"second block run");
    });
    dispatch_async(serialQueue, firstBlock);
    dispatch_async(serialQueue, secondBlock);
    //取消secondBlock
    dispatch_block_cancel(secondBlock);
}

使用dispatch block object（調(diào)度塊）在任務(wù)執(zhí)行前進(jìn)行取消

dispatch block object可以為隊(duì)列中的對象設(shè)置
示例讹俊，下載圖片中途進(jìn)行取消

func downloadPhotosWithCompletion(completion: BatchPhotoDownloadingCompletionClosure?) {
     var storedError: NSError!
     let downloadGroup = dispatch_group_create()
     var addresses = [OverlyAttachedGirlfriendURLString,
          SuccessKidURLString,
          LotsOfFacesURLString]
     addresses += addresses + addresses // 擴(kuò)展address數(shù)組垦沉，復(fù)制3份
     var blocks: [dispatch_block_t] = [] // 一個(gè)保存block的數(shù)組

     for i in 0 ..< addresses.count {
          dispatch_group_enter(downloadGroup)
          let block = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS) { // 創(chuàng)建一個(gè)block，block的標(biāo)志是DISPATCH_BLOCK_INHERIT_QOS_CLASS
               let index = Int(i)
               let address = addresses[index]
               let url = NSURL(string: address)
               let photo = DownloadPhoto(url: url!) {
                    image, error in
                    if let error = error {
                         storedError = error
                    }
                    dispatch_group_leave(downloadGroup)
               }
               PhotoManager.sharedManager.addPhoto(photo)
          }
          blocks.append(block)
          dispatch_async(GlobalMainQueue, block) // 把這個(gè)block放到GlobalMainQueue上異步調(diào)用仍劈。因?yàn)槿株?duì)列是一個(gè)順序隊(duì)列所以方便取消對象block厕倍，同時(shí)可以保證下載任務(wù)在downloadPhotosWithCompletion返回后才開始執(zhí)行。
     }

     for block in blocks[3 ..< blocks.count] {
          let cancel = arc4random_uniform(2) // 隨機(jī)返回一個(gè)整數(shù)贩疙，會返回0或1
          if cancel == 1 {
               dispatch_block_cancel(block) // 如果是1就取消block讹弯，這個(gè)只能發(fā)生在block還在隊(duì)列中并沒有開始的情況下。因?yàn)榘裝lock已經(jīng)放到了GlobalMainQueue中这溅，所以這個(gè)地方會先執(zhí)行组民，執(zhí)行完了才會執(zhí)行block。
               dispatch_group_leave(downloadGroup) // 因?yàn)橐呀?jīng)dispatch_group_enter了悲靴，所以取消時(shí)也要將其都leave掉臭胜。
          }
     }

     dispatch_group_notify(downloadGroup, GlobalMainQueue) {
          if let completion = completion {
               completion(error: storedError)
          }
     }
}

Dispatch IO 文件操作

dispatch io讀取文件的方式類似于下面的方式，多個(gè)線程去讀取文件的切片數(shù)據(jù)，對于大的數(shù)據(jù)文件這樣會比單線程要快很多耸三。

dispatch_async(queue,^{/*read 0-99 bytes*/});
dispatch_async(queue,^{/*read 100-199 bytes*/});
dispatch_async(queue,^{/*read 200-299 bytes*/});

• dispatch_io_create：創(chuàng)建dispatch io
• dispatch_io_set_low_water：指定切割文件大小
• dispatch_io_read：讀取切割的文件然后合并乱陡。

蘋果系統(tǒng)日志API里用到了這個(gè)技術(shù)，可以在這里查看：https://github.com/Apple-FOSS-Mirror/Libc/blob/2ca2ae74647714acfc18674c3114b1a5d3325d7d/gen/asl.c

pipe_q = dispatch_queue_create("PipeQ", NULL);
//創(chuàng)建
pipe_channel = dispatch_io_create(DISPATCH_IO_STREAM, fd, pipe_q, ^(int err){
    close(fd);
});

*out_fd = fdpair[1];
//設(shè)置切割大小
dispatch_io_set_low_water(pipe_channel, SIZE_MAX);

dispatch_io_read(pipe_channel, 0, SIZE_MAX, pipe_q, ^(bool done, dispatch_data_t pipedata, int err){
    if (err == 0)
    {
        size_t len = dispatch_data_get_size(pipedata);
        if (len > 0)
        {
            //對每次切塊數(shù)據(jù)的處理
            const char *bytes = NULL;
            char *encoded;
            uint32_t eval;

            dispatch_data_t md = dispatch_data_create_map(pipedata, (const void **)&bytes, &len);
            encoded = asl_core_encode_buffer(bytes, len);
            asl_msg_set_key_val(aux, ASL_KEY_AUX_DATA, encoded);
            free(encoded);
            eval = _asl_evaluate_send(NULL, (aslmsg)aux, -1);
            _asl_send_message(NULL, eval, aux, NULL);
            asl_msg_release(aux);
            dispatch_release(md);
        }
    }

    if (done)
    {
        //semaphore +1使得不需要再等待繼續(xù)執(zhí)行下去仪壮。
        dispatch_semaphore_signal(sem);
        dispatch_release(pipe_channel);
        dispatch_release(pipe_q);
    }
});

Dispatch Source 用GCD監(jiān)視進(jìn)程

Dispatch Source用于監(jiān)聽系統(tǒng)的底層對象蛋褥，比如文件描述符，Mach端口睛驳，信號量等。主要處理的事件如下表

方法

• dispatch_source_create：創(chuàng)建dispatch source膜廊，創(chuàng)建后會處于掛起狀態(tài)進(jìn)行事件接收乏沸，需要設(shè)置事件處理handler進(jìn)行事件處理。
• dispatch_source_set_event_handler：設(shè)置事件處理handler
• dispatch_source_set_cancel_handler：事件取消handler爪瓜，就是在dispatch source釋放前做些清理的事蹬跃。
• dispatch_source_cancel：關(guān)閉dispatch source，設(shè)置的事件處理handler不會被執(zhí)行铆铆，已經(jīng)執(zhí)行的事件handler不會取消蝶缀。

NSRunningApplication *mail = [NSRunningApplication runningApplicationsWithBundleIdentifier:@"com.apple.mail"];
if (mail == nil) {
     return;
}
pid_t const pid = mail.processIdentifier;
self.source = dispatch_source_create(DISPATCH_SOURCE_TYPE_PROC, pid, DISPATCH_PROC_EXIT, DISPATCH_TARGET_QUEUE_DEFAULT);
dispatch_source_set_event_handler(self.source, ^(){
     NSLog(@"Mail quit.");
});
//在事件源傳到你的事件處理前需要調(diào)用dispatch_resume()這個(gè)方法
dispatch_resume(self.source);

監(jiān)視文件夾內(nèi)文件變化

NSURL *directoryURL; // assume this is set to a directory
int const fd = open([[directoryURL path] fileSystemRepresentation], O_EVTONLY);
if (fd < 0) {
     char buffer[80];
     strerror_r(errno, buffer, sizeof(buffer));
     NSLog(@"Unable to open \"%@\": %s (%d)", [directoryURL path], buffer, errno);
     return;
}
dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE, fd,
DISPATCH_VNODE_WRITE | DISPATCH_VNODE_DELETE, DISPATCH_TARGET_QUEUE_DEFAULT);
dispatch_source_set_event_handler(source, ^(){
     unsigned long const data = dispatch_source_get_data(source);
     if (data & DISPATCH_VNODE_WRITE) {
          NSLog(@"The directory changed.");
     }
     if (data & DISPATCH_VNODE_DELETE) {
          NSLog(@"The directory has been deleted.");
     }
});
dispatch_source_set_cancel_handler(source, ^(){
     close(fd);
});
self.source = source;
dispatch_resume(self.source);
//還要注意需要用DISPATCH_VNODE_DELETE 去檢查監(jiān)視的文件或文件夾是否被刪除，如果刪除了就停止監(jiān)聽

NSTimer在主線程的runloop里會在runloop切換其它模式時(shí)停止薄货，這時(shí)就需要手動在子線程開啟一個(gè)模式為NSRunLoopCommonModes的runloop翁都，如果不想開啟一個(gè)新的runloop可以用不跟runloop關(guān)聯(lián)的dispatch source timer，如下谅猾。

dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER,0, 0, DISPATCH_TARGET_QUEUE_DEFAULT);
dispatch_source_set_event_handler(source, ^(){
     NSLog(@"Time flies.");
});
dispatch_time_t start
dispatch_source_set_timer(source, DISPATCH_TIME_NOW, 5ull * NSEC_PER_SEC,100ull * NSEC_PER_MSEC);
self.source = source;
dispatch_resume(self.source);

Dispatch Semaphore和的介紹

另外一種保證同步的方法柄慰。使用dispatch_semaphore_signal加1dispatch_semaphore_wait減1，為0時(shí)等待的設(shè)置方式來達(dá)到線程同步的目的和同步鎖一樣能夠解決資源搶占的問題税娜。

//dispatch semaphore
- (void)dispatchSemaphoreDemo {
    //創(chuàng)建semaphore
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"start");
        [NSThread sleepForTimeInterval:1.f];
        NSLog(@"semaphore +1");
        dispatch_semaphore_signal(semaphore); //+1 semaphore
    });
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    NSLog(@"continue");
}

鎖

這里簡單介紹下iOS中常用的各種鎖和他們的性能坐搔。

• NSRecursiveLock：遞歸鎖，可以在一個(gè)線程中反復(fù)獲取鎖不會造成死鎖敬矩，這個(gè)過程會記錄獲取鎖和釋放鎖的次數(shù)來達(dá)到何時(shí)釋放的作用概行。
• NSDistributedLock：分布鎖，基于文件方式的鎖機(jī)制弧岳，可以跨進(jìn)程訪問凳忙。
• NSConditionLock：條件鎖，用戶定義條件禽炬，確保一個(gè)線程可以獲取滿足一定條件的鎖消略。因?yàn)榫€程間競爭會涉及到條件鎖檢測，系統(tǒng)調(diào)用上下切換頻繁導(dǎo)致耗時(shí)是幾個(gè)鎖里最長的瞎抛。
• OSSpinLock：自旋鎖艺演，不進(jìn)入內(nèi)核，減少上下文切換，性能最高胎撤，但搶占多時(shí)會占用較多cpu晓殊，好點(diǎn)多，這時(shí)使用pthread_mutex較好伤提。
• pthread_mutex_t：同步鎖基于C語言巫俺，底層api性能高，使用方法和其它的類似肿男。
• @synchronized：更加簡單介汹。

dispatch_suspend和dispatch_resume掛起和恢復(fù)隊(duì)列

dispatch_suspend這里掛起不會暫停正在執(zhí)行的block，只是能夠暫停還沒執(zhí)行的block舶沛。

dispatch_set_context和dispatch_get_context

GCD深入操作

• 緩沖區(qū)：dispatch_data_t基于零碎的內(nèi)存區(qū)域嘹承，使用dispatch_data_apply來遍歷，還可以用dispatch_data_create_subrange來創(chuàng)建一個(gè)不做任何拷貝的子區(qū)域
• I/O調(diào)度：使用GCD提供的dispatch_io_read如庭，dispatch_io_write和dispatch_io_close
• 測試：使用dispatch_benchmark小工具
• 原子操作： libkern/OSAtomic.h里可以查看那些函數(shù)叹卷，用于底層多線程編程。

GCD死鎖

當(dāng)前串行隊(duì)列里面同步執(zhí)行當(dāng)前串行隊(duì)列就會死鎖坪它，解決的方法就是將同步的串行隊(duì)列放到另外一個(gè)線程就能夠解決骤竹。

- (void)deadLockCase1 {
    NSLog(@"1");
    //主隊(duì)列的同步線程，按照FIFO的原則（先入先出）往毡，2排在3后面會等3執(zhí)行完蒙揣，但因?yàn)橥骄€程，3又要等2執(zhí)行完开瞭，相互等待成為死鎖鸣奔。
    dispatch_sync(dispatch_get_main_queue(), ^{
        NSLog(@"2");
    });
    NSLog(@"3");
}
- (void)deadLockCase2 {
    NSLog(@"1");
    //3會等2，因?yàn)?在全局并行隊(duì)列里惩阶，不需要等待3挎狸，這樣2執(zhí)行完回到主隊(duì)列，3就開始執(zhí)行
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        NSLog(@"2");
    });
    NSLog(@"3");
}
- (void)deadLockCase3 {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.starming.gcddemo.serialqueue", DISPATCH_QUEUE_SERIAL);
    NSLog(@"1");
    dispatch_async(serialQueue, ^{
        NSLog(@"2");
        //串行隊(duì)列里面同步一個(gè)串行隊(duì)列就會死鎖
        dispatch_sync(serialQueue, ^{
            NSLog(@"3");
        });
        NSLog(@"4");
    });
    NSLog(@"5");
}
- (void)deadLockCase4 {
    NSLog(@"1");
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"2");
        //將同步的串行隊(duì)列放到另外一個(gè)線程就能夠解決
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"3");
        });
        NSLog(@"4");
    });
    NSLog(@"5");
}
- (void)deadLockCase5 {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"1");
        //回到主線程發(fā)現(xiàn)死循環(huán)后面就沒法執(zhí)行了
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"2");
        });
        NSLog(@"3");
    });
    NSLog(@"4");
    //死循環(huán)
    while (1) {
        //
    }
}

GCD實(shí)際使用

FMDB如何使用dispatch_queue_set_specific和dispatch_get_specific來防止死鎖

作用類似objc_setAssociatedObject跟objc_getAssociatedObject

static const void * const kDispatchQueueSpecificKey = &kDispatchQueueSpecificKey;
//創(chuàng)建串行隊(duì)列断楷，所有數(shù)據(jù)庫的操作都在這個(gè)隊(duì)列里
_queue = dispatch_queue_create([[NSString stringWithFormat:@"fmdb.%@", self] UTF8String], NULL);
//標(biāo)記隊(duì)列
dispatch_queue_set_specific(_queue, kDispatchQueueSpecificKey, (__bridge void *)self, NULL);

//檢查是否是同一個(gè)隊(duì)列來避免死鎖的方法
- (void)inDatabase:(void (^)(FMDatabase *db))block {
    FMDatabaseQueue *currentSyncQueue = (__bridge id)dispatch_get_specific(kDispatchQueueSpecificKey);
    assert(currentSyncQueue != self && "inDatabase: was called reentrantly on the same queue, which would lead to a deadlock");
}

iOS系統(tǒng)版本新特性

iOS8

iOS8新加了一個(gè)功能叫Quality of Service(QoS)锨匆，里面提供了一下幾個(gè)更容易理解的枚舉名來使用user interactive，user initiated冬筒，utility和background恐锣。下面的表做了對比

參考資料

WWDC

• Building Responsive and Efficient Apps with GCD：https://developer.apple.com/videos/play/wwdc2015-718/

文檔

• 官方文檔：https://developer.apple.com/library/prerelease/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/