IOS中GCD的隊(duì)列分為串行隊(duì)列和并行隊(duì)列，任務(wù)分為同步任務(wù)和異步任務(wù)，他們的排列組合有四種情況，下面分析這四種情況的工作方式。

同步任務(wù)，使用GCD dispatch_sync 進(jìn)行派發(fā)任務(wù)

- (void)testSync {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.zyt.queue", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.zyt.queue", DISPATCH_QUEUE_CONCURRENT);

    NSLog(@"====serialQueue====");
    for (int i = 0; i<10; i++) {
        dispatch_sync(serialQueue, ^{
            [NSThread  sleepForTimeInterval:0.3];
            NSLog(@"==>%@  sync serial XXX>%d", [NSThread currentThread], i);
        });
    }
    
    NSLog(@"====concurrentQueue====");
    for (int i = 0; i<10; i++) {
        dispatch_sync(concurrentQueue, ^{
            [NSThread  sleepForTimeInterval:0.3];
            NSLog(@"==>%@  sync concurrent ====>%d", [NSThread currentThread], i*i);
        });
    }
}

結(jié)果如下：

2017-03-01 01:36:22.835 Demo ====serialQueue====
2017-03-01 01:36:23.207 {number = 1, name = main}  sync serial XXX>0
2017-03-01 01:36:23.578 {number = 1, name = main}  sync serial XXX>1
2017-03-01 01:36:23.952 {number = 1, name = main}  sync serial XXX>2
2017-03-01 01:36:24.325 {number = 1, name = main}  sync serial XXX>3
2017-03-01 01:36:24.699 {number = 1, name = main}  sync serial XXX>4
2017-03-01 01:36:25.072 {number = 1, name = main}  sync serial XXX>5
2017-03-01 01:36:25.446 {number = 1, name = main}  sync serial XXX>6
2017-03-01 01:36:25.746 {number = 1, name = main}  sync serial XXX>7
2017-03-01 01:36:26.122 {number = 1, name = main}  sync serial XXX>8
2017-03-01 01:36:26.489 {number = 1, name = main}  sync serial XXX>9
2017-03-01 01:36:26.489 Demo ====concurrentQueue====
2017-03-01 01:36:26.864 {number = 1, name = main}  sync concurrent ====>0
2017-03-01 01:36:27.236 {number = 1, name = main}  sync concurrent ====>1
2017-03-01 01:36:27.611 {number = 1, name = main}  sync concurrent ====>4
2017-03-01 01:36:27.985 {number = 1, name = main}  sync concurrent ====>9
2017-03-01 01:36:28.354 {number = 1, name = main}  sync concurrent ====>16
2017-03-01 01:36:28.726 {number = 1, name = main}  sync concurrent ====>25
2017-03-01 01:36:29.100 {number = 1, name = main}  sync concurrent ====>36
2017-03-01 01:36:29.474 {number = 1, name = main}  sync concurrent ====>49
2017-03-01 01:36:29.849 {number = 1, name = main}  sync concurrent ====>64
2017-03-01 01:36:30.223 {number = 1, name = main}  sync concurrent ====>81

testSync方法是在主線程中調(diào)用的，結(jié)果看到使用的串行隊(duì)列和使用并行隊(duì)列看到的結(jié)果都是發(fā)生在當(dāng)前線程：主線程中，沒有開啟新的線程處理任務(wù)，任務(wù)的調(diào)度也是串行調(diào)度的。

異步任務(wù)，使用GCD dispatch_async 進(jìn)行派發(fā)任務(wù)

- (void)testAsync {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.zyt.queue", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.zyt.queue", DISPATCH_QUEUE_CONCURRENT);
    
    NSLog(@"====serialQueue====");
    for (int i = 0; i<10; i++) {
        dispatch_async(serialQueue, ^{
            [NSThread sleepForTimeInterval:0.3];
            NSLog(@"==>%@  async serial XXX>%d", [NSThread currentThread], i);
        });
    }
    
    NSLog(@"====concurrentQueue====");
    for (int i = 0; i<10; i++) {
        dispatch_async(concurrentQueue, ^{
            [NSThread  sleepForTimeInterval:0.3];
            NSLog(@"==>%@  async concurrent ====>%d", [NSThread currentThread], i*i);
        });
    }
}

結(jié)果如下：

2017-03-01 01:45:36.125 Demo ====serialQueue====
2017-03-01 01:45:36.125 Demo ====concurrentQueue====
2017-03-01 01:45:36.494 {number = 3, name = (null)}  async concurrent ====>0
2017-03-01 01:45:36.494 {number = 5, name = (null)}  async concurrent ====>4
2017-03-01 01:45:36.494 {number = 4, name = (null)}  async concurrent ====>1
2017-03-01 01:45:36.494 {number = 6, name = (null)}  async concurrent ====>16
2017-03-01 01:45:36.494 {number = 8, name = (null)}  async serial XXX>0
2017-03-01 01:45:36.494 {number = 7, name = (null)}  async concurrent ====>9
2017-03-01 01:45:36.494 {number = 9, name = (null)}  async concurrent ====>25
2017-03-01 01:45:36.494 {number = 11, name = (null)}  async concurrent ====>49
2017-03-01 01:45:36.494 {number = 10, name = (null)}  async concurrent ====>36
2017-03-01 01:45:36.501 {number = 13, name = (null)}  async concurrent ====>81
2017-03-01 01:45:36.501 {number = 12, name = (null)}  async concurrent ====>64
2017-03-01 01:45:36.869 {number = 8, name = (null)}  async serial XXX>1
2017-03-01 01:45:37.244 {number = 8, name = (null)}  async serial XXX>2
2017-03-01 01:45:37.615 {number = 8, name = (null)}  async serial XXX>3
2017-03-01 01:45:37.986 {number = 8, name = (null)}  async serial XXX>4
2017-03-01 01:45:38.358 {number = 8, name = (null)}  async serial XXX>5
2017-03-01 01:45:38.730 {number = 8, name = (null)}  async serial XXX>6
2017-03-01 01:45:39.103 {number = 8, name = (null)}  async serial XXX>7
2017-03-01 01:45:39.472 {number = 8, name = (null)}  async serial XXX>8
2017-03-01 01:45:39.842 {number = 8, name = (null)}  async serial XXX>9

testSync方法是在主線程中調(diào)用的，結(jié)果看到使用的串行隊(duì)列的異步任務(wù)會(huì)開啟一個(gè)子線程執(zhí)行任務(wù)，任務(wù)的調(diào)度是串行的
使用并行隊(duì)列的異步任務(wù)會(huì)開啟多個(gè)子線程并行的處理任務(wù)，任務(wù)的先后順序是不固定的，任務(wù)的調(diào)度方式是并行的

總結(jié)

同步任務(wù)：和使用的隊(duì)列無關(guān)，不會(huì)開啟子線程處理任務(wù)，會(huì)在當(dāng)前的線程中串行的調(diào)度任務(wù)，即一個(gè)任務(wù)完成之后繼續(xù)下一個(gè)任務(wù)，如果同步任務(wù)在主線程中調(diào)用，會(huì)阻塞主線程
異步任務(wù)：a. 使用串行隊(duì)列，會(huì)開啟一個(gè)子線程串行的調(diào)度任務(wù) b. 使用并行隊(duì)列，會(huì)開啟多個(gè)子線程并行的調(diào)度任務(wù)，這種情況用的是最多的。