詳解IOS串行隊列與並行隊列進行同步或者異步的實例

IOS中GCD的隊列分為串行隊列和並行隊列，任務分為同步任務和異步任務，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，使用GCD dispatch_sync 進行派發任務

- (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用的，結果看到使用的串行隊列和使用並行隊列看到的結果都是發生在當前線程：主線程中，沒有開啟新的線程處理任務，任務的調度也是串行調度的。

異步任務，使用GCD dispatch_async 進行派發任務

- (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用的，結果看到使用的串行隊列的異步任務會開啟一個子線程執行任務，任務的調度是串行的
使用並行隊列的異步任務會開啟多個子線程並行的處理任務，任務的先後順序是不固定的，任務的調度方式是並行的

總結

同步任務：和使用的隊列無關，不會開啟子線程處理任務，會在當前的線程中串行的調度任務，即一個任務完成之後繼續下一個任務，如果同步任務在主線程中調用，會阻塞主線程

異步任務：a. 使用串行隊列，會開啟一個子線程串行的調度任務 b. 使用並行隊列，會開啟多個子線程並行的調度任務，這種情況用的是最多的。

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