IOS中GCD的队列分为串行队列和并行队列，任务分为同步任务和异步任务，他们的排列组合有四种情况，下面分析这四种情况的工作方式。
同步任务，使用GCD dispatch_sync 进行派发任务
- (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);
});
}
}
结果如下：
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方法是在主线程中调用的，结果看到使用的串行队列和使用并行队列看到的结果都是发生在当前线程：主线程中，没有开启新的线程处理任务，任务的调度也是串行调度的。
异步任务，使用GCD dispatch_async 进行派发任务
- (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);
});
}
}
结果如下：
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方法是在主线程中调用的，结果看到使用的串行队列的异步任务会开启一个子线程执行任务，任务的调度是串行的
使用并行队列的异步任务会开启多个子线程并行的处理任务，任务的先后顺序是不固定的，任务的调度方式是并行的
总结
同步任务：和使用的队列无关，不会开启子线程处理任务，会在当前的线程中串行的调度任务，即一个任务完成之后继续下一个任务，如果同步任务在主线程中调用，会阻塞主线程
异步任务：a. 使用串行队列，会开启一个子线程串行的调度任务 b. 使用并行队列，会开启多个子线程并行的调度任务，这种情况用的是最多的。
