异步执行 + 串行队列

/**
 * 异步执行 + 串行队列
 * 特点：会开启新线程，但是因为任务是串行的，执行完一个任务，再执行下一个任务。
 */
- (void)asyncSerial {
    //打印当前线程
    NSLog(@"currentThread---%@",[NSThread currentThread]);
    NSLog(@"asyncSerial---begin");
    dispatch_queue_t queue = dispatch_queue_create("net.bujige.testQueue", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        // 追加任务1
        for (int i = 0; i < 2; ++i) {
            //模拟耗时操作
            [NSThread sleepForTimeInterval:2];
            //打印当前线程
            NSLog(@"1---%@",[NSThread currentThread]);
        }
    });
    dispatch_async(queue, ^{
        // 追加任务2
        for (int i = 0; i < 2; ++i) {
            //模拟耗时操作
            [NSThread sleepForTimeInterval:2];
            //打印当前线程
            NSLog(@"2---%@",[NSThread currentThread]);
        }
    });
    dispatch_async(queue, ^{
        // 追加任务3
        for (int i = 0; i < 2; ++i) {
            //模拟耗时操作
            [NSThread sleepForTimeInterval:2];
            //打印当前线程
            NSLog(@"3---%@",[NSThread currentThread]);
        }
    });
    NSLog(@"asyncSerial---end");
}
运行结果：
2020-07-06 20:55:53.020073+0800 GCD[16251:3139547] currentThread---<NSThread: 0x6000005b0180>{number = 1, name = main}
2020-07-06 20:55:53.020181+0800 GCD[16251:3139547] asyncSerial---begin
2020-07-06 20:55:53.020276+0800 GCD[16251:3139547] asyncSerial---end
2020-07-06 20:55:55.022406+0800 GCD[16251:3139638] 1---<NSThread: 0x6000005da900>{number = 7, name = (null)}
2020-07-06 20:55:57.024138+0800 GCD[16251:3139638] 1---<NSThread: 0x6000005da900>{number = 7, name = (null)}
2020-07-06 20:55:59.029368+0800 GCD[16251:3139638] 2---<NSThread: 0x6000005da900>{number = 7, name = (null)}
2020-07-06 20:56:01.032945+0800 GCD[16251:3139638] 2---<NSThread: 0x6000005da900>{number = 7, name = (null)}
2020-07-06 20:56:03.033199+0800 GCD[16251:3139638] 3---<NSThread: 0x6000005da900>{number = 7, name = (null)}
2020-07-06 20:56:05.033421+0800 GCD[16251:3139638] 3---<NSThread: 0x6000005da900>{number = 7, name = (null)}