同步执行 + 主队列

使用 NSThread 的 detachNewThreadSelector 方法会创建线程，并自动启动线程执行 selector 任务
[NSThread detachNewThreadSelector:@selector(syncMain) toTarget:self withObject:nil];

/**
 * 同步执行 + 主队列
 * 特点(主线程调用)：互等卡主不执行。
 * 特点(其他线程调用)：不会开启新线程，执行完一个任务，再执行下一个任务。
 */
- (void)syncMain {
    //打印当前线程
    NSLog(@"currentThread---%@",[NSThread currentThread]);
    NSLog(@"syncMain---begin");
    dispatch_queue_t queue = dispatch_get_main_queue();
    dispatch_sync(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_sync(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_sync(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(@"syncMain---end");
}
运行结果：
2020-07-06 21:05:31.879924+0800 GCD[16452:3198245] currentThread---<NSThread: 0x6000039819c0>{number = 7, name = (null)}
2020-07-06 21:05:31.880029+0800 GCD[16452:3198245] syncMain---begin
2020-07-06 21:05:33.890953+0800 GCD[16452:3198120] 1---<NSThread: 0x6000039eed00>{number = 1, name = main}
2020-07-06 21:05:35.891477+0800 GCD[16452:3198120] 1---<NSThread: 0x6000039eed00>{number = 1, name = main}
2020-07-06 21:05:37.902365+0800 GCD[16452:3198120] 2---<NSThread: 0x6000039eed00>{number = 1, name = main}
2020-07-06 21:05:39.902559+0800 GCD[16452:3198120] 2---<NSThread: 0x6000039eed00>{number = 1, name = main}
2020-07-06 21:05:41.903112+0800 GCD[16452:3198120] 3---<NSThread: 0x6000039eed00>{number = 1, name = main}
2020-07-06 21:05:43.903433+0800 GCD[16452:3198120] 3---<NSThread: 0x6000039eed00>{number = 1, name = main}
2020-07-06 21:05:43.903644+0800 GCD[16452:3198245] syncMain---end