iOS 按顺序异步执行线程

GCD方式，关键是信号量的三个重要用法:
创建信号量为0dispatch_semaphore_t sema = dispatch_semaphore_create(0);
进入信号量dispatch_semaphore_signal(sema);这里请求信号量 +1 为1
等待信号量dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);走到这里如果信号量为0 则不再执行下面的代码 一直等待 信号量不是0 出现 才会执行下面代码,然后信号量为 - 1

//创建线程池
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    //创建group
    dispatch_group_t group = dispatch_group_create();
    //创建信号量
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    
    for (int i = 0; i < 10; i++) {
        dispatch_group_async(group, queue, ^{
            NSLog(@"task%i",i);
            //进入信号量
            dispatch_semaphore_signal(sema);
        });
        //等待信号量
        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    }
//结束后
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        dispatch_group_async(group, queue, ^{
            NSLog(@"task11");
            //进入信号量
        });
    });