Martin总结：多线程题

多线程题：A、B完成之后去做D，B、C完成之后去做E

第一种思路：利用GCD的group来完成。

把ABC放进一个队列里面，然后group1来监听AB任务，group2监听BC任务，最后执行notify任务

 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    dispatch_group_t group1  = dispatch_group_create();
    dispatch_group_t group2  = dispatch_group_create();
    
    dispatch_group_enter(group1);
    dispatch_async(queue, ^{
        NSLog(@"A开始");
        sleep(arc4random_uniform(10));
        NSLog(@"A结束");
        dispatch_group_leave(group1);
    });
    
    dispatch_group_enter(group1);
    dispatch_group_enter(group2);
    dispatch_async(queue, ^{
        NSLog(@"B开始");
        sleep(arc4random_uniform(3));
        NSLog(@"B结束");
        dispatch_group_leave(group2);
        dispatch_group_leave(group1);
    });
    
    dispatch_group_enter(group2);
    dispatch_async(queue, ^{
        NSLog(@"C开始");
        sleep(arc4random_uniform(10));
        NSLog(@"C结束");
        dispatch_group_leave(group2);
    });
    
    dispatch_group_notify(group1, queue, ^{
        NSLog(@"D开始");
        NSLog(@"D结束");
    });
    
    dispatch_group_notify(group2, queue, ^{
        NSLog(@"E开始");
        NSLog(@"E结束");
    });

第二种思路：利用信号量来完成

dispatch_semaphore_t a = dispatch_semaphore_create(0);
dispatch_semaphore_t b = dispatch_semaphore_create(0);
dispatch_semaphore_t c = dispatch_semaphore_create(0);

dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(queue, ^{
    NSLog(@"A开始");
    sleep(arc4random_uniform(10));
    NSLog(@"A结束");
    dispatch_semaphore_signal(a);
});
dispatch_async(queue, ^{
    NSLog(@"B开始");
    sleep(arc4random_uniform(10));
    NSLog(@"B结束");
    dispatch_semaphore_signal(b);
    dispatch_semaphore_signal(b);
});
dispatch_async(queue, ^{
    NSLog(@"C开始");
    sleep(arc4random_uniform(10));
    NSLog(@"C结束");
    dispatch_semaphore_signal(c);
});


dispatch_async(queue, ^{
    
    dispatch_semaphore_wait(a, DISPATCH_TIME_FOREVER);
    dispatch_semaphore_wait(b, DISPATCH_TIME_FOREVER);
    NSLog(@"D开始");
    NSLog(@"D结束");
});
dispatch_async(queue, ^{
    dispatch_semaphore_wait(b, DISPATCH_TIME_FOREVER);
    dispatch_semaphore_wait(c, DISPATCH_TIME_FOREVER);
    NSLog(@"E开始");
    NSLog(@"E结束");
});