dispatch_semaphore 信号量基于计数器的一种多线程同步机制。在多个线程访问共有资源时候，会因为多线程的特性而引发数据出错的问题。
和普通锁不一样，这个可以精细的控制并发量；
dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);
会创建三个信号量
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
每次wait都会消耗一个信号量，等待时间可以设置，这里设置为一直等待，如果当前信号量（semaphore）小于1，则一直等待
dispatch_semaphore_signal(semaphore);
发送信号量，发送一次，信号量+1
下面的demo可以并发执行 ，但每次只能并发3个线程；
因为消耗3次信号量后，就进入等待状态，直到sleep 2s完成后，会发送信号量dispatch_semaphore_signal
然后继续执行；
所以每次输出3个，直到循环完成；
dispatch_group_t group = dispatch_group_create();
dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
for (int i = 0; i < 90; i++)
{
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
dispatch_group_async(group, queue, ^{
NSLog(@"%i",i);
sleep(2);
dispatch_semaphore_signal(semaphore);
});
}
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dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
[[[NSURLSession sharedSession] downloadTaskWithURL:[NSURL URLWithString:@"http://xxxxxx.com"]
completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error)
{
//下载完后会发送一个信号量
dispatch_semaphore_signal(semaphore);
}] resume];
//设置超时时间
double timeout = 5;
dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, timeout*NSEC_PER_SEC);
//这里会等待，如果5秒内下载完成，会执行下去，返回0
//如果5秒没下载完成，也不会继续等待下去，但是返回值不为0
long result = dispatch_semaphore_wait(semaphore, time);
NSLog(@"result:----%ld",result);
- 上面一段代码可以控制多个下载任务的并发数量，并且可以通过 dispatch_semaphore_wait 可以对任务进行超时操作；
信号量属于底层工具，很强大，性能很高；
Paste_Image.png
- 第一个是自旋锁，苹果发现这个是有个bug，高级锁和低级锁争夺资源的时候会很消耗性能，所以已经很少用了，那么信号量作为锁来说性能是最高了；
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