1、GCD知识讲解
1）dispatch_semaphore_create(long value) //创建一个信号量，总量为value，value不能小于0
2）dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout) //如果信号总量为0，进入等待状态，信号量大于0时，继续执行代码，同时将 信号量减1
3）dispatch_semaphore_signal(dispatch_semaphore_t desma) //发送信号量，信号量就会加1
2、例子
dispatch_semaphore_t lock = dispatch_semaphore_create();
for(int i=;i<;i++){
dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
dispatch_async(dispatch_get_global_queue(, ), ^{
NSLog(@"我是第%d进入",i);
dispatch_semaphore_signal(lock);
});
}