1、dispatch_semaphore_create：创建一个Semaphore并初始化信号的总量
2、dispatch_semaphore_signal：发送一个信号，让信号总量加1
3、dispatch_semaphore_wait：可以使总信号量减1，当信号量为0时就是一直等待（阻塞当前线程），否则就可以正常执行。
注意：信号量的使用前提是：想清楚需要处理哪个线程等待（阻塞），又要哪个线程继续执行，然后使用信号量。
一、GCD 信号量实现线程同步，将异步操作转换成同步操作
信号量
初始化信号量
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
NSLog(@"2-------");
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:5];
NSLog(@"异步结果------");
dispatch_semaphore_signal(semaphore);
});
//信号量+1
NSLog(@"3-------");
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"4-------");
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"5-------");
截屏2022-09-07 下午4.10.11.png
二、线程安全 -- 多数据保护同一数据安全操作
dispatch_semaphore_t semaphoreLock = dispatch_semaphore_create(1);
self.ticketSurplusCount = 50;
//队列任务
dispatch_queue_t queue1 = dispatch_queue_create("net.bujige.testQueue1",
DISPATCH_QUEUE_SERIAL);
// 队列任务
dispatch_queue_t queue2 = dispatch_queue_create("net.bujige.testQueue2",
DISPATCH_QUEUE_SERIAL);
__weak typeof(self) weakSelf = self;
dispatch_async(queue1, ^{
[weakSelf saleTicketSafe];
});
dispatch_async(queue2, ^{
[weakSelf saleTicketSafe];
});
- (void)saleTicketSafe {
while (1) {
dispatch_semaphore_wait(semaphoreLock, DISPATCH_TIME_FOREVER);
if (self.ticketSurplusCount > 0) {
self.ticketSurplusCount--;
[NSThread sleepForTimeInterval:0.2];
} else {
}
dispatch_semaphore_signal(semaphoreLock);
break;
}
dispatch_semaphore_signal(semaphoreLock);
}
}
