一、简介:
dispatch_semaphore_t:表示信号,生成信号的方法是
dispatch_semaphore_t semaphore= dispatch_semaphore_create(0);
其中参数0表示该新生成信号的总的信号量为0个。
dispatch_semaphore_wait:信号等待,它像一个安保,比如小区规定最多只能进入3辆车,而进入一辆车后名额就会减少一个,当剩下的名额为0的时候,再有汽车说要进去时,就只能在外面等待了,直到有名额闲置出来了,才能开进小区。
dispatch_semaphore_signal:信号释放,当有一辆车从小区出来时,就腾出来了一个名额。
二、验证
这三个常用方法解释就暂且到这里,下面用代码来验证下上面介绍的作用。
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
[self syncAction1];
}
- (void)syncAction1{
//使用GCD的信号量 dispatch_semaphore_t 创建同步请求
dispatch_group_t group =dispatch_group_create();
dispatch_queue_t globalQueue=dispatch_get_global_queue(0, 0);
dispatch_group_async(group, globalQueue, ^{
dispatch_semaphore_t semaphore= dispatch_semaphore_create(0);
//模拟网络多线程耗时操作
dispatch_group_async(group, globalQueue, ^{
sleep(3);
NSLog(@"%@---block1结束。。。",[NSThread currentThread]);
dispatch_semaphore_signal(semaphore);
});
NSLog(@"%@---1结束。。。",[NSThread currentThread]);
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
});
dispatch_group_async(group, globalQueue, ^{
dispatch_semaphore_t semaphore= dispatch_semaphore_create(0);
//模拟网络多线程耗时操作
dispatch_group_async(group, globalQueue, ^{
sleep(3);
NSLog(@"%@---block2结束。。。",[NSThread currentThread]);
dispatch_semaphore_signal(semaphore);
});
NSLog(@"%@---2结束。。。",[NSThread currentThread]);
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
});
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
NSLog(@"%@---全部结束。。。",[NSThread currentThread]);
});
}
运行APP,点击页面,打印的结果如下:
2016-12-22 23:45:36.633 GCDTest[852:27326] <NSThread: 0x7fce485154f0>{number = 2, name = (null)}---1结束。。。
2016-12-22 23:45:36.633 GCDTest[852:27328] <NSThread: 0x7fce48606f40>{number = 3, name = (null)}---2结束。。。
2016-12-22 23:45:39.633 GCDTest[852:27329] <NSThread: 0x7fce48512ea0>{number = 4, name = (null)}---block1结束。。。
2016-12-22 23:45:39.633 GCDTest[852:27330] <NSThread: 0x7fce484a9960>{number = 5, name = (null)}---block2结束。。。
2016-12-22 23:45:39.634 GCDTest[852:27328] <NSThread: 0x7fce48606f40>{number = 3, name = (null)}---全部结束。。。
结论:通过打印结果可以验证,
1.只有当剩余信号总量大于0时线程才能通过dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);方法,继续向下执行。
2.当dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);下面的代码无法执行处于等待情况下时,dispatch_semaphore_signal(semaphore);一旦执行,释放出自己的信号,则代码又可以愉快的往下执行了。
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
for (int i = 0; i < 100; i++)
{
dispatch_async(queue, ^{
// 相当于加锁
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"i = %zd semaphore = %@", i, semaphore); // 相当于解锁
dispatch_semaphore_signal(semaphore);
});
}
原文地址:https://www.cnblogs.com/sundaysgarden/p/10371986.html