A，B,C三个任务并发执行，但是C要等A，B执行完成之后再执行。
信号量:
// 创建一个信号，value：信号量
dispatch_semaphore_create(<#long value#>)
// 使某个信号的信号量+1
dispatch_semaphore_signal(<#dispatch_semaphore_t dsema#>)
// 某个信号进行等待或等待降低信号量 timeout：等待时间，永远等待为 DISPATCH_TIME_FOREVER
dispatch_semaphore_wait(<#dispatch_semaphore_t dsema#>, <#dispatch_time_t timeout#>)
正常的使用顺序是先降低然后再提高，这两个函数通常成对使用。
两种方法
-(void)dispatch_group_function1
{
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_queue_create("com.dispatch.test", DISPATCH_QUEUE_CONCURRENT), ^{
NSURLRequest *request = [[NSURLRequest alloc] initWithURL:[NSURL URLWithString:@"https://www.baidu.com"]];
NSURLSessionDownloadTask *task = [[NSURLSession sharedSession] downloadTaskWithRequest:request completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
// 请求完成，可以通知界面刷新界面等操作
NSLog(@"第一步网络请求完成");
// 使信号的信号量+1，这里的信号量本来为0，+1信号量为1(绿灯)
dispatch_semaphore_signal(semaphore);
}];
[task resume];
// 以下还要进行一些其他的耗时操作
NSLog(@"耗时操作继续进行");
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
});
dispatch_group_async(group, dispatch_queue_create("com.dispatch.test", DISPATCH_QUEUE_CONCURRENT), ^{
NSURLRequest *request = [[NSURLRequest alloc] initWithURL:[NSURL URLWithString:@"https://www.github.com"]];
NSURLSessionDownloadTask *task = [[NSURLSession sharedSession] downloadTaskWithRequest:request completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
// 请求完成，可以通知界面刷新界面等操作
NSLog(@"第二步网络请求完成");
// 使信号的信号量+1，这里的信号量本来为0，+1信号量为1(绿灯)
dispatch_semaphore_signal(semaphore);
}];
[task resume];
// 以下还要进行一些其他的耗时操作
NSLog(@"耗时操作继续进行");
dispatch_semaphore_wait(semaphore,DISPATCH_TIME_FOREVER);
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"刷新界面等在主线程的操作");
});
}
2019-02-24 18:19:16.251067+0800 Semaphore[33094:12267734] 耗时操作继续进行
2019-02-24 18:19:16.251071+0800 Semaphore[33094:12267735] 耗时操作继续进行
2019-02-24 18:19:16.549563+0800 Semaphore[33094:12267748] 第一步网络请求完成
2019-02-24 18:19:18.091922+0800 Semaphore[33094:12267737] 第二步网络请求完成
2019-02-24 18:19:18.092222+0800 Semaphore[33094:12267662] 刷新界面等在主线程的操作
设定的信号值为2，先执行两个线程，等执行完一个，才会继续执行下一个，保证同一时间执行的线程数不超过2。
dispatch_semaphore_t semaphore = dispatch_semaphore_create(2);
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//任务1
dispatch_async(queue, ^{
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"run task 1");
sleep(1);
NSLog(@"complete task 1");
dispatch_semaphore_signal(semaphore);
});
//任务2
dispatch_async(queue, ^{
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"run task 2");
sleep(1);
NSLog(@"complete task 2");
dispatch_semaphore_signal(semaphore);
});
//任务3
dispatch_async(queue, ^{
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"run task 3");
sleep(1);
NSLog(@"complete task 3");
dispatch_semaphore_signal(semaphore);
});
2019-02-24 18:31:02.447769+0800 Semaphore[33286:12284312] run task 1
2019-02-24 18:31:02.447767+0800 Semaphore[33286:12284310] run task 2
2019-02-24 18:31:03.450756+0800 Semaphore[33286:12284312] complete task 1
2019-02-24 18:31:03.450756+0800 Semaphore[33286:12284310] complete task 2
2019-02-24 18:31:03.450997+0800 Semaphore[33286:12284311] run task 3
2019-02-24 18:31:04.454259+0800 Semaphore[33286:12284311] complete task 3
