GCD的多线程的组

dispatch_semaphore_t 多任务按照顺序执行，也就是说单个任务一一执行

NSString *str = @"http://www.jianshu.com/p/6930f335adba";
NSURL *url = [NSURL URLWithString:str];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLSession *session = [NSURLSession sharedSession];

dispatch_semaphore_t sem = dispatch_semaphore_create(0);
for (int i=0; i<10; i++) {
    
    NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        
        NSLog(@"%d---%d",i,i);
        dispatch_semaphore_signal(sem);
    }];
    
    [task resume];
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
}

dispatch_async(dispatch_get_main_queue(), ^{
    NSLog(@"end");
});

semaphore

dispatch_group_t，如果单个任务依赖于其他多个任务并发完成后才能进行该任务

    __block int number = 0;
      
      dispatch_group_t group = dispatch_group_create();
      
      //A耗时操作
      dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
          sleep(3);
          
          number += 2222;
          NSLog(@"-----%d", number);

      });
      
      //B网络请求
      dispatch_group_enter(group);
      [self sendRequestWithCompletion:^(id response) {
          number += [response integerValue];
          NSLog(@"-----%d", number);

          dispatch_group_leave(group);

      }];
      
      //C网络请求
      dispatch_group_enter(group);
      [self sendRequestWithCompletion:^(id response) {
          number += [response integerValue];
          NSLog(@"-----%d", number);

          dispatch_group_leave(group);

      }];

    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"%zd", number + 1000);
    });

group

dispatch_barrier_async 如果多个任务并发依赖于其他多个任务并发执行的结果

dispatch_queue_t queue = dispatch_queue_create("test", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(queue, ^{
    NSLog(@"第一次任务的主线程为: %@", [NSThread currentThread]);
});

dispatch_async(queue, ^{
    NSLog(@"第二次任务的主线程为: %@", [NSThread currentThread]);
});

dispatch_barrier_async(queue, ^{
    NSLog(@"第一次任务, 第二次任务执行完毕, 继续执行");
});

dispatch_async(queue, ^{
    NSLog(@"第三次任务的主线程为: %@", [NSThread currentThread]);
});

dispatch_async(queue, ^{
    NSLog(@"第四次任务的主线程为: %@", [NSThread currentThread]);
});

barrier