1、组：dispatch_group
多个网络请求执行完，再执行刷新

dispatch_group_t group = dispatch_group_create();
NSLog(@"开始执行");

dispatch_group_enter(group);
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   NSLog(@"网络请求1");
   dispatch_group_leave(group);
}) ;

dispatch_group_enter(group);
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   NSLog(@"网络请求2");
   dispatch_group_leave(group);
}) ;

dispatch_group_enter(group);
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   NSLog(@"网络请求3");
   dispatch_group_leave(group);
}) ;

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
     NSLog(@"刷新界面");
 });
dispatch_group_wait(group, DISPATCH_TIME_FOREVER) ;
NSLog(@"等我一下");

以上代码运行打印：

开始执行
网络请求1
网络请求2
网络请求3
等我一下
刷新界面

2、dispatch_once 通常在创建单例时使用
3、dispatch_after 延迟执行
4、dispatch_barrier_async 栅栏函数
分割异步任务

dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   for (int i = 0; i< 3; i++) {
         NSLog(@"任务1");
    }
}) ;

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   for (int i = 0; i< 3; i++) {
         NSLog(@"任务2");
    }
}) ;

dispatch_barrier_async(dyConcurrent, ^{
        
        NSLog(@"拦一下");
    });

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   for (int i = 0; i< 3; i++) {
         NSLog(@"任务3");
    }
}) ;

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   for (int i = 0; i< 3; i++) {
         NSLog(@"任务4");
    }
}) ;

打印结果：121212拦一下343434

5、dispatch_apply 快速遍历
6、信号量 dispatch_semaphore
可控制最大并发

dispatch_semaphore_t  cacheSemphore =  dispatch_semaphore_create(5);
for (int i = 0; i< 10; i++) {
     dispatch_async(dispatch_queue_create("dyConcurrent1", DISPATCH_QUEUE_CONCURRENT), {
         dispatch_semaphore_wait(cacheSemphore , DISPATCH_TIME_FOREVER);
         NSLog(@"任务%ld",(long)i);
         sleep(1);
         NSLog(@"到我了");
         dispatch_semaphore_signal(cacheSemphore);
     });
}

打印结果：

任务0
任务1
任务2
任务3
任务4
到我了
到我了
到我了
到我了
到我了
任务5
任务6
任务7
任务8
任务9
到我了
到我了
到我了
到我了
到我了