dispatch_async(dispatch_get_global_queue(0, 0), ^{ 
    //后台程执行 something; 
}); 

dispatch_async(dispatch_get_main_queue(), ^{
    // 主线程执行something;
});

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{ 
    // 主要用于单例
});

double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PE R_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){ 
    // code to be executed on the main queue after delay
});

dispatch_queue_t myQueue = dispatch_queue_create("com.cnblogs.yybz", NULL); dispatch_async(myQueue, ^{
    // your code
});

dispatch_queue_t myQueue = dispatch_queue_create("com.cnblogs.yybz", NULL); 
dispatch_queue_t globalHightQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0); 
dispatch_set_target_queue(myQueue, globalHighQueue);

dispatch_group_async(group, queue, ^{ 
    // code
}); 

dispatch_group_wait(group, DISPATCH_TIME_FOREVER); //超时参数 
dispatch_group_notify(group, queue, ^{
    // code
});

dispatch_group_enter(group);
dispatch_async(queue, ^{
    //code
    dispatch_group_leave(group);
});

/**
@parameter iterations 迭代器数量
@parameter queue 提交任务的队列
@parameter (^block)(size_t) 任务Block,size_t 为迭代器下标
**/
dispatch_apply(size_t iterations, dispatch_queue_t queue, void (^block)(size_t));

dispatch_suspend(queue); 
dispatch_resume(myQueue);

dispatch_semaphore_create; //创建一个semaphore
dispatch_semaphore_signal; //发送一个信号
dispatch_semaphore_wait; //等待信号