系统提供的dispatch方法

 //  后台执行：
     dispatch_async(dispatch_get_global_queue(0, 0), ^{
          // something
     });
    
     // 主线程执行：
     dispatch_async(dispatch_get_main_queue(), ^{
          // something
     });
    
     // 一次性执行：
     static dispatch_once_t onceToken;
     dispatch_once(&onceToken, ^{
         // code to be executed once
     });
    
     // 延迟2秒执行：
     double delayInSeconds = 2.0;
     dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
     dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
         // code to be executed on the main queue after delay
     });

 // 自定义dispatch_queue_t
     dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL);
     dispatch_async(urls_queue, ^{  
    　 　// your code
     });
     dispatch_release(urls_queue);

 // 合并汇总结果
     dispatch_group_t group = dispatch_group_create();
     dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
          // 并行执行的线程一
     });
     dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
          // 并行执行的线程二
     });
     dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{
          // 汇总结果
     });

    iOSDevTip

系统提供的dispatch方法

原文：http://blog.csdn.net/iosdevtip/article/details/44589861