系统提供的dispatch方法如下：

  //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, ^{
        //something
    });
    
    // 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){
        // 延迟2s在主线程中执行
    });
    
    //自定义queue
    dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL);
    dispatch_async(urls_queue, ^{
        //something
    });
    
    //让后台2个线程并行执行，然后等2个线程都结束后，再汇总执行结果
    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
    });

 参考资料：

http://www.devtang.com/blog/2012/02/22/use-gcd/

http://www.cnblogs.com/pure/archive/2013/03/31/2977420.html