多线程

    //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, ^{
         //yuro 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
    });