iOS多线程GCD的基本用法

    //  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("com.titan.gcd", 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
    });