GCD延时

在主线程中延迟执行某动作，不会卡主主线程，不影响后面的东做执行

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{

        NSLog(@"%@", [NSThread currentThread]);

    });

在子线程中执行某动作，不会卡主整个线程

    dispatch_queue_t queue  = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), queue, ^{


        NSLog(@"%@", [NSThread currentThread]);

    });