GCD-06

延迟

1》perform

3秒后自动回到当前线程调用self的download：方法，并且传递参数：@"http://555.jpg"

// 一旦定制好延迟任务后，不会卡主当前线程，

    [self performSelector:@selector(download:) withObject:@"http://555.jpg" afterDelay:3];

2》dispatch_after

// 3秒后自动开始新线程执行block中的代码

    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(@"---task---%@", [NSThread currentThread]);

    });