dispatch_queue_t querue= dispatch_get_main_queue();

    dispatch_queue_t querue2=  dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT , 0);

    dispatch_async(querue2, ^{

        NSURL *url=[NSURL URLWithString:@"http://c.hiphotos.baidu.com/image/pic/item/cc11728b4710b912aa84a0bfc0fdfc039245222d.jpg"];

        NSData *data=http://www.mamicode.com/[NSData dataWithContentsOfURL:url];

                UIImage *image=[UIImage imageWithData:data];

        dispatch_async(querue, ^{

                self.imageview.image=image;

        });

    });

gcd