//当触摸屏幕的时候，从网络上下载一张图片到控制器的view上显示

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event

{

     //获取一个全局并行队列

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, );

     //把任务添加到队列中执行

     dispatch_async(queue, ^{

         //打印当前线程

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

       //从网络上下载图片

         NSURL *urlstr=[NSURL URLWithString:@"http://h.hiphotos.baidu.com/baike/w%3D268/sign=30b3fb747b310a55c424d9f28f444387/1e30e924b899a9018b8d3ab11f950a7b0308f5f9.jpg"];

         NSData *data=[NSData dataWithContentsOfURL:urlstr];

         UIImage *image=[UIImage imageWithData:data];

         //提示

         NSLog(@"图片加载完毕");

         dispatch_async(dispatch_get_main_queue(), ^{

             self.imageView.image=image;

             //打印当前线程

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

         });

     });

}