GCD下载图片

//网络图像获取

-(void)imageDownLoadByUrlASYNC:(NSString *)url
Complete:(complete)finished

{

   //异步并列执行  
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,
0), ^{     
UIImage *image = nil;

 NSURL *myUrl = [NSURL
URLWithString:url];

  // NSData *responseData = [NSData
dataWithContentsOfURL:myUrl];

  NSError *error;

NSData *responseData =
[NSData dataWithContentsOfURL:myUrl
options:NSDataReadingMappedIfSafe
error:&error];

 image = [UIImage
imageWithData:responseData];

  //跳回主队列执行

dispatch_async(dispatch_get_main_queue(),
^{

//在主队列中进行ui操作

 finished(image);

  });

});

}

//接收图片的处理

  UIImageView *imageView =
[imageViewArray objectAtIndex:i];

 UIActivityIndicatorView
*activeView = [activeViewArray
objectAtIndex:i];

  [activeView startAnimating];

 [imageView
imageDownLoadByUrlASYNC:[imageDownLoadURL
objectAtIndex:i]
Complete:^(UIImage *image) {

if(image)

imageView.image = image;

 else

 {

imageView.image = [UIImage
imageNamed:@"no.jpeg"];

 }

  [activeView
stopAnimating];

}];